| Message ID | 62e96e43de6f09e16f36d3d51af766c8fcbbb05f.1484839971.git.jpoimboe@redhat.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
On Thu 2017-01-19 09:46:21, Josh Poimboeuf wrote: > Change livepatch to use a basic per-task consistency model. This is the > foundation which will eventually enable us to patch those ~10% of > security patches which change function or data semantics. This is the > biggest remaining piece needed to make livepatch more generally useful. > > diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt > index 7f04e13..fb00d66 100644 > --- a/Documentation/livepatch/livepatch.txt > +++ b/Documentation/livepatch/livepatch.txt > +3.1 Adding consistency model support to new architectures > +--------------------------------------------------------- > + > +For adding consistency model support to new architectures, there are a > +few options: > + > +1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and > + for non-DWARF unwinders, also making sure there's a way for the stack > + tracing code to detect interrupts on the stack. > + > +2) Alternatively, figure out a way to patch kthreads without stack > + checking. If all kthreads sleep in the same place, then we can > + designate that place as a patching point. I think Petr M has been > + working on that? Alternatively, every kthread has to explicitely switch current->patch_state on a safe place. Kthreads are typically "infinite" loops that do some action repeatedly. The safe location is between the loops when there are no locks taken and all data structures are in a well defined state. The location is well defined when using the workqueues or kthread worker API. These kthreads process "independent" works in a generic loop. It is much more complicated with kthreads using a custom loop. There the safe place must be carefully localized case by case. > + In that case, arches without > + HAVE_RELIABLE_STACKTRACE would still be able to use the > + non-stack-checking parts of the consistency model: > + > + a) patching user tasks when they cross the kernel/user space > + boundary; and > + > + b) patching kthreads and idle tasks at their designated patch points. > + > + This option isn't as good as option 1 because it requires signaling > + most of the tasks to patch them. Kthreads need to be waken because most of them ignore signals. And idle tasks might need to be explicitely scheduled if there is too big load. Mirek knows more about this. Well, I am not sure if you want to get into such details. > + But it could still be a good backup > + option for those architectures which don't have reliable stack traces > + yet. > + > +In the meantime, patches for such architectures can bypass the > +consistency model by setting klp_patch.immediate to true. I would add that this is perfectly fine for patches that do not change semantic of the patched functions. In practice, this is usable in about 90% of security and critical fixes. > 4. Livepatch module > @@ -134,7 +242,7 @@ Documentation/livepatch/module-elf-format.txt for more details. > > > 4.2. Metadata > ------------- > +------------- > > The patch is described by several structures that split the information > into three levels: > @@ -239,9 +347,15 @@ Registered patches might be enabled either by calling klp_enable_patch() or > by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will > start using the new implementation of the patched functions at this stage. > > -In particular, if an original function is patched for the first time, a > -function specific struct klp_ops is created and an universal ftrace handler > -is registered. > +When a patch is enabled, livepatch enters into a transition state where > +tasks are converging to the patched state. This is indicated by a value > +of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks have > +been patched, the 'transition' value changes to '0'. For more > +information about this process, see the "Consistency model" section. > + > +If an original function is patched for the first time, a function > +specific struct klp_ops is created and an universal ftrace handler is > +registered. > > Functions might be patched multiple times. The ftrace handler is registered > only once for the given function. Further patches just add an entry to the > @@ -261,6 +375,12 @@ by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage > either the code from the previously enabled patch or even the original > code gets used. > > +When a patch is disabled, livepatch enters into a transition state where > +tasks are converging to the unpatched state. This is indicated by a > +value of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks > +have been unpatched, the 'transition' value changes to '0'. For more > +information about this process, see the "Consistency model" section. > + > Here all the functions (struct klp_func) associated with the to-be-disabled > patch are removed from the corresponding struct klp_ops. The ftrace handler > is unregistered and the struct klp_ops is freed when the func_stack list > diff --git a/include/linux/init_task.h b/include/linux/init_task.h > index 325f649..25f0360 100644 > --- a/include/linux/init_task.h > +++ b/include/linux/init_task.h > @@ -14,6 +14,7 @@ > #include <linux/rbtree.h> > #include <net/net_namespace.h> > #include <linux/sched/rt.h> > +#include <linux/livepatch.h> > > #include <asm/thread_info.h> > > @@ -185,6 +186,13 @@ extern struct task_group root_task_group; > # define INIT_KASAN(tsk) > #endif > > +#ifdef CONFIG_LIVEPATCH > +# define INIT_LIVEPATCH(tsk) \ > + .patch_state = KLP_UNDEFINED, > +#else > +# define INIT_LIVEPATCH(tsk) > +#endif > + > #ifdef CONFIG_THREAD_INFO_IN_TASK > # define INIT_TASK_TI(tsk) \ > .thread_info = INIT_THREAD_INFO(tsk), \ > @@ -271,6 +279,7 @@ extern struct task_group root_task_group; > INIT_VTIME(tsk) \ > INIT_NUMA_BALANCING(tsk) \ > INIT_KASAN(tsk) \ > + INIT_LIVEPATCH(tsk) \ > } > > > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h > index 6602b34..ed90ad1 100644 > --- a/include/linux/livepatch.h > +++ b/include/linux/livepatch.h > @@ -28,18 +28,40 @@ > > #include <asm/livepatch.h> > > +/* task patch states */ > +#define KLP_UNDEFINED -1 > +#define KLP_UNPATCHED 0 > +#define KLP_PATCHED 1 > + > /** > * struct klp_func - function structure for live patching > * @old_name: name of the function to be patched > * @new_func: pointer to the patched function code > * @old_sympos: a hint indicating which symbol position the old function > * can be found (optional) > + * @immediate: patch the func immediately, bypassing safety mechanisms > * @old_addr: the address of the function being patched > * @kobj: kobject for sysfs resources > * @stack_node: list node for klp_ops func_stack list > * @old_size: size of the old function > * @new_size: size of the new function > * @patched: the func has been added to the klp_ops list > + * @transition: the func is currently being applied or reverted > + * > + * The patched and transition variables define the func's patching state. When > + * patching, a func is always in one of the following states: > + * > + * patched=0 transition=0: unpatched > + * patched=0 transition=1: unpatched, temporary starting state > + * patched=1 transition=1: patched, may be visible to some tasks > + * patched=1 transition=0: patched, visible to all tasks > + * > + * And when unpatching, it goes in the reverse order: > + * > + * patched=1 transition=0: patched, visible to all tasks > + * patched=1 transition=1: patched, may be visible to some tasks > + * patched=0 transition=1: unpatched, temporary ending state > + * patched=0 transition=0: unpatched > */ > struct klp_func { > /* external */ > @@ -53,6 +75,7 @@ struct klp_func { > * in kallsyms for the given object is used. > */ > unsigned long old_sympos; > + bool immediate; > > /* internal */ > unsigned long old_addr; > @@ -60,6 +83,7 @@ struct klp_func { > struct list_head stack_node; > unsigned long old_size, new_size; > bool patched; > + bool transition; > }; > > /** > @@ -68,7 +92,7 @@ struct klp_func { > * @funcs: function entries for functions to be patched in the object > * @kobj: kobject for sysfs resources > * @mod: kernel module associated with the patched object > - * (NULL for vmlinux) > + * (NULL for vmlinux) > * @patched: the object's funcs have been added to the klp_ops list > */ > struct klp_object { > @@ -86,6 +110,7 @@ struct klp_object { > * struct klp_patch - patch structure for live patching > * @mod: reference to the live patch module > * @objs: object entries for kernel objects to be patched > + * @immediate: patch all funcs immediately, bypassing safety mechanisms > * @list: list node for global list of registered patches > * @kobj: kobject for sysfs resources > * @enabled: the patch is enabled (but operation may be incomplete) > @@ -94,6 +119,7 @@ struct klp_patch { > /* external */ > struct module *mod; > struct klp_object *objs; > + bool immediate; > > /* internal */ > struct list_head list; > @@ -121,13 +147,27 @@ void arch_klp_init_object_loaded(struct klp_patch *patch, > int klp_module_coming(struct module *mod); > void klp_module_going(struct module *mod); > > +void klp_copy_process(struct task_struct *child); > void klp_update_patch_state(struct task_struct *task); > > +static inline bool klp_patch_pending(struct task_struct *task) > +{ > + return test_tsk_thread_flag(task, TIF_PATCH_PENDING); > +} > + > +static inline bool klp_have_reliable_stack(void) > +{ > + return IS_ENABLED(CONFIG_STACKTRACE) && > + IS_ENABLED(CONFIG_HAVE_RELIABLE_STACKTRACE); > +} > + > #else /* !CONFIG_LIVEPATCH */ > > static inline int klp_module_coming(struct module *mod) { return 0; } > static inline void klp_module_going(struct module *mod) {} > +static inline bool klp_patch_pending(struct task_struct *task) { return false; } > static inline void klp_update_patch_state(struct task_struct *task) {} > +static inline void klp_copy_process(struct task_struct *child) {} > > #endif /* CONFIG_LIVEPATCH */ > > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 9df1f42..ccbb1fa 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1973,6 +1973,9 @@ struct task_struct { > /* A live task holds one reference. */ > atomic_t stack_refcount; > #endif > +#ifdef CONFIG_LIVEPATCH > + int patch_state; > +#endif > /* CPU-specific state of this task */ > struct thread_struct thread; > /* > diff --git a/kernel/fork.c b/kernel/fork.c > index 7da33cb..8c8de80 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -77,6 +77,7 @@ > #include <linux/compiler.h> > #include <linux/sysctl.h> > #include <linux/kcov.h> > +#include <linux/livepatch.h> > > #include <asm/pgtable.h> > #include <asm/pgalloc.h> > @@ -1765,6 +1766,8 @@ static __latent_entropy struct task_struct *copy_process( > p->parent_exec_id = current->self_exec_id; > } > > + klp_copy_process(p); > + > spin_lock(¤t->sighand->siglock); > > /* > diff --git a/kernel/livepatch/Makefile b/kernel/livepatch/Makefile > index e136dad..2b8bdb1 100644 > --- a/kernel/livepatch/Makefile > +++ b/kernel/livepatch/Makefile > @@ -1,3 +1,3 @@ > obj-$(CONFIG_LIVEPATCH) += livepatch.o > > -livepatch-objs := core.o patch.o > +livepatch-objs := core.o patch.o transition.o > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c > index 10ba3a1..4c5a816 100644 > --- a/kernel/livepatch/core.c > +++ b/kernel/livepatch/core.c > @@ -31,22 +31,22 @@ > #include <linux/moduleloader.h> > #include <asm/cacheflush.h> > #include "patch.h" > +#include "transition.h" > > /* > - * The klp_mutex protects the global lists and state transitions of any > - * structure reachable from them. References to any structure must be obtained > - * under mutex protection (except in klp_ftrace_handler(), which uses RCU to > - * ensure it gets consistent data). > + * klp_mutex is a coarse lock which serializes access to klp data. All > + * accesses to klp-related variables and structures must have mutex protection, > + * except within the following functions which carefully avoid the need for it: > + * > + * - klp_ftrace_handler() > + * - klp_update_patch_state() > */ > -static DEFINE_MUTEX(klp_mutex); > +DEFINE_MUTEX(klp_mutex); > > static LIST_HEAD(klp_patches); > > static struct kobject *klp_root_kobj; > > -/* TODO: temporary stub */ > -void klp_update_patch_state(struct task_struct *task) {} > - > static bool klp_is_module(struct klp_object *obj) > { > return obj->name; > @@ -85,7 +85,6 @@ static void klp_find_object_module(struct klp_object *obj) > mutex_unlock(&module_mutex); > } > > -/* klp_mutex must be held by caller */ > static bool klp_is_patch_registered(struct klp_patch *patch) > { > struct klp_patch *mypatch; > @@ -281,20 +280,25 @@ static int klp_write_object_relocations(struct module *pmod, > > static int __klp_disable_patch(struct klp_patch *patch) > { > - struct klp_object *obj; > + if (klp_transition_patch) > + return -EBUSY; > > /* enforce stacking: only the last enabled patch can be disabled */ > if (!list_is_last(&patch->list, &klp_patches) && > list_next_entry(patch, list)->enabled) > return -EBUSY; > > - pr_notice("disabling patch '%s'\n", patch->mod->name); > + klp_init_transition(patch, KLP_UNPATCHED); > > - klp_for_each_object(patch, obj) { > - if (obj->patched) > - klp_unpatch_object(obj); > - } > + /* > + * Enforce the order of the klp_target_state write in > + * klp_init_transition() and the TIF_PATCH_PENDING writes in > + * klp_start_transition() to ensure that klp_update_patch_state() > + * doesn't set a task->patch_state to KLP_UNDEFINED. > + */ > + smp_wmb(); The description is not clear. The klp_target_state manipulation is synchronized by another barrier inside klp_init_transition(). Do we need this barrier at all? It is not needed when the patch is immediate. klp_target_state and task->patch_state are synchronized againts func->transition by the other barrier in klp_init_transition(). The only thing left is synchronizing func->transition against TIF_PATCH_PENDING. But the compiler barrier should be enough here. The task must not be migrated when it is inside klp_ftrace_handler(). At least I am not able to find the place for the corresponding read barrier. Note that we need the write barrier in __klp_enable_patch() because ops->func_stack writes in klp_patch_object(). It is correctly described there. So, I would the write barrier here with something like: /* * Enforce the order of func->transtion write in * klp_init_transition() against TIF_PATCH_PENDING writes * in klp_start_transition(). The compiler barrier is * enough because TIF_PATCH_PENDING flag has no effect * when a task is inside klp_ftrace_handler(). It is * the place where func->transition is read without lock. * It is not a safe state to migrate a task, definitely. */ barrier(); > + klp_start_transition(); > patch->enabled = false; > > return 0; > @@ -337,6 +341,9 @@ static int __klp_enable_patch(struct klp_patch *patch) > struct klp_object *obj; > int ret; > > + if (klp_transition_patch) > + return -EBUSY; > + > if (WARN_ON(patch->enabled)) > return -EINVAL; > > @@ -347,22 +354,37 @@ static int __klp_enable_patch(struct klp_patch *patch) > > pr_notice("enabling patch '%s'\n", patch->mod->name); > > + klp_init_transition(patch, KLP_PATCHED); > + > + /* > + * Enforce the order of the func->transition writes in > + * klp_init_transition() and the ops->func_stack writes in > + * klp_patch_object(), so that klp_ftrace_handler() will see the > + * func->transition updates before the handler is registered and the > + * new funcs become visible to the handler. > + */ > + smp_wmb(); > + > klp_for_each_object(patch, obj) { > if (!klp_is_object_loaded(obj)) > continue; > > ret = klp_patch_object(obj); > - if (ret) > - goto unregister; > + if (ret) { > + pr_warn("failed to enable patch '%s'\n", > + patch->mod->name); > + > + klp_unpatch_objects(patch); We should call here synchronize_rcu() here as we do in klp_try_complete_transition. Some of the affected functions might have more patches on the stack and we need to make sure that klp_ftrace_handler() will see the removed patch on the stack. In fact, I would put/move the synchronize_rcu() into klp_unpatch_objects(). > + klp_complete_transition(); > + > + return ret; > + } > } > > + klp_start_transition(); > patch->enabled = true; > > return 0; > - > -unregister: > - WARN_ON(__klp_disable_patch(patch)); > - return ret; > } > > /** > @@ -399,6 +421,7 @@ EXPORT_SYMBOL_GPL(klp_enable_patch); > * /sys/kernel/livepatch > * /sys/kernel/livepatch/<patch> > * /sys/kernel/livepatch/<patch>/enabled > + * /sys/kernel/livepatch/<patch>/transition > * /sys/kernel/livepatch/<patch>/<object> > * /sys/kernel/livepatch/<patch>/<object>/<function,sympos> > */ > @@ -424,7 +447,9 @@ static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr, > goto err; > } > > - if (enabled) { > + if (patch == klp_transition_patch) { > + klp_reverse_transition(); > + } else if (enabled) { > ret = __klp_enable_patch(patch); > if (ret) > goto err; > @@ -452,9 +477,21 @@ static ssize_t enabled_show(struct kobject *kobj, > return snprintf(buf, PAGE_SIZE-1, "%d\n", patch->enabled); > } > > +static ssize_t transition_show(struct kobject *kobj, > + struct kobj_attribute *attr, char *buf) > +{ > + struct klp_patch *patch; > + > + patch = container_of(kobj, struct klp_patch, kobj); > + return snprintf(buf, PAGE_SIZE-1, "%d\n", > + patch == klp_transition_patch); > +} > + > static struct kobj_attribute enabled_kobj_attr = __ATTR_RW(enabled); > +static struct kobj_attribute transition_kobj_attr = __ATTR_RO(transition); > static struct attribute *klp_patch_attrs[] = { > &enabled_kobj_attr.attr, > + &transition_kobj_attr.attr, > NULL > }; > > @@ -544,6 +581,7 @@ static int klp_init_func(struct klp_object *obj, struct klp_func *func) > > INIT_LIST_HEAD(&func->stack_node); > func->patched = false; > + func->transition = false; > > /* The format for the sysfs directory is <function,sympos> where sympos > * is the nth occurrence of this symbol in kallsyms for the patched > @@ -740,6 +778,16 @@ int klp_register_patch(struct klp_patch *patch) > return -ENODEV; > > /* > + * Architectures without reliable stack traces have to set > + * patch->immediate because there's currently no way to patch kthreads > + * with the consistency model. > + */ > + if (!klp_have_reliable_stack() && !patch->immediate) { > + pr_err("This architecture doesn't have support for the livepatch consistency model.\n"); > + return -ENOSYS; > + } > + > + /* > * A reference is taken on the patch module to prevent it from being > * unloaded. Right now, we don't allow patch modules to unload since > * there is currently no method to determine if a thread is still > @@ -788,7 +836,11 @@ int klp_module_coming(struct module *mod) > goto err; > } > > - if (!patch->enabled) > + /* > + * Only patch the module if the patch is enabled or is > + * in transition. > + */ > + if (!patch->enabled && patch != klp_transition_patch) > break; > > pr_notice("applying patch '%s' to loading module '%s'\n", > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c > index 5efa262..1a77f05 100644 > --- a/kernel/livepatch/patch.c > +++ b/kernel/livepatch/patch.c > @@ -29,6 +29,7 @@ > #include <linux/bug.h> > #include <linux/printk.h> > #include "patch.h" > +#include "transition.h" > > static LIST_HEAD(klp_ops); > > @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip, > { > struct klp_ops *ops; > struct klp_func *func; > + int patch_state; > > ops = container_of(fops, struct klp_ops, fops); > > rcu_read_lock(); > + > func = list_first_or_null_rcu(&ops->func_stack, struct klp_func, > stack_node); > + > + /* > + * func should never be NULL because preemption should be disabled here > + * and unregister_ftrace_function() does the equivalent of a > + * synchronize_sched() before the func_stack removal. > + */ > + if (WARN_ON_ONCE(!func)) > + goto unlock; > + > + /* > + * Enforce the order of the ops->func_stack and func->transition reads. > + * The corresponding write barrier is in __klp_enable_patch(). > + */ > + smp_rmb(); I was curious why the comment did not mention __klp_disable_patch(). It helped me to find that we did not need the write barrier there. The counter piece in the disable path would be in klp_try_complete_transition() and klp_unpatch_objects(). But there we need rcu_sychronize as a bigger hammer. > + if (unlikely(func->transition)) { > + > + /* > + * Enforce the order of the func->transition and > + * current->patch_state reads. Otherwise we could read an > + * out-of-date task state and pick the wrong function. The > + * corresponding write barriers are in klp_init_transition() > + * and __klp_disable_patch(). > + */ > + smp_rmb(); We actually do not need the barrier in __klp_disable_patch(). The func->transition and task->patch_state manipulation are synchronized by the barrier in klp_init_transition(). The manipulation related to TIF_PATCH_PENDING is secure. klp_update_patch() must not be called when the task is inside this ftrace handler. I am sorry if I repeate myself. I try to check comments about memory barriers. The things are complicated. Every location helps to me to see it from a different point and use a bit different wording. > + patch_state = current->patch_state; > + > + WARN_ON_ONCE(patch_state == KLP_UNDEFINED); > + > + if (patch_state == KLP_UNPATCHED) { > + /* > + * Use the previously patched version of the function. > + * If no previous patches exist, continue with the > + * original function. > + */ > + func = list_entry_rcu(func->stack_node.next, > + struct klp_func, stack_node); > + > + if (&func->stack_node == &ops->func_stack) > + goto unlock; > + } > + } > + > klp_arch_set_pc(regs, (unsigned long)func->new_func); > unlock: > rcu_read_unlock(); > @@ -211,3 +255,12 @@ int klp_patch_object(struct klp_object *obj) > > return 0; > } > + > +void klp_unpatch_objects(struct klp_patch *patch) > +{ > + struct klp_object *obj; > + > + klp_for_each_object(patch, obj) > + if (obj->patched) > + klp_unpatch_object(obj); > +} > diff --git a/kernel/livepatch/patch.h b/kernel/livepatch/patch.h > index 2d0cce0..0db2271 100644 > --- a/kernel/livepatch/patch.h > +++ b/kernel/livepatch/patch.h > @@ -28,5 +28,6 @@ struct klp_ops *klp_find_ops(unsigned long old_addr); > > int klp_patch_object(struct klp_object *obj); > void klp_unpatch_object(struct klp_object *obj); > +void klp_unpatch_objects(struct klp_patch *patch); > > #endif /* _LIVEPATCH_PATCH_H */ > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c > new file mode 100644 > index 0000000..2b87ff9 > --- /dev/null > +++ b/kernel/livepatch/transition.c > @@ -0,0 +1,525 @@ > +/* > + * transition.c - Kernel Live Patching transition functions > + * > + * Copyright (C) 2015-2016 Josh Poimboeuf <jpoimboe@redhat.com> > + * > + * This program is free software; you can redistribute it and/or > + * modify it under the terms of the GNU General Public License > + * as published by the Free Software Foundation; either version 2 > + * of the License, or (at your option) any later version. > + * > + * This program is distributed in the hope that it will be useful, > + * but WITHOUT ANY WARRANTY; without even the implied warranty of > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > + * GNU General Public License for more details. > + * > + * You should have received a copy of the GNU General Public License > + * along with this program; if not, see <http://www.gnu.org/licenses/>. > + */ > + > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt > + > +#include <linux/cpu.h> > +#include <linux/stacktrace.h> > +#include "patch.h" > +#include "transition.h" > +#include "../sched/sched.h" > + > +#define MAX_STACK_ENTRIES 100 > +#define STACK_ERR_BUF_SIZE 128 > + > +extern struct mutex klp_mutex; > + > +struct klp_patch *klp_transition_patch; > + > +static int klp_target_state = KLP_UNDEFINED; > + > +/* > + * This work can be performed periodically to finish patching or unpatching any > + * "straggler" tasks which failed to transition in the first attempt. > + */ > +static void klp_try_complete_transition(void); > +static void klp_transition_work_fn(struct work_struct *work) > +{ > + mutex_lock(&klp_mutex); > + > + if (klp_transition_patch) > + klp_try_complete_transition(); > + > + mutex_unlock(&klp_mutex); > +} > +static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn); > + > +/* > + * The transition to the target patch state is complete. Clean up the data > + * structures. > + */ > +void klp_complete_transition(void) > +{ > + struct klp_object *obj; > + struct klp_func *func; > + struct task_struct *g, *task; > + unsigned int cpu; > + > + if (klp_transition_patch->immediate) > + goto done; > + > + klp_for_each_object(klp_transition_patch, obj) > + klp_for_each_func(obj, func) > + func->transition = false; > + > + /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */ > + if (klp_target_state == KLP_PATCHED) > + synchronize_rcu(); > + > + read_lock(&tasklist_lock); > + for_each_process_thread(g, task) { > + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); > + task->patch_state = KLP_UNDEFINED; > + } > + read_unlock(&tasklist_lock); > + > + for_each_possible_cpu(cpu) { > + task = idle_task(cpu); > + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); > + task->patch_state = KLP_UNDEFINED; > + } > + > +done: > + klp_target_state = KLP_UNDEFINED; > + klp_transition_patch = NULL; > +} > + > +/* > + * Switch the patched state of the task to the set of functions in the target > + * patch state. > + * > + * NOTE: If task is not 'current', the caller must ensure the task is inactive. > + * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value. > + */ > +void klp_update_patch_state(struct task_struct *task) > +{ > + rcu_read_lock(); > + > + /* > + * This test_and_clear_tsk_thread_flag() call also serves as a read > + * barrier to enforce the order of the TIF_PATCH_PENDING and > + * klp_target_state reads. The corresponding write barrier is in > + * __klp_disable_patch(). > + */ > + if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING)) > + task->patch_state = READ_ONCE(klp_target_state); > + > + rcu_read_unlock(); > +} > + > +/* > + * Determine whether the given stack trace includes any references to a > + * to-be-patched or to-be-unpatched function. > + */ > +static int klp_check_stack_func(struct klp_func *func, > + struct stack_trace *trace) > +{ > + unsigned long func_addr, func_size, address; > + struct klp_ops *ops; > + int i; > + > + if (func->immediate) > + return 0; > + > + for (i = 0; i < trace->nr_entries; i++) { > + address = trace->entries[i]; > + > + if (klp_target_state == KLP_UNPATCHED) { > + /* > + * Check for the to-be-unpatched function > + * (the func itself). > + */ > + func_addr = (unsigned long)func->new_func; > + func_size = func->new_size; > + } else { > + /* > + * Check for the to-be-patched function > + * (the previous func). > + */ > + ops = klp_find_ops(func->old_addr); > + > + if (list_is_singular(&ops->func_stack)) { > + /* original function */ > + func_addr = func->old_addr; > + func_size = func->old_size; > + } else { > + /* previously patched function */ > + struct klp_func *prev; > + > + prev = list_next_entry(func, stack_node); > + func_addr = (unsigned long)prev->new_func; > + func_size = prev->new_size; > + } > + } > + > + if (address >= func_addr && address < func_addr + func_size) > + return -EAGAIN; > + } > + > + return 0; > +} > + > +/* > + * Determine whether it's safe to transition the task to the target patch state > + * by looking for any to-be-patched or to-be-unpatched functions on its stack. > + */ > +static int klp_check_stack(struct task_struct *task, char *err_buf) > +{ > + static unsigned long entries[MAX_STACK_ENTRIES]; > + struct stack_trace trace; > + struct klp_object *obj; > + struct klp_func *func; > + int ret; > + > + trace.skip = 0; > + trace.nr_entries = 0; > + trace.max_entries = MAX_STACK_ENTRIES; > + trace.entries = entries; > + ret = save_stack_trace_tsk_reliable(task, &trace); > + WARN_ON_ONCE(ret == -ENOSYS); > + if (ret) { > + snprintf(err_buf, STACK_ERR_BUF_SIZE, > + "%s: %s:%d has an unreliable stack\n", > + __func__, task->comm, task->pid); > + return ret; > + } > + > + klp_for_each_object(klp_transition_patch, obj) { > + if (!obj->patched) > + continue; > + klp_for_each_func(obj, func) { > + ret = klp_check_stack_func(func, &trace); > + if (ret) { > + snprintf(err_buf, STACK_ERR_BUF_SIZE, > + "%s: %s:%d is sleeping on function %s\n", > + __func__, task->comm, task->pid, > + func->old_name); > + return ret; > + } > + } > + } > + > + return 0; > +} > + > +/* > + * Try to safely switch a task to the target patch state. If it's currently > + * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or > + * if the stack is unreliable, return false. > + */ > +static bool klp_try_switch_task(struct task_struct *task) > +{ > + struct rq *rq; > + struct rq_flags flags; > + int ret; > + bool success = false; > + char err_buf[STACK_ERR_BUF_SIZE]; > + > + err_buf[0] = '\0'; > + > + /* check if this task has already switched over */ > + if (task->patch_state == klp_target_state) > + return true; > + > + /* > + * For arches which don't have reliable stack traces, we have to rely > + * on other methods (e.g., switching tasks at kernel exit). > + */ > + if (!klp_have_reliable_stack()) > + return false; > + > + /* > + * Now try to check the stack for any to-be-patched or to-be-unpatched > + * functions. If all goes well, switch the task to the target patch > + * state. > + */ > + rq = task_rq_lock(task, &flags); > + > + if (task_running(rq, task) && task != current) { > + snprintf(err_buf, STACK_ERR_BUF_SIZE, > + "%s: %s:%d is running\n", __func__, task->comm, > + task->pid); > + goto done; > + } > + > + ret = klp_check_stack(task, err_buf); > + if (ret) > + goto done; > + > + success = true; > + > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > + task->patch_state = klp_target_state; > + > +done: > + task_rq_unlock(rq, task, &flags); > + > + /* > + * Due to console deadlock issues, pr_debug() can't be used while > + * holding the task rq lock. Instead we have to use a temporary buffer > + * and print the debug message after releasing the lock. > + */ > + if (err_buf[0] != '\0') > + pr_debug("%s", err_buf); > + > + return success; > + > +} > + > +/* > + * Try to switch all remaining tasks to the target patch state by walking the > + * stacks of sleeping tasks and looking for any to-be-patched or > + * to-be-unpatched functions. If such functions are found, the task can't be > + * switched yet. > + * > + * If any tasks are still stuck in the initial patch state, schedule a retry. > + */ > +static void klp_try_complete_transition(void) > +{ > + unsigned int cpu; > + struct task_struct *g, *task; > + bool complete = true; > + > + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED); > + > + /* > + * If the patch can be applied or reverted immediately, skip the > + * per-task transitions. > + */ > + if (klp_transition_patch->immediate) > + goto success; > + > + /* > + * Try to switch the tasks to the target patch state by walking their > + * stacks and looking for any to-be-patched or to-be-unpatched > + * functions. If such functions are found on a stack, or if the stack > + * is deemed unreliable, the task can't be switched yet. > + * > + * Usually this will transition most (or all) of the tasks on a system > + * unless the patch includes changes to a very common function. > + */ > + read_lock(&tasklist_lock); > + for_each_process_thread(g, task) > + if (!klp_try_switch_task(task)) > + complete = false; > + read_unlock(&tasklist_lock); > + > + /* > + * Ditto for the idle "swapper" tasks. > + */ > + get_online_cpus(); > + for_each_possible_cpu(cpu) { > + task = idle_task(cpu); > + if (cpu_online(cpu)) { > + if (!klp_try_switch_task(task)) > + complete = false; > + } else if (task->patch_state != klp_target_state) { > + /* offline idle tasks can be switched immediately */ > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > + task->patch_state = klp_target_state; > + } > + } > + put_online_cpus(); > + > + /* > + * Some tasks weren't able to be switched over. Try again later and/or > + * wait for other methods like kernel exit switching. > + */ > + if (!complete) { > + schedule_delayed_work(&klp_transition_work, > + round_jiffies_relative(HZ)); > + return; > + } > + > +success: > + > + if (klp_target_state == KLP_UNPATCHED) { > + /* > + * All tasks have transitioned to KLP_UNPATCHED so we can now > + * remove the new functions from the func_stack. > + */ > + klp_unpatch_objects(klp_transition_patch); > + > + /* > + * Make sure klp_ftrace_handler() sees that the functions have > + * been removed from ops->func_stack before we clear > + * func->transition. Otherwise it may pick the wrong > + * transition. > + */ > + synchronize_rcu(); > + } > + > + pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name, > + klp_target_state == KLP_PATCHED ? "patching" : "unpatching"); > + > + /* we're done, now cleanup the data structures */ > + klp_complete_transition(); > +} > + > +/* > + * Start the transition to the specified target patch state so tasks can begin > + * switching to it. > + */ > +void klp_start_transition(void) > +{ > + struct task_struct *g, *task; > + unsigned int cpu; > + > + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED); > + > + pr_notice("'%s': %s...\n", klp_transition_patch->mod->name, > + klp_target_state == KLP_PATCHED ? "patching" : "unpatching"); > + > + /* > + * If the patch can be applied or reverted immediately, skip the > + * per-task transitions. > + */ > + if (klp_transition_patch->immediate) > + return; > + > + /* > + * Mark all normal tasks as needing a patch state update. They'll > + * switch either in klp_try_complete_transition() or as they exit the > + * kernel. > + */ > + read_lock(&tasklist_lock); > + for_each_process_thread(g, task) > + if (task->patch_state != klp_target_state) > + set_tsk_thread_flag(task, TIF_PATCH_PENDING); > + read_unlock(&tasklist_lock); > + > + /* > + * Mark all idle tasks as needing a patch state update. They'll switch > + * either in klp_try_complete_transition() or at the idle loop switch > + * point. > + */ > + for_each_possible_cpu(cpu) { > + task = idle_task(cpu); > + if (task->patch_state != klp_target_state) > + set_tsk_thread_flag(task, TIF_PATCH_PENDING); > + } > + > + klp_try_complete_transition(); > +} > + > +/* > + * Initialize the global target patch state and all tasks to the initial patch > + * state, and initialize all function transition states to true in preparation > + * for patching or unpatching. > + */ > +void klp_init_transition(struct klp_patch *patch, int state) > +{ > + struct task_struct *g, *task; > + unsigned int cpu; > + struct klp_object *obj; > + struct klp_func *func; > + int initial_state = !state; > + > + WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED); > + > + klp_transition_patch = patch; > + > + /* > + * Set the global target patch state which tasks will switch to. This > + * has no effect until the TIF_PATCH_PENDING flags get set later. > + */ > + klp_target_state = state; > + > + /* > + * If the patch can be applied or reverted immediately, skip the > + * per-task transitions. > + */ > + if (patch->immediate) > + return; > + > + /* > + * Initialize all tasks to the initial patch state to prepare them for > + * switching to the target state. > + */ > + read_lock(&tasklist_lock); > + for_each_process_thread(g, task) { > + WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED); > + task->patch_state = initial_state; > + } > + read_unlock(&tasklist_lock); > + > + /* > + * Ditto for the idle "swapper" tasks. > + */ > + for_each_possible_cpu(cpu) { > + task = idle_task(cpu); > + WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED); > + task->patch_state = initial_state; > + } > + > + /* > + * Enforce the order of the task->patch_state initializations and the > + * func->transition updates to ensure that, in the enable path, > + * klp_ftrace_handler() doesn't see a func in transition with a > + * task->patch_state of KLP_UNDEFINED. > + */ > + smp_wmb(); > + > + /* > + * Set the func transition states so klp_ftrace_handler() will know to > + * switch to the transition logic. > + * > + * When patching, the funcs aren't yet in the func_stack and will be > + * made visible to the ftrace handler shortly by the calls to > + * klp_patch_object(). > + * > + * When unpatching, the funcs are already in the func_stack and so are > + * already visible to the ftrace handler. > + */ > + klp_for_each_object(patch, obj) > + klp_for_each_func(obj, func) > + func->transition = true; > +} > + > +/* > + * This function can be called in the middle of an existing transition to > + * reverse the direction of the target patch state. This can be done to > + * effectively cancel an existing enable or disable operation if there are any > + * tasks which are stuck in the initial patch state. > + */ > +void klp_reverse_transition(void) > +{ > + unsigned int cpu; > + struct task_struct *g, *task; > + > + klp_transition_patch->enabled = !klp_transition_patch->enabled; > + > + klp_target_state = !klp_target_state; > + > + /* > + * Clear all TIF_PATCH_PENDING flags to prevent races caused by > + * klp_update_patch_state() running in parallel with > + * klp_start_transition(). > + */ > + read_lock(&tasklist_lock); > + for_each_process_thread(g, task) > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > + read_unlock(&tasklist_lock); > + > + for_each_possible_cpu(cpu) > + clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING); > + > + /* Let any remaining calls to klp_update_patch_state() complete */ > + synchronize_rcu(); > + > + klp_start_transition(); > +} > + > +/* Called from copy_process() during fork */ > +void klp_copy_process(struct task_struct *child) > +{ > + child->patch_state = current->patch_state; > + > + /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */ > +} > diff --git a/kernel/livepatch/transition.h b/kernel/livepatch/transition.h > new file mode 100644 > index 0000000..3ee625f > --- /dev/null > +++ b/kernel/livepatch/transition.h > @@ -0,0 +1,13 @@ > +#ifndef _LIVEPATCH_TRANSITION_H > +#define _LIVEPATCH_TRANSITION_H > + > +#include <linux/livepatch.h> > + > +extern struct klp_patch *klp_transition_patch; > + > +void klp_init_transition(struct klp_patch *patch, int state); > +void klp_start_transition(void); > +void klp_reverse_transition(void); > +void klp_complete_transition(void); > + > +#endif /* _LIVEPATCH_TRANSITION_H */ > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > index 6a4bae0..a8b3f1a 100644 > --- a/kernel/sched/idle.c > +++ b/kernel/sched/idle.c > @@ -9,6 +9,7 @@ > #include <linux/mm.h> > #include <linux/stackprotector.h> > #include <linux/suspend.h> > +#include <linux/livepatch.h> > > #include <asm/tlb.h> > > @@ -264,6 +265,9 @@ static void do_idle(void) > > sched_ttwu_pending(); > schedule_preempt_disabled(); > + > + if (unlikely(klp_patch_pending(current))) > + klp_update_patch_state(current); > } > > bool cpu_in_idle(unsigned long pc) > diff --git a/samples/livepatch/livepatch-sample.c b/samples/livepatch/livepatch-sample.c > index e34f871..629e0dc 100644 > --- a/samples/livepatch/livepatch-sample.c > +++ b/samples/livepatch/livepatch-sample.c > @@ -17,6 +17,8 @@ > * along with this program; if not, see <http://www.gnu.org/licenses/>. > */ > > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt > + > #include <linux/module.h> > #include <linux/kernel.h> > #include <linux/livepatch.h> > @@ -69,6 +71,21 @@ static int livepatch_init(void) > { > int ret; > > + if (!klp_have_reliable_stack() && !patch.immediate) { > + /* > + * WARNING: Be very careful when using 'patch.immediate' in > + * your patches. It's ok to use it for simple patches like > + * this, but for more complex patches which change function > + * semantics, locking semantics, or data structures, it may not > + * be safe. Use of this option will also prevent removal of > + * the patch. > + * > + * See Documentation/livepatch/livepatch.txt for more details. > + */ > + patch.immediate = true; > + pr_notice("The consistency model isn't supported for your architecture. Bypassing safety mechanisms and applying the patch immediately.\n"); > + } > + > ret = klp_register_patch(&patch); > if (ret) > return ret; > -- > 2.7.4 >
IMPORTANT: Please, forget this version. It is few days old and incomplete and probably wrong. I am sorry for confusion. Best Regards, Petr
!!! This is the right version. I am sorry again for the confusion. !!! > Change livepatch to use a basic per-task consistency model. This is the > foundation which will eventually enable us to patch those ~10% of > security patches which change function or data semantics. This is the > biggest remaining piece needed to make livepatch more generally useful. > > diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt > index 7f04e13..fb00d66 100644 > --- a/Documentation/livepatch/livepatch.txt > +++ b/Documentation/livepatch/livepatch.txt > +3.1 Adding consistency model support to new architectures > +--------------------------------------------------------- > + > +For adding consistency model support to new architectures, there are a > +few options: > + > +1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and > + for non-DWARF unwinders, also making sure there's a way for the stack > + tracing code to detect interrupts on the stack. > + > +2) Alternatively, figure out a way to patch kthreads without stack > + checking. If all kthreads sleep in the same place, then we can > + designate that place as a patching point. I think Petr M has been > + working on that? Here is version with some more details: Alternatively, every kthread has to explicitely switch current->patch_state on a safe place. Kthreads are typically "infinite" loops that do some action repeatedly. The safe location is between the loops when there are no locks taken and all data structures are in a well defined state. The location is clear when using the workqueues or the kthread worker API. These kthreads process "independent" works in a generic loop. It is much more complicated with kthreads using a custom loop. There the safe place must be carefully localized case by case. > + In that case, arches without > + HAVE_RELIABLE_STACKTRACE would still be able to use the > + non-stack-checking parts of the consistency model: > + > + a) patching user tasks when they cross the kernel/user space > + boundary; and > + > + b) patching kthreads and idle tasks at their designated patch points. > + > + This option isn't as good as option 1 because it requires signaling > + most of the tasks to patch them. Kthreads need to be waken because most of them ignore signals. And idle tasks might need to be explicitely scheduled if there is too big load. Mirek knows more about this. Well, I am not sure if you want to get into such details. > + But it could still be a good backup > + option for those architectures which don't have reliable stack traces > + yet. > + > +In the meantime, patches for such architectures can bypass the > +consistency model by setting klp_patch.immediate to true. I would add that this is perfectly fine for patches that do not change semantic of the patched functions. In practice, this is usable in about 90% of security and critical fixes. > 4. Livepatch module > @@ -134,7 +242,7 @@ Documentation/livepatch/module-elf-format.txt for more details. > > > 4.2. Metadata > ------------- > +------------- > > The patch is described by several structures that split the information > into three levels: > @@ -239,9 +347,15 @@ Registered patches might be enabled either by calling klp_enable_patch() or > by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will > start using the new implementation of the patched functions at this stage. > > -In particular, if an original function is patched for the first time, a > -function specific struct klp_ops is created and an universal ftrace handler > -is registered. > +When a patch is enabled, livepatch enters into a transition state where > +tasks are converging to the patched state. This is indicated by a value > +of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks have > +been patched, the 'transition' value changes to '0'. For more > +information about this process, see the "Consistency model" section. > + > +If an original function is patched for the first time, a function > +specific struct klp_ops is created and an universal ftrace handler is > +registered. > > Functions might be patched multiple times. The ftrace handler is registered > only once for the given function. Further patches just add an entry to the > @@ -261,6 +375,12 @@ by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage > either the code from the previously enabled patch or even the original > code gets used. > > +When a patch is disabled, livepatch enters into a transition state where > +tasks are converging to the unpatched state. This is indicated by a > +value of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks > +have been unpatched, the 'transition' value changes to '0'. For more > +information about this process, see the "Consistency model" section. > + > Here all the functions (struct klp_func) associated with the to-be-disabled > patch are removed from the corresponding struct klp_ops. The ftrace handler > is unregistered and the struct klp_ops is freed when the func_stack list > diff --git a/include/linux/init_task.h b/include/linux/init_task.h > index 325f649..25f0360 100644 > --- a/include/linux/init_task.h > +++ b/include/linux/init_task.h > @@ -14,6 +14,7 @@ > #include <linux/rbtree.h> > #include <net/net_namespace.h> > #include <linux/sched/rt.h> > +#include <linux/livepatch.h> > > #include <asm/thread_info.h> > > @@ -185,6 +186,13 @@ extern struct task_group root_task_group; > # define INIT_KASAN(tsk) > #endif > > +#ifdef CONFIG_LIVEPATCH > +# define INIT_LIVEPATCH(tsk) \ > + .patch_state = KLP_UNDEFINED, > +#else > +# define INIT_LIVEPATCH(tsk) > +#endif > + > #ifdef CONFIG_THREAD_INFO_IN_TASK > # define INIT_TASK_TI(tsk) \ > .thread_info = INIT_THREAD_INFO(tsk), \ > @@ -271,6 +279,7 @@ extern struct task_group root_task_group; > INIT_VTIME(tsk) \ > INIT_NUMA_BALANCING(tsk) \ > INIT_KASAN(tsk) \ > + INIT_LIVEPATCH(tsk) \ > } > > > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h > index 6602b34..ed90ad1 100644 > --- a/include/linux/livepatch.h > +++ b/include/linux/livepatch.h > @@ -28,18 +28,40 @@ > > #include <asm/livepatch.h> > > +/* task patch states */ > +#define KLP_UNDEFINED -1 > +#define KLP_UNPATCHED 0 > +#define KLP_PATCHED 1 > + > /** > * struct klp_func - function structure for live patching > * @old_name: name of the function to be patched > * @new_func: pointer to the patched function code > * @old_sympos: a hint indicating which symbol position the old function > * can be found (optional) > + * @immediate: patch the func immediately, bypassing safety mechanisms > * @old_addr: the address of the function being patched > * @kobj: kobject for sysfs resources > * @stack_node: list node for klp_ops func_stack list > * @old_size: size of the old function > * @new_size: size of the new function > * @patched: the func has been added to the klp_ops list > + * @transition: the func is currently being applied or reverted > + * > + * The patched and transition variables define the func's patching state. When > + * patching, a func is always in one of the following states: > + * > + * patched=0 transition=0: unpatched > + * patched=0 transition=1: unpatched, temporary starting state > + * patched=1 transition=1: patched, may be visible to some tasks > + * patched=1 transition=0: patched, visible to all tasks > + * > + * And when unpatching, it goes in the reverse order: > + * > + * patched=1 transition=0: patched, visible to all tasks > + * patched=1 transition=1: patched, may be visible to some tasks > + * patched=0 transition=1: unpatched, temporary ending state > + * patched=0 transition=0: unpatched > */ > struct klp_func { > /* external */ > @@ -53,6 +75,7 @@ struct klp_func { > * in kallsyms for the given object is used. > */ > unsigned long old_sympos; > + bool immediate; > > /* internal */ > unsigned long old_addr; > @@ -60,6 +83,7 @@ struct klp_func { > struct list_head stack_node; > unsigned long old_size, new_size; > bool patched; > + bool transition; > }; > > /** > @@ -68,7 +92,7 @@ struct klp_func { > * @funcs: function entries for functions to be patched in the object > * @kobj: kobject for sysfs resources > * @mod: kernel module associated with the patched object > - * (NULL for vmlinux) > + * (NULL for vmlinux) > * @patched: the object's funcs have been added to the klp_ops list > */ > struct klp_object { > @@ -86,6 +110,7 @@ struct klp_object { > * struct klp_patch - patch structure for live patching > * @mod: reference to the live patch module > * @objs: object entries for kernel objects to be patched > + * @immediate: patch all funcs immediately, bypassing safety mechanisms > * @list: list node for global list of registered patches > * @kobj: kobject for sysfs resources > * @enabled: the patch is enabled (but operation may be incomplete) > @@ -94,6 +119,7 @@ struct klp_patch { > /* external */ > struct module *mod; > struct klp_object *objs; > + bool immediate; > > /* internal */ > struct list_head list; > @@ -121,13 +147,27 @@ void arch_klp_init_object_loaded(struct klp_patch *patch, > int klp_module_coming(struct module *mod); > void klp_module_going(struct module *mod); > > +void klp_copy_process(struct task_struct *child); > void klp_update_patch_state(struct task_struct *task); > > +static inline bool klp_patch_pending(struct task_struct *task) > +{ > + return test_tsk_thread_flag(task, TIF_PATCH_PENDING); > +} > + > +static inline bool klp_have_reliable_stack(void) > +{ > + return IS_ENABLED(CONFIG_STACKTRACE) && > + IS_ENABLED(CONFIG_HAVE_RELIABLE_STACKTRACE); > +} > + > #else /* !CONFIG_LIVEPATCH */ > > static inline int klp_module_coming(struct module *mod) { return 0; } > static inline void klp_module_going(struct module *mod) {} > +static inline bool klp_patch_pending(struct task_struct *task) { return false; } > static inline void klp_update_patch_state(struct task_struct *task) {} > +static inline void klp_copy_process(struct task_struct *child) {} > > #endif /* CONFIG_LIVEPATCH */ > > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 9df1f42..ccbb1fa 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1973,6 +1973,9 @@ struct task_struct { > /* A live task holds one reference. */ > atomic_t stack_refcount; > #endif > +#ifdef CONFIG_LIVEPATCH > + int patch_state; > +#endif > /* CPU-specific state of this task */ > struct thread_struct thread; > /* > diff --git a/kernel/fork.c b/kernel/fork.c > index 7da33cb..8c8de80 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -77,6 +77,7 @@ > #include <linux/compiler.h> > #include <linux/sysctl.h> > #include <linux/kcov.h> > +#include <linux/livepatch.h> > > #include <asm/pgtable.h> > #include <asm/pgalloc.h> > @@ -1765,6 +1766,8 @@ static __latent_entropy struct task_struct *copy_process( > p->parent_exec_id = current->self_exec_id; > } > > + klp_copy_process(p); > + > spin_lock(¤t->sighand->siglock); > > /* > diff --git a/kernel/livepatch/Makefile b/kernel/livepatch/Makefile > index e136dad..2b8bdb1 100644 > --- a/kernel/livepatch/Makefile > +++ b/kernel/livepatch/Makefile > @@ -1,3 +1,3 @@ > obj-$(CONFIG_LIVEPATCH) += livepatch.o > > -livepatch-objs := core.o patch.o > +livepatch-objs := core.o patch.o transition.o > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c > index 10ba3a1..4c5a816 100644 > --- a/kernel/livepatch/core.c > +++ b/kernel/livepatch/core.c > @@ -31,22 +31,22 @@ > #include <linux/moduleloader.h> > #include <asm/cacheflush.h> > #include "patch.h" > +#include "transition.h" > > /* > - * The klp_mutex protects the global lists and state transitions of any > - * structure reachable from them. References to any structure must be obtained > - * under mutex protection (except in klp_ftrace_handler(), which uses RCU to > - * ensure it gets consistent data). > + * klp_mutex is a coarse lock which serializes access to klp data. All > + * accesses to klp-related variables and structures must have mutex protection, > + * except within the following functions which carefully avoid the need for it: > + * > + * - klp_ftrace_handler() > + * - klp_update_patch_state() > */ > -static DEFINE_MUTEX(klp_mutex); > +DEFINE_MUTEX(klp_mutex); > > static LIST_HEAD(klp_patches); > > static struct kobject *klp_root_kobj; > > -/* TODO: temporary stub */ > -void klp_update_patch_state(struct task_struct *task) {} > - > static bool klp_is_module(struct klp_object *obj) > { > return obj->name; > @@ -85,7 +85,6 @@ static void klp_find_object_module(struct klp_object *obj) > mutex_unlock(&module_mutex); > } > > -/* klp_mutex must be held by caller */ > static bool klp_is_patch_registered(struct klp_patch *patch) > { > struct klp_patch *mypatch; > @@ -281,20 +280,25 @@ static int klp_write_object_relocations(struct module *pmod, > > static int __klp_disable_patch(struct klp_patch *patch) > { > - struct klp_object *obj; > + if (klp_transition_patch) > + return -EBUSY; > > /* enforce stacking: only the last enabled patch can be disabled */ > if (!list_is_last(&patch->list, &klp_patches) && > list_next_entry(patch, list)->enabled) > return -EBUSY; > > - pr_notice("disabling patch '%s'\n", patch->mod->name); > + klp_init_transition(patch, KLP_UNPATCHED); > > - klp_for_each_object(patch, obj) { > - if (obj->patched) > - klp_unpatch_object(obj); > - } > + /* > + * Enforce the order of the klp_target_state write in > + * klp_init_transition() and the TIF_PATCH_PENDING writes in > + * klp_start_transition() to ensure that klp_update_patch_state() > + * doesn't set a task->patch_state to KLP_UNDEFINED. > + */ > + smp_wmb(); The description is not clear. The klp_target_state manipulation is synchronized by another barrier inside klp_init_transition(). A similar barrier is in __klp_enable_patch() and it is correctly described there: It enforces the order of the func->transition writes in klp_init_transition() and the ops->func_stack writes in klp_patch_object(). The corresponding barrier is in klp_ftrace_handler(). But we do not modify ops->func_stack in __klp_disable_patch(). So we need another explanation. Huh, I spent few hours thinking about it. I am almost sure that it is not needed. But I am not 100% sure. The many times rewriten summary looks like: /* * Enforce the order of func->transtion write in * klp_init_transition() against TIF_PATCH_PENDING writes * in klp_start_transition(). It makes sure that * klp_ftrace_hadler() will see func->transition set * after the task is migrated by klp_update_patch_state(). * * The barrier probably is not needed because the task * must not be migrated when being inside klp_ftrace_handler() * and there is another full barrier in * klp_update_patch_state(). * But this is slow path and better be safe than sorry. */ smp_wmb(); > + klp_start_transition(); > patch->enabled = false; > > return 0; > @@ -337,6 +341,9 @@ static int __klp_enable_patch(struct klp_patch *patch) > struct klp_object *obj; > int ret; > > + if (klp_transition_patch) > + return -EBUSY; > + > if (WARN_ON(patch->enabled)) > return -EINVAL; > > @@ -347,22 +354,37 @@ static int __klp_enable_patch(struct klp_patch *patch) > > pr_notice("enabling patch '%s'\n", patch->mod->name); > > + klp_init_transition(patch, KLP_PATCHED); > + > + /* > + * Enforce the order of the func->transition writes in > + * klp_init_transition() and the ops->func_stack writes in > + * klp_patch_object(), so that klp_ftrace_handler() will see the > + * func->transition updates before the handler is registered and the > + * new funcs become visible to the handler. > + */ > + smp_wmb(); > + > klp_for_each_object(patch, obj) { > if (!klp_is_object_loaded(obj)) > continue; > > ret = klp_patch_object(obj); > - if (ret) > - goto unregister; > + if (ret) { > + pr_warn("failed to enable patch '%s'\n", > + patch->mod->name); > + > + klp_unpatch_objects(patch); We should call here synchronize_rcu() here as we do in klp_try_complete_transition(). Some of the affected functions might have more versions on the stack and we need to make sure that klp_ftrace_handler() will _not_ see the removed patch on the stack. Alternatively, we could move all the code around klp_unpatch_objects() from klp_try_complete_transition() into klp_complete_transition(), see below. > + klp_complete_transition(); > + > + return ret; > + } > } > > + klp_start_transition(); > patch->enabled = true; > > return 0; > - > -unregister: > - WARN_ON(__klp_disable_patch(patch)); > - return ret; > } > > /** > @@ -399,6 +421,7 @@ EXPORT_SYMBOL_GPL(klp_enable_patch); > * /sys/kernel/livepatch > * /sys/kernel/livepatch/<patch> > * /sys/kernel/livepatch/<patch>/enabled > + * /sys/kernel/livepatch/<patch>/transition > * /sys/kernel/livepatch/<patch>/<object> > * /sys/kernel/livepatch/<patch>/<object>/<function,sympos> > */ > @@ -424,7 +447,9 @@ static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr, > goto err; > } > > - if (enabled) { > + if (patch == klp_transition_patch) { > + klp_reverse_transition(); > + } else if (enabled) { > ret = __klp_enable_patch(patch); > if (ret) > goto err; > @@ -452,9 +477,21 @@ static ssize_t enabled_show(struct kobject *kobj, > return snprintf(buf, PAGE_SIZE-1, "%d\n", patch->enabled); > } > > +static ssize_t transition_show(struct kobject *kobj, > + struct kobj_attribute *attr, char *buf) > +{ > + struct klp_patch *patch; > + > + patch = container_of(kobj, struct klp_patch, kobj); > + return snprintf(buf, PAGE_SIZE-1, "%d\n", > + patch == klp_transition_patch); > +} > + > static struct kobj_attribute enabled_kobj_attr = __ATTR_RW(enabled); > +static struct kobj_attribute transition_kobj_attr = __ATTR_RO(transition); > static struct attribute *klp_patch_attrs[] = { > &enabled_kobj_attr.attr, > + &transition_kobj_attr.attr, > NULL > }; > > @@ -544,6 +581,7 @@ static int klp_init_func(struct klp_object *obj, struct klp_func *func) > > INIT_LIST_HEAD(&func->stack_node); > func->patched = false; > + func->transition = false; > > /* The format for the sysfs directory is <function,sympos> where sympos > * is the nth occurrence of this symbol in kallsyms for the patched > @@ -740,6 +778,16 @@ int klp_register_patch(struct klp_patch *patch) > return -ENODEV; > > /* > + * Architectures without reliable stack traces have to set > + * patch->immediate because there's currently no way to patch kthreads > + * with the consistency model. > + */ > + if (!klp_have_reliable_stack() && !patch->immediate) { > + pr_err("This architecture doesn't have support for the livepatch consistency model.\n"); > + return -ENOSYS; > + } > + > + /* > * A reference is taken on the patch module to prevent it from being > * unloaded. Right now, we don't allow patch modules to unload since > * there is currently no method to determine if a thread is still > @@ -788,7 +836,11 @@ int klp_module_coming(struct module *mod) > goto err; > } > > - if (!patch->enabled) > + /* > + * Only patch the module if the patch is enabled or is > + * in transition. > + */ > + if (!patch->enabled && patch != klp_transition_patch) We need the same change also in klp_module_going(). > break; > > pr_notice("applying patch '%s' to loading module '%s'\n", > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c > index 5efa262..1a77f05 100644 > --- a/kernel/livepatch/patch.c > +++ b/kernel/livepatch/patch.c > @@ -29,6 +29,7 @@ > #include <linux/bug.h> > #include <linux/printk.h> > #include "patch.h" > +#include "transition.h" > > static LIST_HEAD(klp_ops); > > @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip, > { > struct klp_ops *ops; > struct klp_func *func; > + int patch_state; > > ops = container_of(fops, struct klp_ops, fops); > > rcu_read_lock(); > + > func = list_first_or_null_rcu(&ops->func_stack, struct klp_func, > stack_node); > + > + /* > + * func should never be NULL because preemption should be disabled here > + * and unregister_ftrace_function() does the equivalent of a > + * synchronize_sched() before the func_stack removal. > + */ > + if (WARN_ON_ONCE(!func)) > + goto unlock; > + > + /* > + * Enforce the order of the ops->func_stack and func->transition reads. > + * The corresponding write barrier is in __klp_enable_patch(). > + */ > + smp_rmb(); I was curious why the comment did not mention __klp_disable_patch(). It was related to the hours of thinking. I would like to avoid this in the future and add a comment like. * This barrier probably is not needed when the patch is being * disabled. The patch is removed from the stack in * klp_try_complete_transition() and there we need to call * rcu_synchronize() to prevent seeing the patch on the stack * at all. * * Well, it still might be needed to see func->transition * when the patch is removed and the task is migrated. See * the write barrier in __klp_disable_patch(). > + if (unlikely(func->transition)) { > + > + /* > + * Enforce the order of the func->transition and > + * current->patch_state reads. Otherwise we could read an > + * out-of-date task state and pick the wrong function. The > + * corresponding write barriers are in klp_init_transition() > + * and __klp_disable_patch(). > + */ > + smp_rmb(); IMHO, the corresponding barrier is in klp_init_transition(). The barrier in __klp_disable_patch() is questionable. Anyway, it has a different purpose. > + patch_state = current->patch_state; > + > + WARN_ON_ONCE(patch_state == KLP_UNDEFINED); > + > + if (patch_state == KLP_UNPATCHED) { > + /* > + * Use the previously patched version of the function. > + * If no previous patches exist, continue with the > + * original function. > + */ > + func = list_entry_rcu(func->stack_node.next, > + struct klp_func, stack_node); > + > + if (&func->stack_node == &ops->func_stack) > + goto unlock; > + } > + } > + > klp_arch_set_pc(regs, (unsigned long)func->new_func); > unlock: > rcu_read_unlock(); > @@ -211,3 +255,12 @@ int klp_patch_object(struct klp_object *obj) > > return 0; > } > + > +void klp_unpatch_objects(struct klp_patch *patch) > +{ > + struct klp_object *obj; > + > + klp_for_each_object(patch, obj) > + if (obj->patched) > + klp_unpatch_object(obj); > +} > diff --git a/kernel/livepatch/patch.h b/kernel/livepatch/patch.h > index 2d0cce0..0db2271 100644 > --- a/kernel/livepatch/patch.h > +++ b/kernel/livepatch/patch.h > @@ -28,5 +28,6 @@ struct klp_ops *klp_find_ops(unsigned long old_addr); > > int klp_patch_object(struct klp_object *obj); > void klp_unpatch_object(struct klp_object *obj); > +void klp_unpatch_objects(struct klp_patch *patch); > > #endif /* _LIVEPATCH_PATCH_H */ > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c > new file mode 100644 > index 0000000..2b87ff9 > --- /dev/null > +++ b/kernel/livepatch/transition.c > @@ -0,0 +1,525 @@ > +/* > + * transition.c - Kernel Live Patching transition functions > + * > + * Copyright (C) 2015-2016 Josh Poimboeuf <jpoimboe@redhat.com> > + * > + * This program is free software; you can redistribute it and/or > + * modify it under the terms of the GNU General Public License > + * as published by the Free Software Foundation; either version 2 > + * of the License, or (at your option) any later version. > + * > + * This program is distributed in the hope that it will be useful, > + * but WITHOUT ANY WARRANTY; without even the implied warranty of > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > + * GNU General Public License for more details. > + * > + * You should have received a copy of the GNU General Public License > + * along with this program; if not, see <http://www.gnu.org/licenses/>. > + */ > + > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt > + > +#include <linux/cpu.h> > +#include <linux/stacktrace.h> > +#include "patch.h" > +#include "transition.h" > +#include "../sched/sched.h" > + > +#define MAX_STACK_ENTRIES 100 > +#define STACK_ERR_BUF_SIZE 128 > + > +extern struct mutex klp_mutex; > + > +struct klp_patch *klp_transition_patch; > + > +static int klp_target_state = KLP_UNDEFINED; > + > +/* > + * This work can be performed periodically to finish patching or unpatching any > + * "straggler" tasks which failed to transition in the first attempt. > + */ > +static void klp_try_complete_transition(void); > +static void klp_transition_work_fn(struct work_struct *work) > +{ > + mutex_lock(&klp_mutex); > + > + if (klp_transition_patch) > + klp_try_complete_transition(); > + > + mutex_unlock(&klp_mutex); > +} > +static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn); > + > +/* > + * The transition to the target patch state is complete. Clean up the data > + * structures. > + */ > +void klp_complete_transition(void) > +{ > + struct klp_object *obj; > + struct klp_func *func; > + struct task_struct *g, *task; > + unsigned int cpu; > + > + if (klp_transition_patch->immediate) > + goto done; > + > + klp_for_each_object(klp_transition_patch, obj) > + klp_for_each_func(obj, func) > + func->transition = false; > + > + /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */ > + if (klp_target_state == KLP_PATCHED) > + synchronize_rcu(); I forgot why this is done only when the patch is beeing enabled. It is because klp_unpatch_objects() and rcu_synchronize() is called in klp_try_complete_transition() when klp_target_state == KLP_UNPATCHED. I would suggest to move the code below the "success" label from klp_try_complete_transtion() to klp_complete_transition(). It will get the two related things close to each other. Also it would fix the missing rcu_synchronize() in the error path in __klp_enable_patch(), see above. > + read_lock(&tasklist_lock); > + for_each_process_thread(g, task) { > + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); > + task->patch_state = KLP_UNDEFINED; > + } > + read_unlock(&tasklist_lock); > + > + for_each_possible_cpu(cpu) { > + task = idle_task(cpu); > + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); > + task->patch_state = KLP_UNDEFINED; > + } > + > +done: > + klp_target_state = KLP_UNDEFINED; > + klp_transition_patch = NULL; > +} > + > +/* > + * Switch the patched state of the task to the set of functions in the target > + * patch state. > + * > + * NOTE: If task is not 'current', the caller must ensure the task is inactive. > + * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value. > + */ > +void klp_update_patch_state(struct task_struct *task) > +{ > + rcu_read_lock(); > + > + /* > + * This test_and_clear_tsk_thread_flag() call also serves as a read > + * barrier to enforce the order of the TIF_PATCH_PENDING and > + * klp_target_state reads. The corresponding write barrier is in > + * __klp_disable_patch(). The corresponding barrier is in klp_init_transition(), see below. In each case, we need the corresponding write barrier in both enable and disable paths. They both set klp_target_state and TIF_PATCH_PENDING. The write barrier in klp_init_transition() perfectly fits the purpose. > + */ > + if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING)) > + task->patch_state = READ_ONCE(klp_target_state); > + > + rcu_read_unlock(); > +} > + > +/* > + * Determine whether the given stack trace includes any references to a > + * to-be-patched or to-be-unpatched function. > + */ > +static int klp_check_stack_func(struct klp_func *func, > + struct stack_trace *trace) > +{ > + unsigned long func_addr, func_size, address; > + struct klp_ops *ops; > + int i; > + > + if (func->immediate) > + return 0; > + > + for (i = 0; i < trace->nr_entries; i++) { > + address = trace->entries[i]; > + > + if (klp_target_state == KLP_UNPATCHED) { > + /* > + * Check for the to-be-unpatched function > + * (the func itself). > + */ > + func_addr = (unsigned long)func->new_func; > + func_size = func->new_size; > + } else { > + /* > + * Check for the to-be-patched function > + * (the previous func). > + */ > + ops = klp_find_ops(func->old_addr); > + > + if (list_is_singular(&ops->func_stack)) { > + /* original function */ > + func_addr = func->old_addr; > + func_size = func->old_size; > + } else { > + /* previously patched function */ > + struct klp_func *prev; > + > + prev = list_next_entry(func, stack_node); > + func_addr = (unsigned long)prev->new_func; > + func_size = prev->new_size; > + } > + } > + > + if (address >= func_addr && address < func_addr + func_size) > + return -EAGAIN; > + } > + > + return 0; > +} > + > +/* > + * Determine whether it's safe to transition the task to the target patch state > + * by looking for any to-be-patched or to-be-unpatched functions on its stack. > + */ > +static int klp_check_stack(struct task_struct *task, char *err_buf) > +{ > + static unsigned long entries[MAX_STACK_ENTRIES]; > + struct stack_trace trace; > + struct klp_object *obj; > + struct klp_func *func; > + int ret; > + > + trace.skip = 0; > + trace.nr_entries = 0; > + trace.max_entries = MAX_STACK_ENTRIES; > + trace.entries = entries; > + ret = save_stack_trace_tsk_reliable(task, &trace); > + WARN_ON_ONCE(ret == -ENOSYS); > + if (ret) { > + snprintf(err_buf, STACK_ERR_BUF_SIZE, > + "%s: %s:%d has an unreliable stack\n", > + __func__, task->comm, task->pid); > + return ret; > + } > + > + klp_for_each_object(klp_transition_patch, obj) { > + if (!obj->patched) > + continue; > + klp_for_each_func(obj, func) { > + ret = klp_check_stack_func(func, &trace); > + if (ret) { > + snprintf(err_buf, STACK_ERR_BUF_SIZE, > + "%s: %s:%d is sleeping on function %s\n", > + __func__, task->comm, task->pid, > + func->old_name); > + return ret; > + } > + } > + } > + > + return 0; > +} > + > +/* > + * Try to safely switch a task to the target patch state. If it's currently > + * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or > + * if the stack is unreliable, return false. > + */ > +static bool klp_try_switch_task(struct task_struct *task) > +{ > + struct rq *rq; > + struct rq_flags flags; > + int ret; > + bool success = false; > + char err_buf[STACK_ERR_BUF_SIZE]; > + > + err_buf[0] = '\0'; > + > + /* check if this task has already switched over */ > + if (task->patch_state == klp_target_state) > + return true; > + > + /* > + * For arches which don't have reliable stack traces, we have to rely > + * on other methods (e.g., switching tasks at kernel exit). > + */ > + if (!klp_have_reliable_stack()) > + return false; > + > + /* > + * Now try to check the stack for any to-be-patched or to-be-unpatched > + * functions. If all goes well, switch the task to the target patch > + * state. > + */ > + rq = task_rq_lock(task, &flags); > + > + if (task_running(rq, task) && task != current) { > + snprintf(err_buf, STACK_ERR_BUF_SIZE, > + "%s: %s:%d is running\n", __func__, task->comm, > + task->pid); > + goto done; > + } > + > + ret = klp_check_stack(task, err_buf); > + if (ret) > + goto done; > + > + success = true; > + > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > + task->patch_state = klp_target_state; > + > +done: > + task_rq_unlock(rq, task, &flags); > + > + /* > + * Due to console deadlock issues, pr_debug() can't be used while > + * holding the task rq lock. Instead we have to use a temporary buffer > + * and print the debug message after releasing the lock. > + */ > + if (err_buf[0] != '\0') > + pr_debug("%s", err_buf); > + > + return success; > + > +} > + > +/* > + * Try to switch all remaining tasks to the target patch state by walking the > + * stacks of sleeping tasks and looking for any to-be-patched or > + * to-be-unpatched functions. If such functions are found, the task can't be > + * switched yet. > + * > + * If any tasks are still stuck in the initial patch state, schedule a retry. > + */ > +static void klp_try_complete_transition(void) > +{ > + unsigned int cpu; > + struct task_struct *g, *task; > + bool complete = true; > + > + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED); > + > + /* > + * If the patch can be applied or reverted immediately, skip the > + * per-task transitions. > + */ > + if (klp_transition_patch->immediate) > + goto success; > + > + /* > + * Try to switch the tasks to the target patch state by walking their > + * stacks and looking for any to-be-patched or to-be-unpatched > + * functions. If such functions are found on a stack, or if the stack > + * is deemed unreliable, the task can't be switched yet. > + * > + * Usually this will transition most (or all) of the tasks on a system > + * unless the patch includes changes to a very common function. > + */ > + read_lock(&tasklist_lock); > + for_each_process_thread(g, task) > + if (!klp_try_switch_task(task)) > + complete = false; > + read_unlock(&tasklist_lock); > + > + /* > + * Ditto for the idle "swapper" tasks. > + */ > + get_online_cpus(); > + for_each_possible_cpu(cpu) { > + task = idle_task(cpu); > + if (cpu_online(cpu)) { > + if (!klp_try_switch_task(task)) > + complete = false; > + } else if (task->patch_state != klp_target_state) { > + /* offline idle tasks can be switched immediately */ > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > + task->patch_state = klp_target_state; > + } > + } > + put_online_cpus(); > + > + /* > + * Some tasks weren't able to be switched over. Try again later and/or > + * wait for other methods like kernel exit switching. > + */ > + if (!complete) { > + schedule_delayed_work(&klp_transition_work, > + round_jiffies_relative(HZ)); > + return; > + } > + > +success: > + > + if (klp_target_state == KLP_UNPATCHED) { > + /* > + * All tasks have transitioned to KLP_UNPATCHED so we can now > + * remove the new functions from the func_stack. > + */ > + klp_unpatch_objects(klp_transition_patch); > + > + /* > + * Make sure klp_ftrace_handler() sees that the functions have > + * been removed from ops->func_stack before we clear > + * func->transition. Otherwise it may pick the wrong > + * transition. > + */ I was a bit confused by the comment. It might be my problem. I just wonder if might be more clear with somethink like: /* * Make sure klp_ftrace_handler() can not longer see functions * from this patch on the stack. Otherwise it may use * the being-removed function after func->transition is cleared. */ > + synchronize_rcu(); > + } > + > + pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name, > + klp_target_state == KLP_PATCHED ? "patching" : "unpatching"); > + > + /* we're done, now cleanup the data structures */ > + klp_complete_transition(); > +} > + > +/* > + * Start the transition to the specified target patch state so tasks can begin > + * switching to it. > + */ > +void klp_start_transition(void) > +{ > + struct task_struct *g, *task; > + unsigned int cpu; > + > + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED); > + > + pr_notice("'%s': %s...\n", klp_transition_patch->mod->name, > + klp_target_state == KLP_PATCHED ? "patching" : "unpatching"); > + > + /* > + * If the patch can be applied or reverted immediately, skip the > + * per-task transitions. > + */ > + if (klp_transition_patch->immediate) We should call klp_try_complete_transition() here. Otherwise, it will never be called and the transition will never get completed. Alternative solution would be to move klp_try_complete_transition() from klp_start_transition() and explicitely call it from __klp_disable_patch() and klp_enable_patch(). It would actually solve one issue with klp_revert_patch(), see below. I kind of like the alternative solution. I hope that it was not me who suggested to move klp_try_complete_transition() into klp_start_transtion(). > + return; > + > + /* > + * Mark all normal tasks as needing a patch state update. They'll > + * switch either in klp_try_complete_transition() or as they exit the > + * kernel. > + */ > + read_lock(&tasklist_lock); > + for_each_process_thread(g, task) > + if (task->patch_state != klp_target_state) > + set_tsk_thread_flag(task, TIF_PATCH_PENDING); > + read_unlock(&tasklist_lock); > + > + /* > + * Mark all idle tasks as needing a patch state update. They'll switch > + * either in klp_try_complete_transition() or at the idle loop switch > + * point. > + */ > + for_each_possible_cpu(cpu) { > + task = idle_task(cpu); > + if (task->patch_state != klp_target_state) > + set_tsk_thread_flag(task, TIF_PATCH_PENDING); > + } > + > + klp_try_complete_transition(); > +} > + > +/* > + * Initialize the global target patch state and all tasks to the initial patch > + * state, and initialize all function transition states to true in preparation > + * for patching or unpatching. > + */ > +void klp_init_transition(struct klp_patch *patch, int state) > +{ > + struct task_struct *g, *task; > + unsigned int cpu; > + struct klp_object *obj; > + struct klp_func *func; > + int initial_state = !state; > + > + WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED); > + > + klp_transition_patch = patch; > + > + /* > + * Set the global target patch state which tasks will switch to. This > + * has no effect until the TIF_PATCH_PENDING flags get set later. > + */ > + klp_target_state = state; > + > + /* > + * If the patch can be applied or reverted immediately, skip the > + * per-task transitions. > + */ > + if (patch->immediate) > + return; > + > + /* > + * Initialize all tasks to the initial patch state to prepare them for > + * switching to the target state. > + */ > + read_lock(&tasklist_lock); > + for_each_process_thread(g, task) { > + WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED); > + task->patch_state = initial_state; > + } > + read_unlock(&tasklist_lock); > + > + /* > + * Ditto for the idle "swapper" tasks. > + */ > + for_each_possible_cpu(cpu) { > + task = idle_task(cpu); > + WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED); > + task->patch_state = initial_state; > + } > + > + /* > + * Enforce the order of the task->patch_state initializations and the > + * func->transition updates to ensure that, in the enable path, > + * klp_ftrace_handler() doesn't see a func in transition with a > + * task->patch_state of KLP_UNDEFINED. It has one more purpose: * * Also enforce the order between klp_target_state and * TIF_PATCH_PENDING. The corresponding read barrier is in * klp_update_patch_state(). > + */ > + smp_wmb(); > + > + /* > + * Set the func transition states so klp_ftrace_handler() will know to > + * switch to the transition logic. > + * > + * When patching, the funcs aren't yet in the func_stack and will be > + * made visible to the ftrace handler shortly by the calls to > + * klp_patch_object(). > + * > + * When unpatching, the funcs are already in the func_stack and so are > + * already visible to the ftrace handler. > + */ > + klp_for_each_object(patch, obj) > + klp_for_each_func(obj, func) > + func->transition = true; > +} > + > +/* > + * This function can be called in the middle of an existing transition to > + * reverse the direction of the target patch state. This can be done to > + * effectively cancel an existing enable or disable operation if there are any > + * tasks which are stuck in the initial patch state. > + */ > +void klp_reverse_transition(void) > +{ > + unsigned int cpu; > + struct task_struct *g, *task; > + > + klp_transition_patch->enabled = !klp_transition_patch->enabled; > + > + klp_target_state = !klp_target_state; > + > + /* > + * Clear all TIF_PATCH_PENDING flags to prevent races caused by > + * klp_update_patch_state() running in parallel with > + * klp_start_transition(). > + */ > + read_lock(&tasklist_lock); > + for_each_process_thread(g, task) > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > + read_unlock(&tasklist_lock); > + > + for_each_possible_cpu(cpu) > + clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING); > + > + /* Let any remaining calls to klp_update_patch_state() complete */ > + synchronize_rcu(); > + > + klp_start_transition(); Hmm, we should not call klp_try_complete_transition() when klp_start_transition() is called from here. I can't find a safe way to cancel klp_transition_work() when we own klp_mutex. It smells with a possible deadlock. I suggest to move move klp_try_complete_transition() outside klp_start_transition() and explicitely call it from __klp_disable_patch() and __klp_enabled_patch(). This would fix also the problem with immediate patches, see klp_start_transition(). > +} > + > +/* Called from copy_process() during fork */ > +void klp_copy_process(struct task_struct *child) > +{ > + child->patch_state = current->patch_state; > + > + /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */ > +} > diff --git a/kernel/livepatch/transition.h b/kernel/livepatch/transition.h > new file mode 100644 > index 0000000..3ee625f > --- /dev/null > +++ b/kernel/livepatch/transition.h > @@ -0,0 +1,13 @@ > +#ifndef _LIVEPATCH_TRANSITION_H > +#define _LIVEPATCH_TRANSITION_H > + > +#include <linux/livepatch.h> > + > +extern struct klp_patch *klp_transition_patch; > + > +void klp_init_transition(struct klp_patch *patch, int state); > +void klp_start_transition(void); > +void klp_reverse_transition(void); > +void klp_complete_transition(void); > + > +#endif /* _LIVEPATCH_TRANSITION_H */ > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > index 6a4bae0..a8b3f1a 100644 > --- a/kernel/sched/idle.c > +++ b/kernel/sched/idle.c > @@ -9,6 +9,7 @@ > #include <linux/mm.h> > #include <linux/stackprotector.h> > #include <linux/suspend.h> > +#include <linux/livepatch.h> > > #include <asm/tlb.h> > > @@ -264,6 +265,9 @@ static void do_idle(void) > > sched_ttwu_pending(); > schedule_preempt_disabled(); > + > + if (unlikely(klp_patch_pending(current))) > + klp_update_patch_state(current); > } > > bool cpu_in_idle(unsigned long pc) > diff --git a/samples/livepatch/livepatch-sample.c b/samples/livepatch/livepatch-sample.c > index e34f871..629e0dc 100644 > --- a/samples/livepatch/livepatch-sample.c > +++ b/samples/livepatch/livepatch-sample.c > @@ -17,6 +17,8 @@ > * along with this program; if not, see <http://www.gnu.org/licenses/>. > */ > > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt > + > #include <linux/module.h> > #include <linux/kernel.h> > #include <linux/livepatch.h> > @@ -69,6 +71,21 @@ static int livepatch_init(void) > { > int ret; > > + if (!klp_have_reliable_stack() && !patch.immediate) { > + /* > + * WARNING: Be very careful when using 'patch.immediate' in > + * your patches. It's ok to use it for simple patches like > + * this, but for more complex patches which change function > + * semantics, locking semantics, or data structures, it may not > + * be safe. Use of this option will also prevent removal of > + * the patch. > + * > + * See Documentation/livepatch/livepatch.txt for more details. > + */ > + patch.immediate = true; > + pr_notice("The consistency model isn't supported for your architecture. Bypassing safety mechanisms and applying the patch immediately.\n"); > + } > + > ret = klp_register_patch(&patch); > if (ret) > return ret; I am sorry that the review took me so long. I was interrupted several times by other tasks. The logic is very complex. I wanted to make sure that my comments are consistent and make sense. Anyway, I think that we are getting close. Best Regards, Petr
On Thu, 2 Feb 2017, Petr Mladek wrote: > > diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt > > index 7f04e13..fb00d66 100644 > > --- a/Documentation/livepatch/livepatch.txt > > +++ b/Documentation/livepatch/livepatch.txt > > > + In that case, arches without > > + HAVE_RELIABLE_STACKTRACE would still be able to use the > > + non-stack-checking parts of the consistency model: > > + > > + a) patching user tasks when they cross the kernel/user space > > + boundary; and > > + > > + b) patching kthreads and idle tasks at their designated patch points. > > + > > + This option isn't as good as option 1 because it requires signaling > > + most of the tasks to patch them. > > Kthreads need to be waken because most of them ignore signals. > > And idle tasks might need to be explicitely scheduled if there > is too big load. Mirek knows more about this. Yes, and we've already discussed it with Josh. The plan is not to do anything now, because described situations should be rare and/or theoretical only. It should be on our TODO lists though. > Well, I am not sure if you want to get into such details. I would not bother about it. [...] > > +/* > > + * Start the transition to the specified target patch state so tasks can begin > > + * switching to it. > > + */ > > +void klp_start_transition(void) > > +{ > > + struct task_struct *g, *task; > > + unsigned int cpu; > > + > > + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED); > > + > > + pr_notice("'%s': %s...\n", klp_transition_patch->mod->name, > > + klp_target_state == KLP_PATCHED ? "patching" : "unpatching"); > > + > > + /* > > + * If the patch can be applied or reverted immediately, skip the > > + * per-task transitions. > > + */ > > + if (klp_transition_patch->immediate) > > We should call klp_try_complete_transition() here. Otherwise, it will > never be called and the transition will never get completed. > > Alternative solution would be to move klp_try_complete_transition() > from klp_start_transition() and explicitely call it from > __klp_disable_patch() and klp_enable_patch(). It would actually > solve one issue with klp_revert_patch(), see below. > > I kind of like the alternative solution. I hope that it was not > me who suggested to move klp_try_complete_transition() into > klp_start_transtion(). [...] > Hmm, we should not call klp_try_complete_transition() when > klp_start_transition() is called from here. I can't find a safe > way to cancel klp_transition_work() when we own klp_mutex. > It smells with a possible deadlock. > > I suggest to move move klp_try_complete_transition() outside > klp_start_transition() and explicitely call it from > __klp_disable_patch() and __klp_enabled_patch(). > This would fix also the problem with immediate patches, see > klp_start_transition(). I agree. I think the best would be to move klp_try_complete_transition() out from klp_start_transition() and call it explicitly. This would solve the immediate problem for free. I agree we should not call klp_try_complete_transition() from klp_reverse_transition() and leave it entirely to our delayed work. We discussed this in the past and the counter argument was that explicit call to klp_try_complete_transition() could make the process a bit faster. Possibly true, but reversion is really slow path and I would not care about speed at all. I think it is cleaner and perhaps safer. Thanks, Miroslav
Petr has already mentioned majority of things I too found out, so only couple of nits... > diff --git a/Documentation/ABI/testing/sysfs-kernel-livepatch b/Documentation/ABI/testing/sysfs-kernel-livepatch > index da87f43..24b6570 100644 > --- a/Documentation/ABI/testing/sysfs-kernel-livepatch > +++ b/Documentation/ABI/testing/sysfs-kernel-livepatch > @@ -25,6 +25,14 @@ Description: > code is currently applied. Writing 0 will disable the patch > while writing 1 will re-enable the patch. > > +What: /sys/kernel/livepatch/<patch>/transition > +Date: May 2016 'May 2016' looks strange, but maybe nobody cares about it... > diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt > index 7f04e13..fb00d66 100644 > --- a/Documentation/livepatch/livepatch.txt > +++ b/Documentation/livepatch/livepatch.txt > 4. Livepatch module > @@ -134,7 +242,7 @@ Documentation/livepatch/module-elf-format.txt for more details. > > > 4.2. Metadata > ------------- > +------------- klp_func and klp_patch have new members - immediate. Should be documented here in "4.2. Metadata" section. The section also contains this text under klp_patch bullet. It seems oudated: "Also if a more complex consistency model is supported then a selected unit (thread, kernel as a whole) will see the new code from the entire patch only when it is in a safe state." We now have a more complex consistency model. And finally, the section "Limitations" has this text under the first bullet: + The patch must not change the semantic of the patched functions. The current implementation guarantees only that either the old or the new function is called. The functions are patched one by one. It means that the patch must _not_ change the semantic of the function. I think it is confusing. The consistency model allows us to change the semantic of a function. To certain degree. Of course, there are cases that cannot be patched, or have to be patched carefully. For example if a function takes a lock by calling foo_lock(), foo_lock() is not on a stack afterwards. Then the locking semantics may be changed with a livepatch. One has to make sure to patch also the caller foo_lock() to enforce the consistency. And so on... But I do not consider a limitation of livepatch. It is a feature of the consistency model, which is weaker than kGraft's or kpatch's (or stronger. It depends on your point of view.) So, I propose to remove this text and better describe the properties of the consistency model above in the section 3. Maybe a quote from an old mail thread (Nov 2014) would be sufficient. I don't remember what was mentioned and what not. What do you think? > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h > index 6602b34..ed90ad1 100644 > --- a/include/linux/livepatch.h > +++ b/include/linux/livepatch.h > @@ -68,7 +92,7 @@ struct klp_func { > * @funcs: function entries for functions to be patched in the object > * @kobj: kobject for sysfs resources > * @mod: kernel module associated with the patched object > - * (NULL for vmlinux) > + * (NULL for vmlinux) This looks superfluous. (checking my notes)... and that's it. Aside from the discussion in Petr's subthread it looks good to me. Great job. Thanks, Miroslav
On Thu, Feb 02, 2017 at 12:51:16PM +0100, Petr Mladek wrote: > !!! This is the right version. I am sorry again for the confusion. !!! > > > Change livepatch to use a basic per-task consistency model. This is the > > foundation which will eventually enable us to patch those ~10% of > > security patches which change function or data semantics. This is the > > biggest remaining piece needed to make livepatch more generally useful. > > > > diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt > > index 7f04e13..fb00d66 100644 > > --- a/Documentation/livepatch/livepatch.txt > > +++ b/Documentation/livepatch/livepatch.txt > > +3.1 Adding consistency model support to new architectures > > +--------------------------------------------------------- > > + > > +For adding consistency model support to new architectures, there are a > > +few options: > > + > > +1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and > > + for non-DWARF unwinders, also making sure there's a way for the stack > > + tracing code to detect interrupts on the stack. > > + > > +2) Alternatively, figure out a way to patch kthreads without stack > > + checking. If all kthreads sleep in the same place, then we can > > + designate that place as a patching point. I think Petr M has been > > + working on that? > > Here is version with some more details: > > Alternatively, every kthread has to explicitely switch > current->patch_state on a safe place. Kthreads are typically > "infinite" loops that do some action repeatedly. The safe > location is between the loops when there are no locks > taken and all data structures are in a well defined state. > > The location is clear when using the workqueues or the kthread worker > API. These kthreads process "independent" works in a generic loop. > > It is much more complicated with kthreads using a custom loop. > There the safe place must be carefully localized case by case. Good clarification. > > + In that case, arches without > > + HAVE_RELIABLE_STACKTRACE would still be able to use the > > + non-stack-checking parts of the consistency model: > > + > > + a) patching user tasks when they cross the kernel/user space > > + boundary; and > > + > > + b) patching kthreads and idle tasks at their designated patch points. > > + > > + This option isn't as good as option 1 because it requires signaling > > + most of the tasks to patch them. > > Kthreads need to be waken because most of them ignore signals. > > And idle tasks might need to be explicitely scheduled if there > is too big load. Mirek knows more about this. > > Well, I am not sure if you want to get into such details. > > > > + But it could still be a good backup > > + option for those architectures which don't have reliable stack traces > > + yet. > > + > > +In the meantime, patches for such architectures can bypass the > > +consistency model by setting klp_patch.immediate to true. > > I would add that this is perfectly fine for patches that do not > change semantic of the patched functions. In practice, this is > usable in about 90% of security and critical fixes. Another good one. > > 4. Livepatch module > > @@ -134,7 +242,7 @@ Documentation/livepatch/module-elf-format.txt for more details. > > > > > > 4.2. Metadata > > ------------- > > +------------- > > > > The patch is described by several structures that split the information > > into three levels: > > @@ -239,9 +347,15 @@ Registered patches might be enabled either by calling klp_enable_patch() or > > by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will > > start using the new implementation of the patched functions at this stage. > > > > -In particular, if an original function is patched for the first time, a > > -function specific struct klp_ops is created and an universal ftrace handler > > -is registered. > > +When a patch is enabled, livepatch enters into a transition state where > > +tasks are converging to the patched state. This is indicated by a value > > +of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks have > > +been patched, the 'transition' value changes to '0'. For more > > +information about this process, see the "Consistency model" section. > > + > > +If an original function is patched for the first time, a function > > +specific struct klp_ops is created and an universal ftrace handler is > > +registered. > > > > Functions might be patched multiple times. The ftrace handler is registered > > only once for the given function. Further patches just add an entry to the > > @@ -261,6 +375,12 @@ by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage > > either the code from the previously enabled patch or even the original > > code gets used. > > > > +When a patch is disabled, livepatch enters into a transition state where > > +tasks are converging to the unpatched state. This is indicated by a > > +value of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks > > +have been unpatched, the 'transition' value changes to '0'. For more > > +information about this process, see the "Consistency model" section. > > + > > Here all the functions (struct klp_func) associated with the to-be-disabled > > patch are removed from the corresponding struct klp_ops. The ftrace handler > > is unregistered and the struct klp_ops is freed when the func_stack list > > diff --git a/include/linux/init_task.h b/include/linux/init_task.h > > index 325f649..25f0360 100644 > > --- a/include/linux/init_task.h > > +++ b/include/linux/init_task.h > > @@ -14,6 +14,7 @@ > > #include <linux/rbtree.h> > > #include <net/net_namespace.h> > > #include <linux/sched/rt.h> > > +#include <linux/livepatch.h> > > > > #include <asm/thread_info.h> > > > > @@ -185,6 +186,13 @@ extern struct task_group root_task_group; > > # define INIT_KASAN(tsk) > > #endif > > > > +#ifdef CONFIG_LIVEPATCH > > +# define INIT_LIVEPATCH(tsk) \ > > + .patch_state = KLP_UNDEFINED, > > +#else > > +# define INIT_LIVEPATCH(tsk) > > +#endif > > + > > #ifdef CONFIG_THREAD_INFO_IN_TASK > > # define INIT_TASK_TI(tsk) \ > > .thread_info = INIT_THREAD_INFO(tsk), \ > > @@ -271,6 +279,7 @@ extern struct task_group root_task_group; > > INIT_VTIME(tsk) \ > > INIT_NUMA_BALANCING(tsk) \ > > INIT_KASAN(tsk) \ > > + INIT_LIVEPATCH(tsk) \ > > } > > > > > > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h > > index 6602b34..ed90ad1 100644 > > --- a/include/linux/livepatch.h > > +++ b/include/linux/livepatch.h > > @@ -28,18 +28,40 @@ > > > > #include <asm/livepatch.h> > > > > +/* task patch states */ > > +#define KLP_UNDEFINED -1 > > +#define KLP_UNPATCHED 0 > > +#define KLP_PATCHED 1 > > + > > /** > > * struct klp_func - function structure for live patching > > * @old_name: name of the function to be patched > > * @new_func: pointer to the patched function code > > * @old_sympos: a hint indicating which symbol position the old function > > * can be found (optional) > > + * @immediate: patch the func immediately, bypassing safety mechanisms > > * @old_addr: the address of the function being patched > > * @kobj: kobject for sysfs resources > > * @stack_node: list node for klp_ops func_stack list > > * @old_size: size of the old function > > * @new_size: size of the new function > > * @patched: the func has been added to the klp_ops list > > + * @transition: the func is currently being applied or reverted > > + * > > + * The patched and transition variables define the func's patching state. When > > + * patching, a func is always in one of the following states: > > + * > > + * patched=0 transition=0: unpatched > > + * patched=0 transition=1: unpatched, temporary starting state > > + * patched=1 transition=1: patched, may be visible to some tasks > > + * patched=1 transition=0: patched, visible to all tasks > > + * > > + * And when unpatching, it goes in the reverse order: > > + * > > + * patched=1 transition=0: patched, visible to all tasks > > + * patched=1 transition=1: patched, may be visible to some tasks > > + * patched=0 transition=1: unpatched, temporary ending state > > + * patched=0 transition=0: unpatched > > */ > > struct klp_func { > > /* external */ > > @@ -53,6 +75,7 @@ struct klp_func { > > * in kallsyms for the given object is used. > > */ > > unsigned long old_sympos; > > + bool immediate; > > > > /* internal */ > > unsigned long old_addr; > > @@ -60,6 +83,7 @@ struct klp_func { > > struct list_head stack_node; > > unsigned long old_size, new_size; > > bool patched; > > + bool transition; > > }; > > > > /** > > @@ -68,7 +92,7 @@ struct klp_func { > > * @funcs: function entries for functions to be patched in the object > > * @kobj: kobject for sysfs resources > > * @mod: kernel module associated with the patched object > > - * (NULL for vmlinux) > > + * (NULL for vmlinux) > > * @patched: the object's funcs have been added to the klp_ops list > > */ > > struct klp_object { > > @@ -86,6 +110,7 @@ struct klp_object { > > * struct klp_patch - patch structure for live patching > > * @mod: reference to the live patch module > > * @objs: object entries for kernel objects to be patched > > + * @immediate: patch all funcs immediately, bypassing safety mechanisms > > * @list: list node for global list of registered patches > > * @kobj: kobject for sysfs resources > > * @enabled: the patch is enabled (but operation may be incomplete) > > @@ -94,6 +119,7 @@ struct klp_patch { > > /* external */ > > struct module *mod; > > struct klp_object *objs; > > + bool immediate; > > > > /* internal */ > > struct list_head list; > > @@ -121,13 +147,27 @@ void arch_klp_init_object_loaded(struct klp_patch *patch, > > int klp_module_coming(struct module *mod); > > void klp_module_going(struct module *mod); > > > > +void klp_copy_process(struct task_struct *child); > > void klp_update_patch_state(struct task_struct *task); > > > > +static inline bool klp_patch_pending(struct task_struct *task) > > +{ > > + return test_tsk_thread_flag(task, TIF_PATCH_PENDING); > > +} > > + > > +static inline bool klp_have_reliable_stack(void) > > +{ > > + return IS_ENABLED(CONFIG_STACKTRACE) && > > + IS_ENABLED(CONFIG_HAVE_RELIABLE_STACKTRACE); > > +} > > + > > #else /* !CONFIG_LIVEPATCH */ > > > > static inline int klp_module_coming(struct module *mod) { return 0; } > > static inline void klp_module_going(struct module *mod) {} > > +static inline bool klp_patch_pending(struct task_struct *task) { return false; } > > static inline void klp_update_patch_state(struct task_struct *task) {} > > +static inline void klp_copy_process(struct task_struct *child) {} > > > > #endif /* CONFIG_LIVEPATCH */ > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h > > index 9df1f42..ccbb1fa 100644 > > --- a/include/linux/sched.h > > +++ b/include/linux/sched.h > > @@ -1973,6 +1973,9 @@ struct task_struct { > > /* A live task holds one reference. */ > > atomic_t stack_refcount; > > #endif > > +#ifdef CONFIG_LIVEPATCH > > + int patch_state; > > +#endif > > /* CPU-specific state of this task */ > > struct thread_struct thread; > > /* > > diff --git a/kernel/fork.c b/kernel/fork.c > > index 7da33cb..8c8de80 100644 > > --- a/kernel/fork.c > > +++ b/kernel/fork.c > > @@ -77,6 +77,7 @@ > > #include <linux/compiler.h> > > #include <linux/sysctl.h> > > #include <linux/kcov.h> > > +#include <linux/livepatch.h> > > > > #include <asm/pgtable.h> > > #include <asm/pgalloc.h> > > @@ -1765,6 +1766,8 @@ static __latent_entropy struct task_struct *copy_process( > > p->parent_exec_id = current->self_exec_id; > > } > > > > + klp_copy_process(p); > > + > > spin_lock(¤t->sighand->siglock); > > > > /* > > diff --git a/kernel/livepatch/Makefile b/kernel/livepatch/Makefile > > index e136dad..2b8bdb1 100644 > > --- a/kernel/livepatch/Makefile > > +++ b/kernel/livepatch/Makefile > > @@ -1,3 +1,3 @@ > > obj-$(CONFIG_LIVEPATCH) += livepatch.o > > > > -livepatch-objs := core.o patch.o > > +livepatch-objs := core.o patch.o transition.o > > diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c > > index 10ba3a1..4c5a816 100644 > > --- a/kernel/livepatch/core.c > > +++ b/kernel/livepatch/core.c > > @@ -31,22 +31,22 @@ > > #include <linux/moduleloader.h> > > #include <asm/cacheflush.h> > > #include "patch.h" > > +#include "transition.h" > > > > /* > > - * The klp_mutex protects the global lists and state transitions of any > > - * structure reachable from them. References to any structure must be obtained > > - * under mutex protection (except in klp_ftrace_handler(), which uses RCU to > > - * ensure it gets consistent data). > > + * klp_mutex is a coarse lock which serializes access to klp data. All > > + * accesses to klp-related variables and structures must have mutex protection, > > + * except within the following functions which carefully avoid the need for it: > > + * > > + * - klp_ftrace_handler() > > + * - klp_update_patch_state() > > */ > > -static DEFINE_MUTEX(klp_mutex); > > +DEFINE_MUTEX(klp_mutex); > > > > static LIST_HEAD(klp_patches); > > > > static struct kobject *klp_root_kobj; > > > > -/* TODO: temporary stub */ > > -void klp_update_patch_state(struct task_struct *task) {} > > - > > static bool klp_is_module(struct klp_object *obj) > > { > > return obj->name; > > @@ -85,7 +85,6 @@ static void klp_find_object_module(struct klp_object *obj) > > mutex_unlock(&module_mutex); > > } > > > > -/* klp_mutex must be held by caller */ > > static bool klp_is_patch_registered(struct klp_patch *patch) > > { > > struct klp_patch *mypatch; > > @@ -281,20 +280,25 @@ static int klp_write_object_relocations(struct module *pmod, > > > > static int __klp_disable_patch(struct klp_patch *patch) > > { > > - struct klp_object *obj; > > + if (klp_transition_patch) > > + return -EBUSY; > > > > /* enforce stacking: only the last enabled patch can be disabled */ > > if (!list_is_last(&patch->list, &klp_patches) && > > list_next_entry(patch, list)->enabled) > > return -EBUSY; > > > > - pr_notice("disabling patch '%s'\n", patch->mod->name); > > + klp_init_transition(patch, KLP_UNPATCHED); > > > > - klp_for_each_object(patch, obj) { > > - if (obj->patched) > > - klp_unpatch_object(obj); > > - } > > + /* > > + * Enforce the order of the klp_target_state write in > > + * klp_init_transition() and the TIF_PATCH_PENDING writes in > > + * klp_start_transition() to ensure that klp_update_patch_state() > > + * doesn't set a task->patch_state to KLP_UNDEFINED. > > + */ > > + smp_wmb(); > > The description is not clear. The klp_target_state manipulation > is synchronized by another barrier inside klp_init_transition(). Yeah. I should also update the barrier comment in klp_init_transition() to clarify that it also does this. > A similar barrier is in __klp_enable_patch() and it is correctly > described there: > > It enforces the order of the func->transition writes in > klp_init_transition() and the ops->func_stack writes in > klp_patch_object(). The corresponding barrier is in > klp_ftrace_handler(). > > But we do not modify ops->func_stack in __klp_disable_patch(). > So we need another explanation. > > Huh, I spent few hours thinking about it. I am almost sure > that it is not needed. But I am not 100% sure. The many times > rewriten summary looks like: > > /* > * Enforce the order of func->transtion write in > * klp_init_transition() against TIF_PATCH_PENDING writes > * in klp_start_transition(). It makes sure that > * klp_ftrace_hadler() will see func->transition set > * after the task is migrated by klp_update_patch_state(). > * > * The barrier probably is not needed because the task > * must not be migrated when being inside klp_ftrace_handler() > * and there is another full barrier in > * klp_update_patch_state(). > * But this is slow path and better be safe than sorry. > */ > smp_wmb(); This mostly makes sense, but I think the barrier *is* needed for ordering func->transition and TIF_PATCH_PENDING writes for the rare case where klp_ftrace_handler() is called right after klp_update_patch_state(), which could be possible in the idle loop, for example. CPU0 CPU1 __klp_disable_patch() klp_init_transition() func->transition = true; (...no barrier...) klp_start_transition() set TIF_PATCH_PENDING klp_update_patch_state() if (test_and_clear(TIF_PATCH_PENDING)) task->patch_state = KLP_UNPATCHED; ... klp_ftrace_handler() smp_rmb(); if (unlikely(func->transition)) <--- false (patched) ... klp_ftrace_handler() smp_rmb(); if (unlikely(func->transition)) <--- true (unpatched) So how about: /* * Enforce the order of the func->transition writes in * klp_init_transition() and the TIF_PATCH_PENDING writes in * klp_start_transition(). In the rare case where klp_ftrace_handler() * is called shortly after klp_update_patch_state() switches the task, * this ensures the handler sees func->transition is set. */ smp_wmb(); > > + klp_start_transition(); > > patch->enabled = false; > > > > return 0; > > @@ -337,6 +341,9 @@ static int __klp_enable_patch(struct klp_patch *patch) > > struct klp_object *obj; > > int ret; > > > > + if (klp_transition_patch) > > + return -EBUSY; > > + > > if (WARN_ON(patch->enabled)) > > return -EINVAL; > > > > @@ -347,22 +354,37 @@ static int __klp_enable_patch(struct klp_patch *patch) > > > > pr_notice("enabling patch '%s'\n", patch->mod->name); > > > > + klp_init_transition(patch, KLP_PATCHED); > > + > > + /* > > + * Enforce the order of the func->transition writes in > > + * klp_init_transition() and the ops->func_stack writes in > > + * klp_patch_object(), so that klp_ftrace_handler() will see the > > + * func->transition updates before the handler is registered and the > > + * new funcs become visible to the handler. > > + */ > > + smp_wmb(); > > + > > klp_for_each_object(patch, obj) { > > if (!klp_is_object_loaded(obj)) > > continue; > > > > ret = klp_patch_object(obj); > > - if (ret) > > - goto unregister; > > + if (ret) { > > + pr_warn("failed to enable patch '%s'\n", > > + patch->mod->name); > > + > > + klp_unpatch_objects(patch); > > We should call here synchronize_rcu() here as we do > in klp_try_complete_transition(). Some of the affected > functions might have more versions on the stack and we > need to make sure that klp_ftrace_handler() will _not_ > see the removed patch on the stack. Even if the handler sees the new func on the stack, the task->patch_state is still KLP_UNPATCHED, so it will still choose the previous version of the function. Or did I miss your point? > Alternatively, we could move all the code around > klp_unpatch_objects() from klp_try_complete_transition() > into klp_complete_transition(), see below. But klp_target_state is KLP_PATCHED so the synchronize_rcu() at the end of klp_try_complete_transition() wouldn't get called anyway. > > > + klp_complete_transition(); > > + > > + return ret; > > + } > > } > > > > + klp_start_transition(); > > patch->enabled = true; > > > > return 0; > > - > > -unregister: > > - WARN_ON(__klp_disable_patch(patch)); > > - return ret; > > } > > > > /** > > @@ -399,6 +421,7 @@ EXPORT_SYMBOL_GPL(klp_enable_patch); > > * /sys/kernel/livepatch > > * /sys/kernel/livepatch/<patch> > > * /sys/kernel/livepatch/<patch>/enabled > > + * /sys/kernel/livepatch/<patch>/transition > > * /sys/kernel/livepatch/<patch>/<object> > > * /sys/kernel/livepatch/<patch>/<object>/<function,sympos> > > */ > > @@ -424,7 +447,9 @@ static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr, > > goto err; > > } > > > > - if (enabled) { > > + if (patch == klp_transition_patch) { > > + klp_reverse_transition(); > > + } else if (enabled) { > > ret = __klp_enable_patch(patch); > > if (ret) > > goto err; > > @@ -452,9 +477,21 @@ static ssize_t enabled_show(struct kobject *kobj, > > return snprintf(buf, PAGE_SIZE-1, "%d\n", patch->enabled); > > } > > > > +static ssize_t transition_show(struct kobject *kobj, > > + struct kobj_attribute *attr, char *buf) > > +{ > > + struct klp_patch *patch; > > + > > + patch = container_of(kobj, struct klp_patch, kobj); > > + return snprintf(buf, PAGE_SIZE-1, "%d\n", > > + patch == klp_transition_patch); > > +} > > + > > static struct kobj_attribute enabled_kobj_attr = __ATTR_RW(enabled); > > +static struct kobj_attribute transition_kobj_attr = __ATTR_RO(transition); > > static struct attribute *klp_patch_attrs[] = { > > &enabled_kobj_attr.attr, > > + &transition_kobj_attr.attr, > > NULL > > }; > > > > @@ -544,6 +581,7 @@ static int klp_init_func(struct klp_object *obj, struct klp_func *func) > > > > INIT_LIST_HEAD(&func->stack_node); > > func->patched = false; > > + func->transition = false; > > > > /* The format for the sysfs directory is <function,sympos> where sympos > > * is the nth occurrence of this symbol in kallsyms for the patched > > @@ -740,6 +778,16 @@ int klp_register_patch(struct klp_patch *patch) > > return -ENODEV; > > > > /* > > + * Architectures without reliable stack traces have to set > > + * patch->immediate because there's currently no way to patch kthreads > > + * with the consistency model. > > + */ > > + if (!klp_have_reliable_stack() && !patch->immediate) { > > + pr_err("This architecture doesn't have support for the livepatch consistency model.\n"); > > + return -ENOSYS; > > + } > > + > > + /* > > * A reference is taken on the patch module to prevent it from being > > * unloaded. Right now, we don't allow patch modules to unload since > > * there is currently no method to determine if a thread is still > > @@ -788,7 +836,11 @@ int klp_module_coming(struct module *mod) > > goto err; > > } > > > > - if (!patch->enabled) > > + /* > > + * Only patch the module if the patch is enabled or is > > + * in transition. > > + */ > > + if (!patch->enabled && patch != klp_transition_patch) > > We need the same change also in klp_module_going(). Ok. > > break; > > > > pr_notice("applying patch '%s' to loading module '%s'\n", > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c > > index 5efa262..1a77f05 100644 > > --- a/kernel/livepatch/patch.c > > +++ b/kernel/livepatch/patch.c > > @@ -29,6 +29,7 @@ > > #include <linux/bug.h> > > #include <linux/printk.h> > > #include "patch.h" > > +#include "transition.h" > > > > static LIST_HEAD(klp_ops); > > > > @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip, > > { > > struct klp_ops *ops; > > struct klp_func *func; > > + int patch_state; > > > > ops = container_of(fops, struct klp_ops, fops); > > > > rcu_read_lock(); > > + > > func = list_first_or_null_rcu(&ops->func_stack, struct klp_func, > > stack_node); > > + > > + /* > > + * func should never be NULL because preemption should be disabled here > > + * and unregister_ftrace_function() does the equivalent of a > > + * synchronize_sched() before the func_stack removal. > > + */ > > + if (WARN_ON_ONCE(!func)) > > + goto unlock; > > + > > + /* > > + * Enforce the order of the ops->func_stack and func->transition reads. > > + * The corresponding write barrier is in __klp_enable_patch(). > > + */ > > + smp_rmb(); > > I was curious why the comment did not mention __klp_disable_patch(). > It was related to the hours of thinking. I would like to avoid this > in the future and add a comment like. > > * This barrier probably is not needed when the patch is being > * disabled. The patch is removed from the stack in > * klp_try_complete_transition() and there we need to call > * rcu_synchronize() to prevent seeing the patch on the stack > * at all. > * > * Well, it still might be needed to see func->transition > * when the patch is removed and the task is migrated. See > * the write barrier in __klp_disable_patch(). Agreed, though as you mentioned earlier, there's also the implicit barrier in klp_update_patch_state(), which would execute first in such a scenario. So I think I'll update the barrier comments in klp_update_patch_state(). > > + if (unlikely(func->transition)) { > > + > > + /* > > + * Enforce the order of the func->transition and > > + * current->patch_state reads. Otherwise we could read an > > + * out-of-date task state and pick the wrong function. The > > + * corresponding write barriers are in klp_init_transition() > > + * and __klp_disable_patch(). > > + */ > > + smp_rmb(); > > IMHO, the corresponding barrier is in klp_init_transition(). > > The barrier in __klp_disable_patch() is questionable. Anyway, > it has a different purpose. Agreed. > > + patch_state = current->patch_state; > > + > > + WARN_ON_ONCE(patch_state == KLP_UNDEFINED); > > + > > + if (patch_state == KLP_UNPATCHED) { > > + /* > > + * Use the previously patched version of the function. > > + * If no previous patches exist, continue with the > > + * original function. > > + */ > > + func = list_entry_rcu(func->stack_node.next, > > + struct klp_func, stack_node); > > + > > + if (&func->stack_node == &ops->func_stack) > > + goto unlock; > > + } > > + } > > + > > klp_arch_set_pc(regs, (unsigned long)func->new_func); > > unlock: > > rcu_read_unlock(); > > @@ -211,3 +255,12 @@ int klp_patch_object(struct klp_object *obj) > > > > return 0; > > } > > + > > +void klp_unpatch_objects(struct klp_patch *patch) > > +{ > > + struct klp_object *obj; > > + > > + klp_for_each_object(patch, obj) > > + if (obj->patched) > > + klp_unpatch_object(obj); > > +} > > diff --git a/kernel/livepatch/patch.h b/kernel/livepatch/patch.h > > index 2d0cce0..0db2271 100644 > > --- a/kernel/livepatch/patch.h > > +++ b/kernel/livepatch/patch.h > > @@ -28,5 +28,6 @@ struct klp_ops *klp_find_ops(unsigned long old_addr); > > > > int klp_patch_object(struct klp_object *obj); > > void klp_unpatch_object(struct klp_object *obj); > > +void klp_unpatch_objects(struct klp_patch *patch); > > > > #endif /* _LIVEPATCH_PATCH_H */ > > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c > > new file mode 100644 > > index 0000000..2b87ff9 > > --- /dev/null > > +++ b/kernel/livepatch/transition.c > > @@ -0,0 +1,525 @@ > > +/* > > + * transition.c - Kernel Live Patching transition functions > > + * > > + * Copyright (C) 2015-2016 Josh Poimboeuf <jpoimboe@redhat.com> > > + * > > + * This program is free software; you can redistribute it and/or > > + * modify it under the terms of the GNU General Public License > > + * as published by the Free Software Foundation; either version 2 > > + * of the License, or (at your option) any later version. > > + * > > + * This program is distributed in the hope that it will be useful, > > + * but WITHOUT ANY WARRANTY; without even the implied warranty of > > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > > + * GNU General Public License for more details. > > + * > > + * You should have received a copy of the GNU General Public License > > + * along with this program; if not, see <http://www.gnu.org/licenses/>. > > + */ > > + > > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt > > + > > +#include <linux/cpu.h> > > +#include <linux/stacktrace.h> > > +#include "patch.h" > > +#include "transition.h" > > +#include "../sched/sched.h" > > + > > +#define MAX_STACK_ENTRIES 100 > > +#define STACK_ERR_BUF_SIZE 128 > > + > > +extern struct mutex klp_mutex; > > + > > +struct klp_patch *klp_transition_patch; > > + > > +static int klp_target_state = KLP_UNDEFINED; > > + > > +/* > > + * This work can be performed periodically to finish patching or unpatching any > > + * "straggler" tasks which failed to transition in the first attempt. > > + */ > > +static void klp_try_complete_transition(void); > > +static void klp_transition_work_fn(struct work_struct *work) > > +{ > > + mutex_lock(&klp_mutex); > > + > > + if (klp_transition_patch) > > + klp_try_complete_transition(); > > + > > + mutex_unlock(&klp_mutex); > > +} > > +static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn); > > + > > +/* > > + * The transition to the target patch state is complete. Clean up the data > > + * structures. > > + */ > > +void klp_complete_transition(void) > > +{ > > + struct klp_object *obj; > > + struct klp_func *func; > > + struct task_struct *g, *task; > > + unsigned int cpu; > > + > > + if (klp_transition_patch->immediate) > > + goto done; > > + > > + klp_for_each_object(klp_transition_patch, obj) > > + klp_for_each_func(obj, func) > > + func->transition = false; > > + > > + /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */ > > + if (klp_target_state == KLP_PATCHED) > > + synchronize_rcu(); > > I forgot why this is done only when the patch is beeing enabled. > It is because klp_unpatch_objects() and rcu_synchronize() is called > in klp_try_complete_transition() when klp_target_state == > KLP_UNPATCHED. > > I would suggest to move the code below the "success" label from > klp_try_complete_transtion() to klp_complete_transition(). > It will get the two related things close to each other. > Also it would fix the missing rcu_synchronize() in > the error path in __klp_enable_patch(), see above. As I mentioned, I don't see how that will fix the __klp_enable_patch() error path, nor can I see why it needs fixing in the first place. Regardless, it does seem like a good idea to move the end of klp_try_complete_transition() into klp_complete_transition(). > > + read_lock(&tasklist_lock); > > + for_each_process_thread(g, task) { > > + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); > > + task->patch_state = KLP_UNDEFINED; > > + } > > + read_unlock(&tasklist_lock); > > + > > + for_each_possible_cpu(cpu) { > > + task = idle_task(cpu); > > + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); > > + task->patch_state = KLP_UNDEFINED; > > + } > > + > > +done: > > + klp_target_state = KLP_UNDEFINED; > > + klp_transition_patch = NULL; > > +} > > + > > +/* > > + * Switch the patched state of the task to the set of functions in the target > > + * patch state. > > + * > > + * NOTE: If task is not 'current', the caller must ensure the task is inactive. > > + * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value. > > + */ > > +void klp_update_patch_state(struct task_struct *task) > > +{ > > + rcu_read_lock(); > > + > > + /* > > + * This test_and_clear_tsk_thread_flag() call also serves as a read > > + * barrier to enforce the order of the TIF_PATCH_PENDING and > > + * klp_target_state reads. The corresponding write barrier is in > > + * __klp_disable_patch(). > > The corresponding barrier is in klp_init_transition(), see below. > > In each case, we need the corresponding write barrier in both enable > and disable paths. They both set klp_target_state and > TIF_PATCH_PENDING. The write barrier in klp_init_transition() perfectly > fits the purpose. Agreed. > > + */ > > + if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING)) > > + task->patch_state = READ_ONCE(klp_target_state); > > + > > + rcu_read_unlock(); > > +} > > + > > +/* > > + * Determine whether the given stack trace includes any references to a > > + * to-be-patched or to-be-unpatched function. > > + */ > > +static int klp_check_stack_func(struct klp_func *func, > > + struct stack_trace *trace) > > +{ > > + unsigned long func_addr, func_size, address; > > + struct klp_ops *ops; > > + int i; > > + > > + if (func->immediate) > > + return 0; > > + > > + for (i = 0; i < trace->nr_entries; i++) { > > + address = trace->entries[i]; > > + > > + if (klp_target_state == KLP_UNPATCHED) { > > + /* > > + * Check for the to-be-unpatched function > > + * (the func itself). > > + */ > > + func_addr = (unsigned long)func->new_func; > > + func_size = func->new_size; > > + } else { > > + /* > > + * Check for the to-be-patched function > > + * (the previous func). > > + */ > > + ops = klp_find_ops(func->old_addr); > > + > > + if (list_is_singular(&ops->func_stack)) { > > + /* original function */ > > + func_addr = func->old_addr; > > + func_size = func->old_size; > > + } else { > > + /* previously patched function */ > > + struct klp_func *prev; > > + > > + prev = list_next_entry(func, stack_node); > > + func_addr = (unsigned long)prev->new_func; > > + func_size = prev->new_size; > > + } > > + } > > + > > + if (address >= func_addr && address < func_addr + func_size) > > + return -EAGAIN; > > + } > > + > > + return 0; > > +} > > + > > +/* > > + * Determine whether it's safe to transition the task to the target patch state > > + * by looking for any to-be-patched or to-be-unpatched functions on its stack. > > + */ > > +static int klp_check_stack(struct task_struct *task, char *err_buf) > > +{ > > + static unsigned long entries[MAX_STACK_ENTRIES]; > > + struct stack_trace trace; > > + struct klp_object *obj; > > + struct klp_func *func; > > + int ret; > > + > > + trace.skip = 0; > > + trace.nr_entries = 0; > > + trace.max_entries = MAX_STACK_ENTRIES; > > + trace.entries = entries; > > + ret = save_stack_trace_tsk_reliable(task, &trace); > > + WARN_ON_ONCE(ret == -ENOSYS); > > + if (ret) { > > + snprintf(err_buf, STACK_ERR_BUF_SIZE, > > + "%s: %s:%d has an unreliable stack\n", > > + __func__, task->comm, task->pid); > > + return ret; > > + } > > + > > + klp_for_each_object(klp_transition_patch, obj) { > > + if (!obj->patched) > > + continue; > > + klp_for_each_func(obj, func) { > > + ret = klp_check_stack_func(func, &trace); > > + if (ret) { > > + snprintf(err_buf, STACK_ERR_BUF_SIZE, > > + "%s: %s:%d is sleeping on function %s\n", > > + __func__, task->comm, task->pid, > > + func->old_name); > > + return ret; > > + } > > + } > > + } > > + > > + return 0; > > +} > > + > > +/* > > + * Try to safely switch a task to the target patch state. If it's currently > > + * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or > > + * if the stack is unreliable, return false. > > + */ > > +static bool klp_try_switch_task(struct task_struct *task) > > +{ > > + struct rq *rq; > > + struct rq_flags flags; > > + int ret; > > + bool success = false; > > + char err_buf[STACK_ERR_BUF_SIZE]; > > + > > + err_buf[0] = '\0'; > > + > > + /* check if this task has already switched over */ > > + if (task->patch_state == klp_target_state) > > + return true; > > + > > + /* > > + * For arches which don't have reliable stack traces, we have to rely > > + * on other methods (e.g., switching tasks at kernel exit). > > + */ > > + if (!klp_have_reliable_stack()) > > + return false; > > + > > + /* > > + * Now try to check the stack for any to-be-patched or to-be-unpatched > > + * functions. If all goes well, switch the task to the target patch > > + * state. > > + */ > > + rq = task_rq_lock(task, &flags); > > + > > + if (task_running(rq, task) && task != current) { > > + snprintf(err_buf, STACK_ERR_BUF_SIZE, > > + "%s: %s:%d is running\n", __func__, task->comm, > > + task->pid); > > + goto done; > > + } > > + > > + ret = klp_check_stack(task, err_buf); > > + if (ret) > > + goto done; > > + > > + success = true; > > + > > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > > + task->patch_state = klp_target_state; > > + > > +done: > > + task_rq_unlock(rq, task, &flags); > > + > > + /* > > + * Due to console deadlock issues, pr_debug() can't be used while > > + * holding the task rq lock. Instead we have to use a temporary buffer > > + * and print the debug message after releasing the lock. > > + */ > > + if (err_buf[0] != '\0') > > + pr_debug("%s", err_buf); > > + > > + return success; > > + > > +} > > + > > +/* > > + * Try to switch all remaining tasks to the target patch state by walking the > > + * stacks of sleeping tasks and looking for any to-be-patched or > > + * to-be-unpatched functions. If such functions are found, the task can't be > > + * switched yet. > > + * > > + * If any tasks are still stuck in the initial patch state, schedule a retry. > > + */ > > +static void klp_try_complete_transition(void) > > +{ > > + unsigned int cpu; > > + struct task_struct *g, *task; > > + bool complete = true; > > + > > + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED); > > + > > + /* > > + * If the patch can be applied or reverted immediately, skip the > > + * per-task transitions. > > + */ > > + if (klp_transition_patch->immediate) > > + goto success; > > + > > + /* > > + * Try to switch the tasks to the target patch state by walking their > > + * stacks and looking for any to-be-patched or to-be-unpatched > > + * functions. If such functions are found on a stack, or if the stack > > + * is deemed unreliable, the task can't be switched yet. > > + * > > + * Usually this will transition most (or all) of the tasks on a system > > + * unless the patch includes changes to a very common function. > > + */ > > + read_lock(&tasklist_lock); > > + for_each_process_thread(g, task) > > + if (!klp_try_switch_task(task)) > > + complete = false; > > + read_unlock(&tasklist_lock); > > + > > + /* > > + * Ditto for the idle "swapper" tasks. > > + */ > > + get_online_cpus(); > > + for_each_possible_cpu(cpu) { > > + task = idle_task(cpu); > > + if (cpu_online(cpu)) { > > + if (!klp_try_switch_task(task)) > > + complete = false; > > + } else if (task->patch_state != klp_target_state) { > > + /* offline idle tasks can be switched immediately */ > > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > > + task->patch_state = klp_target_state; > > + } > > + } > > + put_online_cpus(); > > + > > + /* > > + * Some tasks weren't able to be switched over. Try again later and/or > > + * wait for other methods like kernel exit switching. > > + */ > > + if (!complete) { > > + schedule_delayed_work(&klp_transition_work, > > + round_jiffies_relative(HZ)); > > + return; > > + } > > + > > +success: > > + > > + if (klp_target_state == KLP_UNPATCHED) { > > + /* > > + * All tasks have transitioned to KLP_UNPATCHED so we can now > > + * remove the new functions from the func_stack. > > + */ > > + klp_unpatch_objects(klp_transition_patch); > > + > > + /* > > + * Make sure klp_ftrace_handler() sees that the functions have > > + * been removed from ops->func_stack before we clear > > + * func->transition. Otherwise it may pick the wrong > > + * transition. > > + */ > > I was a bit confused by the comment. It might be my problem. I just > wonder if might be more clear with somethink like: > > /* > * Make sure klp_ftrace_handler() can not longer see functions > * from this patch on the stack. Otherwise it may use > * the being-removed function after func->transition is cleared. > */ Ok. > > + synchronize_rcu(); > > + } > > + > > + pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name, > > + klp_target_state == KLP_PATCHED ? "patching" : "unpatching"); > > + > > + /* we're done, now cleanup the data structures */ > > + klp_complete_transition(); > > +} > > + > > +/* > > + * Start the transition to the specified target patch state so tasks can begin > > + * switching to it. > > + */ > > +void klp_start_transition(void) > > +{ > > + struct task_struct *g, *task; > > + unsigned int cpu; > > + > > + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED); > > + > > + pr_notice("'%s': %s...\n", klp_transition_patch->mod->name, > > + klp_target_state == KLP_PATCHED ? "patching" : "unpatching"); > > + > > + /* > > + * If the patch can be applied or reverted immediately, skip the > > + * per-task transitions. > > + */ > > + if (klp_transition_patch->immediate) > > We should call klp_try_complete_transition() here. Otherwise, it will > never be called and the transition will never get completed. > > Alternative solution would be to move klp_try_complete_transition() > from klp_start_transition() and explicitely call it from > __klp_disable_patch() and klp_enable_patch(). It would actually > solve one issue with klp_revert_patch(), see below. > > I kind of like the alternative solution. I hope that it was not > me who suggested to move klp_try_complete_transition() into > klp_start_transtion(). I'll go with your alternative solution. (BTW, moving the klp_try_complete_transition() call was my idea. It was an "improvement" I made as part of moving the klp_transition_work to transition.c.) > > + return; > > + > > + /* > > + * Mark all normal tasks as needing a patch state update. They'll > > + * switch either in klp_try_complete_transition() or as they exit the > > + * kernel. > > + */ > > + read_lock(&tasklist_lock); > > + for_each_process_thread(g, task) > > + if (task->patch_state != klp_target_state) > > + set_tsk_thread_flag(task, TIF_PATCH_PENDING); > > + read_unlock(&tasklist_lock); > > + > > + /* > > + * Mark all idle tasks as needing a patch state update. They'll switch > > + * either in klp_try_complete_transition() or at the idle loop switch > > + * point. > > + */ > > + for_each_possible_cpu(cpu) { > > + task = idle_task(cpu); > > + if (task->patch_state != klp_target_state) > > + set_tsk_thread_flag(task, TIF_PATCH_PENDING); > > + } > > + > > + klp_try_complete_transition(); > > +} > > + > > +/* > > + * Initialize the global target patch state and all tasks to the initial patch > > + * state, and initialize all function transition states to true in preparation > > + * for patching or unpatching. > > + */ > > +void klp_init_transition(struct klp_patch *patch, int state) > > +{ > > + struct task_struct *g, *task; > > + unsigned int cpu; > > + struct klp_object *obj; > > + struct klp_func *func; > > + int initial_state = !state; > > + > > + WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED); > > + > > + klp_transition_patch = patch; > > + > > + /* > > + * Set the global target patch state which tasks will switch to. This > > + * has no effect until the TIF_PATCH_PENDING flags get set later. > > + */ > > + klp_target_state = state; > > + > > + /* > > + * If the patch can be applied or reverted immediately, skip the > > + * per-task transitions. > > + */ > > + if (patch->immediate) > > + return; > > + > > + /* > > + * Initialize all tasks to the initial patch state to prepare them for > > + * switching to the target state. > > + */ > > + read_lock(&tasklist_lock); > > + for_each_process_thread(g, task) { > > + WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED); > > + task->patch_state = initial_state; > > + } > > + read_unlock(&tasklist_lock); > > + > > + /* > > + * Ditto for the idle "swapper" tasks. > > + */ > > + for_each_possible_cpu(cpu) { > > + task = idle_task(cpu); > > + WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED); > > + task->patch_state = initial_state; > > + } > > + > > + /* > > + * Enforce the order of the task->patch_state initializations and the > > + * func->transition updates to ensure that, in the enable path, > > + * klp_ftrace_handler() doesn't see a func in transition with a > > + * task->patch_state of KLP_UNDEFINED. > > It has one more purpose: > > * > * Also enforce the order between klp_target_state and > * TIF_PATCH_PENDING. The corresponding read barrier is in > * klp_update_patch_state(). Yes. > > + */ > > + smp_wmb(); > > + > > + /* > > + * Set the func transition states so klp_ftrace_handler() will know to > > + * switch to the transition logic. > > + * > > + * When patching, the funcs aren't yet in the func_stack and will be > > + * made visible to the ftrace handler shortly by the calls to > > + * klp_patch_object(). > > + * > > + * When unpatching, the funcs are already in the func_stack and so are > > + * already visible to the ftrace handler. > > + */ > > + klp_for_each_object(patch, obj) > > + klp_for_each_func(obj, func) > > + func->transition = true; > > +} > > + > > +/* > > + * This function can be called in the middle of an existing transition to > > + * reverse the direction of the target patch state. This can be done to > > + * effectively cancel an existing enable or disable operation if there are any > > + * tasks which are stuck in the initial patch state. > > + */ > > +void klp_reverse_transition(void) > > +{ > > + unsigned int cpu; > > + struct task_struct *g, *task; > > + > > + klp_transition_patch->enabled = !klp_transition_patch->enabled; > > + > > + klp_target_state = !klp_target_state; > > + > > + /* > > + * Clear all TIF_PATCH_PENDING flags to prevent races caused by > > + * klp_update_patch_state() running in parallel with > > + * klp_start_transition(). > > + */ > > + read_lock(&tasklist_lock); > > + for_each_process_thread(g, task) > > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > > + read_unlock(&tasklist_lock); > > + > > + for_each_possible_cpu(cpu) > > + clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING); > > + > > + /* Let any remaining calls to klp_update_patch_state() complete */ > > + synchronize_rcu(); > > + > > + klp_start_transition(); > > Hmm, we should not call klp_try_complete_transition() when > klp_start_transition() is called from here. I can't find a safe > way to cancel klp_transition_work() when we own klp_mutex. > It smells with a possible deadlock. > > I suggest to move move klp_try_complete_transition() outside > klp_start_transition() and explicitely call it from > __klp_disable_patch() and __klp_enabled_patch(). > This would fix also the problem with immediate patches, see > klp_start_transition(). Agreed. I'll fix it as you suggest and I'll put the mod_delayed_work() call in klp_reverse_transition() again. > > +} > > + > > +/* Called from copy_process() during fork */ > > +void klp_copy_process(struct task_struct *child) > > +{ > > + child->patch_state = current->patch_state; > > + > > + /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */ > > +} > > diff --git a/kernel/livepatch/transition.h b/kernel/livepatch/transition.h > > new file mode 100644 > > index 0000000..3ee625f > > --- /dev/null > > +++ b/kernel/livepatch/transition.h > > @@ -0,0 +1,13 @@ > > +#ifndef _LIVEPATCH_TRANSITION_H > > +#define _LIVEPATCH_TRANSITION_H > > + > > +#include <linux/livepatch.h> > > + > > +extern struct klp_patch *klp_transition_patch; > > + > > +void klp_init_transition(struct klp_patch *patch, int state); > > +void klp_start_transition(void); > > +void klp_reverse_transition(void); > > +void klp_complete_transition(void); > > + > > +#endif /* _LIVEPATCH_TRANSITION_H */ > > diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c > > index 6a4bae0..a8b3f1a 100644 > > --- a/kernel/sched/idle.c > > +++ b/kernel/sched/idle.c > > @@ -9,6 +9,7 @@ > > #include <linux/mm.h> > > #include <linux/stackprotector.h> > > #include <linux/suspend.h> > > +#include <linux/livepatch.h> > > > > #include <asm/tlb.h> > > > > @@ -264,6 +265,9 @@ static void do_idle(void) > > > > sched_ttwu_pending(); > > schedule_preempt_disabled(); > > + > > + if (unlikely(klp_patch_pending(current))) > > + klp_update_patch_state(current); > > } > > > > bool cpu_in_idle(unsigned long pc) > > diff --git a/samples/livepatch/livepatch-sample.c b/samples/livepatch/livepatch-sample.c > > index e34f871..629e0dc 100644 > > --- a/samples/livepatch/livepatch-sample.c > > +++ b/samples/livepatch/livepatch-sample.c > > @@ -17,6 +17,8 @@ > > * along with this program; if not, see <http://www.gnu.org/licenses/>. > > */ > > > > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt > > + > > #include <linux/module.h> > > #include <linux/kernel.h> > > #include <linux/livepatch.h> > > @@ -69,6 +71,21 @@ static int livepatch_init(void) > > { > > int ret; > > > > + if (!klp_have_reliable_stack() && !patch.immediate) { > > + /* > > + * WARNING: Be very careful when using 'patch.immediate' in > > + * your patches. It's ok to use it for simple patches like > > + * this, but for more complex patches which change function > > + * semantics, locking semantics, or data structures, it may not > > + * be safe. Use of this option will also prevent removal of > > + * the patch. > > + * > > + * See Documentation/livepatch/livepatch.txt for more details. > > + */ > > + patch.immediate = true; > > + pr_notice("The consistency model isn't supported for your architecture. Bypassing safety mechanisms and applying the patch immediately.\n"); > > + } > > + > > ret = klp_register_patch(&patch); > > if (ret) > > return ret; > > I am sorry that the review took me so long. I was interrupted several > times by other tasks. The logic is very complex. I wanted to > make sure that my comments are consistent and make sense. > > Anyway, I think that we are getting close. Yeah, it does take a while to wrap your head around the code, especially with respect to synchronization. If there's any way to simplify things, I'm all for it. Thanks again for your excellent reviews.
On Fri, Feb 03, 2017 at 05:41:28PM +0100, Miroslav Benes wrote: > > Petr has already mentioned majority of things I too found out, so only > couple of nits... > > > diff --git a/Documentation/ABI/testing/sysfs-kernel-livepatch b/Documentation/ABI/testing/sysfs-kernel-livepatch > > index da87f43..24b6570 100644 > > --- a/Documentation/ABI/testing/sysfs-kernel-livepatch > > +++ b/Documentation/ABI/testing/sysfs-kernel-livepatch > > @@ -25,6 +25,14 @@ Description: > > code is currently applied. Writing 0 will disable the patch > > while writing 1 will re-enable the patch. > > > > +What: /sys/kernel/livepatch/<patch>/transition > > +Date: May 2016 > > 'May 2016' looks strange, but maybe nobody cares about it... Will update the date to Feb 2017 and also the affected kernel version to 4.12. > > > diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt > > index 7f04e13..fb00d66 100644 > > --- a/Documentation/livepatch/livepatch.txt > > +++ b/Documentation/livepatch/livepatch.txt > > 4. Livepatch module > > @@ -134,7 +242,7 @@ Documentation/livepatch/module-elf-format.txt for more details. > > > > > > 4.2. Metadata > > ------------- > > +------------- > > klp_func and klp_patch have new members - immediate. Should be documented > here in "4.2. Metadata" section. Agreed. > The section also contains this text under klp_patch bullet. It seems > oudated: > > "Also if a more complex consistency model is supported then a selected > unit (thread, kernel as a whole) will see the new code from the entire > patch only when it is in a safe state." > > We now have a more complex consistency model. Agreed. > And finally, the section "Limitations" has this text under the first > bullet: > > + The patch must not change the semantic of the patched functions. > > The current implementation guarantees only that either the old > or the new function is called. The functions are patched one > by one. It means that the patch must _not_ change the semantic > of the function. > > I think it is confusing. The consistency model allows us to change the > semantic of a function. To certain degree. Of course, there are cases that > cannot be patched, or have to be patched carefully. For example if a > function takes a lock by calling foo_lock(), foo_lock() is not on a stack > afterwards. Then the locking semantics may be changed with a livepatch. > One has to make sure to patch also the caller foo_lock() to enforce the > consistency. And so on... But I do not consider a limitation of livepatch. > It is a feature of the consistency model, which is weaker than kGraft's or > kpatch's (or stronger. It depends on your point of view.) > > So, I propose to remove this text and better describe the properties of > the consistency model above in the section 3. Maybe a quote from an old > mail thread (Nov 2014) would be sufficient. I don't remember what was > mentioned and what not. > > What do you think? I'll remove the above limitation. I'm not sure how to improve the consistency model section. It already has at least some mentions of changed function semantics and locking semantics. I'll leave it alone for now, unless you have a specific suggestion. > > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h > > index 6602b34..ed90ad1 100644 > > --- a/include/linux/livepatch.h > > +++ b/include/linux/livepatch.h > > @@ -68,7 +92,7 @@ struct klp_func { > > * @funcs: function entries for functions to be patched in the object > > * @kobj: kobject for sysfs resources > > * @mod: kernel module associated with the patched object > > - * (NULL for vmlinux) > > + * (NULL for vmlinux) > > This looks superfluous. This is a minor whitespace fix -- remove a space before tab. I figured I'd go ahead and fix it since I'm already changing some of the surrounding code. > (checking my notes)... and that's it. Aside from the discussion in Petr's > subthread it looks good to me. Great job. Thanks!
On Fri 2017-02-03 14:39:16, Josh Poimboeuf wrote: > On Thu, Feb 02, 2017 at 12:51:16PM +0100, Petr Mladek wrote: > > !!! This is the right version. I am sorry again for the confusion. !!! > > > > > static int __klp_disable_patch(struct klp_patch *patch) > > > { > > > - struct klp_object *obj; > > > + if (klp_transition_patch) > > > + return -EBUSY; > > > > > > /* enforce stacking: only the last enabled patch can be disabled */ > > > if (!list_is_last(&patch->list, &klp_patches) && > > > list_next_entry(patch, list)->enabled) > > > return -EBUSY; > > > > > > - pr_notice("disabling patch '%s'\n", patch->mod->name); > > > + klp_init_transition(patch, KLP_UNPATCHED); > > > > > > - klp_for_each_object(patch, obj) { > > > - if (obj->patched) > > > - klp_unpatch_object(obj); > > > - } > > > + /* > > > + * Enforce the order of the klp_target_state write in > > > + * klp_init_transition() and the TIF_PATCH_PENDING writes in > > > + * klp_start_transition() to ensure that klp_update_patch_state() > > > + * doesn't set a task->patch_state to KLP_UNDEFINED. > > > + */ > > > + smp_wmb(); > > > > The description is not clear. The klp_target_state manipulation > > is synchronized by another barrier inside klp_init_transition(). > > Yeah. I should also update the barrier comment in klp_init_transition() > to clarify that it also does this. > > > A similar barrier is in __klp_enable_patch() and it is correctly > > described there: > > > > It enforces the order of the func->transition writes in > > klp_init_transition() and the ops->func_stack writes in > > klp_patch_object(). The corresponding barrier is in > > klp_ftrace_handler(). > > > > But we do not modify ops->func_stack in __klp_disable_patch(). > > So we need another explanation. > > > > Huh, I spent few hours thinking about it. I am almost sure > > that it is not needed. But I am not 100% sure. The many times > > rewriten summary looks like: > > > > /* > > * Enforce the order of func->transtion write in > > * klp_init_transition() against TIF_PATCH_PENDING writes > > * in klp_start_transition(). It makes sure that > > * klp_ftrace_hadler() will see func->transition set > > * after the task is migrated by klp_update_patch_state(). > > * > > * The barrier probably is not needed because the task > > * must not be migrated when being inside klp_ftrace_handler() > > * and there is another full barrier in > > * klp_update_patch_state(). > > * But this is slow path and better be safe than sorry. > > */ > > smp_wmb(); > > This mostly makes sense, but I think the barrier *is* needed for > ordering func->transition and TIF_PATCH_PENDING writes for the rare case > where klp_ftrace_handler() is called right after > klp_update_patch_state(), which could be possible in the idle loop, for > example. > > CPU0 CPU1 > __klp_disable_patch() > klp_init_transition() > func->transition = true; > (...no barrier...) > klp_start_transition() > set TIF_PATCH_PENDING > > klp_update_patch_state() > if (test_and_clear(TIF_PATCH_PENDING)) > task->patch_state = KLP_UNPATCHED; > ... > klp_ftrace_handler() > smp_rmb(); > if (unlikely(func->transition)) <--- false (patched) > ... > klp_ftrace_handler() > smp_rmb(); > if (unlikely(func->transition)) <--- true (unpatched) You are right. I was able to find many scenarios where the barrier was not needed. But it is needed in this one. The first paragraph should be enough then: /* * Enforce the order of func->transition write in * klp_init_transition() against TIF_PATCH_PENDING writes * in klp_start_transition(). It makes sure that * klp_ftrace_handler() will see func->transition set * after the task is migrated by klp_update_patch_state(). */ smp_wmb(); > So how about: > > /* > * Enforce the order of the func->transition writes in > * klp_init_transition() and the TIF_PATCH_PENDING writes in > * klp_start_transition(). In the rare case where klp_ftrace_handler() > * is called shortly after klp_update_patch_state() switches the task, > * this ensures the handler sees func->transition is set. > */ > smp_wmb(); Looks good to me. > > > + klp_start_transition(); > > > patch->enabled = false; > > > > > > return 0; > > > @@ -337,6 +341,9 @@ static int __klp_enable_patch(struct klp_patch *patch) > > > struct klp_object *obj; > > > int ret; > > > > > > + if (klp_transition_patch) > > > + return -EBUSY; > > > + > > > if (WARN_ON(patch->enabled)) > > > return -EINVAL; > > > > > > @@ -347,22 +354,37 @@ static int __klp_enable_patch(struct klp_patch *patch) > > > > > > pr_notice("enabling patch '%s'\n", patch->mod->name); > > > > > > + klp_init_transition(patch, KLP_PATCHED); > > > + > > > + /* > > > + * Enforce the order of the func->transition writes in > > > + * klp_init_transition() and the ops->func_stack writes in > > > + * klp_patch_object(), so that klp_ftrace_handler() will see the > > > + * func->transition updates before the handler is registered and the > > > + * new funcs become visible to the handler. > > > + */ > > > + smp_wmb(); > > > + > > > klp_for_each_object(patch, obj) { > > > if (!klp_is_object_loaded(obj)) > > > continue; > > > > > > ret = klp_patch_object(obj); > > > - if (ret) > > > - goto unregister; > > > + if (ret) { > > > + pr_warn("failed to enable patch '%s'\n", > > > + patch->mod->name); > > > + > > > + klp_unpatch_objects(patch); > > > > We should call here synchronize_rcu() here as we do > > in klp_try_complete_transition(). Some of the affected > > functions might have more versions on the stack and we > > need to make sure that klp_ftrace_handler() will _not_ > > see the removed patch on the stack. > > Even if the handler sees the new func on the stack, the > task->patch_state is still KLP_UNPATCHED, so it will still choose the > previous version of the function. Or did I miss your point? The barrier is needed from exactly the same reason as the one in klp_try_complete_transition() CPU0 CPU1 __klp_enable_patch() klp_init_transition() for_each... task->patch_state = KLP_UNPATCHED for_each... func->transition = true klp_for_each_object() klp_patch_object() list_add_rcu() klp_ftrace_handler() func = list_first_...() if (func->transition) ret = klp_patch_object() /* error */ if (ret) { klp_unpatch_objects() list_remove_rcu() klp_complete_transition() for_each_.... func->transition = true for_each_.... task->patch_state = PATCH_UNDEFINED patch_state = current->patch_state; WARN_ON_ONCE(patch_state == KLP_UNDEFINED); BANG: The warning is triggered. => we need to call rcu_synchronize(). It will make sure that no ftrace handled will see the removed func on the stack and we could clear all the other values. > > Alternatively, we could move all the code around > > klp_unpatch_objects() from klp_try_complete_transition() > > into klp_complete_transition(), see below. > > But klp_target_state is KLP_PATCHED so the synchronize_rcu() at the end > of klp_try_complete_transition() wouldn't get called anyway. > > > > > > + klp_complete_transition(); > > > + > > > + return ret; > > > + } > > > } > > > > > > + klp_start_transition(); > > > patch->enabled = true; > > > > > > return 0; > > > - > > > -unregister: > > > - WARN_ON(__klp_disable_patch(patch)); > > > - return ret; > > > } > > > > > > /** [...] > > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c > > > index 5efa262..1a77f05 100644 > > > --- a/kernel/livepatch/patch.c > > > +++ b/kernel/livepatch/patch.c > > > @@ -29,6 +29,7 @@ > > > #include <linux/bug.h> > > > #include <linux/printk.h> > > > #include "patch.h" > > > +#include "transition.h" > > > > > > static LIST_HEAD(klp_ops); > > > > > > @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip, > > > { > > > struct klp_ops *ops; > > > struct klp_func *func; > > > + int patch_state; > > > > > > ops = container_of(fops, struct klp_ops, fops); > > > > > > rcu_read_lock(); > > > + > > > func = list_first_or_null_rcu(&ops->func_stack, struct klp_func, > > > stack_node); > > > + > > > + /* > > > + * func should never be NULL because preemption should be disabled here > > > + * and unregister_ftrace_function() does the equivalent of a > > > + * synchronize_sched() before the func_stack removal. > > > + */ > > > + if (WARN_ON_ONCE(!func)) > > > + goto unlock; > > > + > > > + /* > > > + * Enforce the order of the ops->func_stack and func->transition reads. > > > + * The corresponding write barrier is in __klp_enable_patch(). > > > + */ > > > + smp_rmb(); > > > > I was curious why the comment did not mention __klp_disable_patch(). > > It was related to the hours of thinking. I would like to avoid this > > in the future and add a comment like. > > > > * This barrier probably is not needed when the patch is being > > * disabled. The patch is removed from the stack in > > * klp_try_complete_transition() and there we need to call > > * rcu_synchronize() to prevent seeing the patch on the stack > > * at all. > > * > > * Well, it still might be needed to see func->transition > > * when the patch is removed and the task is migrated. See > > * the write barrier in __klp_disable_patch(). > > Agreed, though as you mentioned earlier, there's also the implicit > barrier in klp_update_patch_state(), which would execute first in such a > scenario. So I think I'll update the barrier comments in > klp_update_patch_state(). You inspired me to a scenario with 3 CPUs: CPU0 CPU1 CPU2 __klp_disable_patch() klp_init_transition() func->transition = true smp_wmb() klp_start_transition() set TIF_PATCH_PATCHPENDING klp_update_patch_state() task->patch_state = KLP_UNPATCHED smp_mb() klp_ftrace_handler() func = list_... smp_rmb() /*needed?*/ if (func->transition) We need to make sure the CPU3 sees func->transition set. Otherwise, it would wrongly use the function from the patch. So, the description might be: * Enforce the order of the ops->func_stack and * func->transition reads when the patch is enabled. * The corresponding write barrier is in __klp_enable_patch(). * * Also make sure that func->transition is visible before * TIF_PATCH_PENDING_FLAG is set and the task might get * migrated to KLP_UNPATCHED state. The corresponding * write barrier is in __klp_disable_patch(). By other words, the read barrier here is needed from the same reason as the write barrier in __klp_disable_patch(). > > > + if (unlikely(func->transition)) { > > > + > > > + /* > > > + * Enforce the order of the func->transition and > > > + * current->patch_state reads. Otherwise we could read an > > > + * out-of-date task state and pick the wrong function. The > > > + * corresponding write barriers are in klp_init_transition() > > > + * and __klp_disable_patch(). > > > + */ > > > + smp_rmb(); > > > > IMHO, the corresponding barrier is in klp_init_transition(). > > > > The barrier in __klp_disable_patch() is questionable. Anyway, > > it has a different purpose. > > Agreed. [...] > > > diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c > > > new file mode 100644 > > > index 0000000..2b87ff9 > > > --- /dev/null > > > +++ b/kernel/livepatch/transition.c > > > @@ -0,0 +1,525 @@ > > > +/* > > > + * transition.c - Kernel Live Patching transition functions > > > + * > > > + * Copyright (C) 2015-2016 Josh Poimboeuf <jpoimboe@redhat.com> > > > + * > > > + * This program is free software; you can redistribute it and/or > > > + * modify it under the terms of the GNU General Public License > > > + * as published by the Free Software Foundation; either version 2 > > > + * of the License, or (at your option) any later version. > > > + * > > > + * This program is distributed in the hope that it will be useful, > > > + * but WITHOUT ANY WARRANTY; without even the implied warranty of > > > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the > > > + * GNU General Public License for more details. > > > + * > > > + * You should have received a copy of the GNU General Public License > > > + * along with this program; if not, see <http://www.gnu.org/licenses/>. > > > + */ > > > + > > > +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt > > > + > > > +#include <linux/cpu.h> > > > +#include <linux/stacktrace.h> > > > +#include "patch.h" > > > +#include "transition.h" > > > +#include "../sched/sched.h" > > > + > > > +#define MAX_STACK_ENTRIES 100 > > > +#define STACK_ERR_BUF_SIZE 128 > > > + > > > +extern struct mutex klp_mutex; > > > + > > > +struct klp_patch *klp_transition_patch; > > > + > > > +static int klp_target_state = KLP_UNDEFINED; > > > + > > > +/* > > > + * This work can be performed periodically to finish patching or unpatching any > > > + * "straggler" tasks which failed to transition in the first attempt. > > > + */ > > > +static void klp_try_complete_transition(void); > > > +static void klp_transition_work_fn(struct work_struct *work) > > > +{ > > > + mutex_lock(&klp_mutex); > > > + > > > + if (klp_transition_patch) > > > + klp_try_complete_transition(); > > > + > > > + mutex_unlock(&klp_mutex); > > > +} > > > +static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn); > > > + > > > +/* > > > + * The transition to the target patch state is complete. Clean up the data > > > + * structures. > > > + */ > > > +void klp_complete_transition(void) > > > +{ > > > + struct klp_object *obj; > > > + struct klp_func *func; > > > + struct task_struct *g, *task; > > > + unsigned int cpu; > > > + > > > + if (klp_transition_patch->immediate) > > > + goto done; > > > + > > > + klp_for_each_object(klp_transition_patch, obj) > > > + klp_for_each_func(obj, func) > > > + func->transition = false; > > > + > > > + /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */ > > > + if (klp_target_state == KLP_PATCHED) > > > + synchronize_rcu(); > > > > I forgot why this is done only when the patch is beeing enabled. > > It is because klp_unpatch_objects() and rcu_synchronize() is called > > in klp_try_complete_transition() when klp_target_state == > > KLP_UNPATCHED. > > > > I would suggest to move the code below the "success" label from > > klp_try_complete_transtion() to klp_complete_transition(). > > It will get the two related things close to each other. > > Also it would fix the missing rcu_synchronize() in > > the error path in __klp_enable_patch(), see above. > > As I mentioned, I don't see how that will fix the __klp_enable_patch() > error path, nor can I see why it needs fixing in the first place. I hope that I have persuaded you :-) > Regardless, it does seem like a good idea to move the end of > klp_try_complete_transition() into klp_complete_transition(). > > > > + read_lock(&tasklist_lock); > > > + for_each_process_thread(g, task) { > > > + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); > > > + task->patch_state = KLP_UNDEFINED; > > > + } > > > + read_unlock(&tasklist_lock); > > > + > > > + for_each_possible_cpu(cpu) { > > > + task = idle_task(cpu); > > > + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); > > > + task->patch_state = KLP_UNDEFINED; > > > + } > > > + > > > +done: > > > + klp_target_state = KLP_UNDEFINED; > > > + klp_transition_patch = NULL; > > > +} > > > + > > > +/* > > > + * Switch the patched state of the task to the set of functions in the target > > > + * patch state. > > > + * > > > + * NOTE: If task is not 'current', the caller must ensure the task is inactive. > > > + * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value. > > > + */ [...] > > > +void klp_reverse_transition(void) > > > +{ > > > + unsigned int cpu; > > > + struct task_struct *g, *task; > > > + > > > + klp_transition_patch->enabled = !klp_transition_patch->enabled; > > > + > > > + klp_target_state = !klp_target_state; > > > + > > > + /* > > > + * Clear all TIF_PATCH_PENDING flags to prevent races caused by > > > + * klp_update_patch_state() running in parallel with > > > + * klp_start_transition(). > > > + */ > > > + read_lock(&tasklist_lock); > > > + for_each_process_thread(g, task) > > > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > > > + read_unlock(&tasklist_lock); > > > + > > > + for_each_possible_cpu(cpu) > > > + clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING); > > > + > > > + /* Let any remaining calls to klp_update_patch_state() complete */ > > > + synchronize_rcu(); > > > + > > > + klp_start_transition(); > > > > Hmm, we should not call klp_try_complete_transition() when > > klp_start_transition() is called from here. I can't find a safe > > way to cancel klp_transition_work() when we own klp_mutex. > > It smells with a possible deadlock. > > > > I suggest to move move klp_try_complete_transition() outside > > klp_start_transition() and explicitely call it from > > __klp_disable_patch() and __klp_enabled_patch(). > > This would fix also the problem with immediate patches, see > > klp_start_transition(). > > Agreed. I'll fix it as you suggest and I'll put the mod_delayed_work() > call in klp_reverse_transition() again. There is one small catch. The mod_delayed_work() might cause that two works might be scheduled: + one already running that is waiting for the klp_mutex + another one scheduled by that mod_delayed_work() A solution would be to cancel the work from klp_transition_work_fn() if the transition succeeds. Another possibility would be to do nothing here. The work is scheduled very often anyway. > > Yeah, it does take a while to wrap your head around the code, especially > with respect to synchronization. Yes. I am looking forward to get this done. But we are really close, IMHO. I hope that my today's comments make sense. I am not in a good condition. I need some rest or so. Best Regards, Petr
On Mon, Feb 06, 2017 at 05:44:31PM +0100, Petr Mladek wrote: > > > > @@ -347,22 +354,37 @@ static int __klp_enable_patch(struct klp_patch *patch) > > > > > > > > pr_notice("enabling patch '%s'\n", patch->mod->name); > > > > > > > > + klp_init_transition(patch, KLP_PATCHED); > > > > + > > > > + /* > > > > + * Enforce the order of the func->transition writes in > > > > + * klp_init_transition() and the ops->func_stack writes in > > > > + * klp_patch_object(), so that klp_ftrace_handler() will see the > > > > + * func->transition updates before the handler is registered and the > > > > + * new funcs become visible to the handler. > > > > + */ > > > > + smp_wmb(); > > > > + > > > > klp_for_each_object(patch, obj) { > > > > if (!klp_is_object_loaded(obj)) > > > > continue; > > > > > > > > ret = klp_patch_object(obj); > > > > - if (ret) > > > > - goto unregister; > > > > + if (ret) { > > > > + pr_warn("failed to enable patch '%s'\n", > > > > + patch->mod->name); > > > > + > > > > + klp_unpatch_objects(patch); > > > > > > We should call here synchronize_rcu() here as we do > > > in klp_try_complete_transition(). Some of the affected > > > functions might have more versions on the stack and we > > > need to make sure that klp_ftrace_handler() will _not_ > > > see the removed patch on the stack. > > > > Even if the handler sees the new func on the stack, the > > task->patch_state is still KLP_UNPATCHED, so it will still choose the > > previous version of the function. Or did I miss your point? > > The barrier is needed from exactly the same reason as the one > in klp_try_complete_transition() > > CPU0 CPU1 > > __klp_enable_patch() > klp_init_transition() > > for_each... > task->patch_state = KLP_UNPATCHED > > for_each... > func->transition = true > > klp_for_each_object() > klp_patch_object() > list_add_rcu() > > klp_ftrace_handler() > func = list_first_...() > > if (func->transition) > > > ret = klp_patch_object() > /* error */ > if (ret) { > klp_unpatch_objects() > > list_remove_rcu() > > klp_complete_transition() > > for_each_.... > func->transition = true > > for_each_.... > task->patch_state = PATCH_UNDEFINED > > patch_state = current->patch_state; > WARN_ON_ONCE(patch_state > == > KLP_UNDEFINED); > > BANG: The warning is triggered. > > => we need to call rcu_synchronize(). It will make sure that > no ftrace handled will see the removed func on the stack > and we could clear all the other values. Makes sense. Notice in this case that klp_target_state is KLP_PATCHED. Which means that klp_complete_transition() would not call synchronize_rcu() at the right time, nor would it call module_put(). It can be fixed with: @@ -387,7 +389,7 @@ static int __klp_enable_patch(struct klp_patch *patch) pr_warn("failed to enable patch '%s'\n", patch->mod->name); - klp_unpatch_objects(patch); + klp_target_state = KLP_UNPATCHED; klp_complete_transition(); return ret; This assumes that the 'if (klp_target_state == KLP_UNPATCHED)' clause in klp_try_complete_transition() gets moved to klp_complete_transition() as you suggested. > > > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c > > > > index 5efa262..1a77f05 100644 > > > > --- a/kernel/livepatch/patch.c > > > > +++ b/kernel/livepatch/patch.c > > > > @@ -29,6 +29,7 @@ > > > > #include <linux/bug.h> > > > > #include <linux/printk.h> > > > > #include "patch.h" > > > > +#include "transition.h" > > > > > > > > static LIST_HEAD(klp_ops); > > > > > > > > @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip, > > > > { > > > > struct klp_ops *ops; > > > > struct klp_func *func; > > > > + int patch_state; > > > > > > > > ops = container_of(fops, struct klp_ops, fops); > > > > > > > > rcu_read_lock(); > > > > + > > > > func = list_first_or_null_rcu(&ops->func_stack, struct klp_func, > > > > stack_node); > > > > + > > > > + /* > > > > + * func should never be NULL because preemption should be disabled here > > > > + * and unregister_ftrace_function() does the equivalent of a > > > > + * synchronize_sched() before the func_stack removal. > > > > + */ > > > > + if (WARN_ON_ONCE(!func)) > > > > + goto unlock; > > > > + > > > > + /* > > > > + * Enforce the order of the ops->func_stack and func->transition reads. > > > > + * The corresponding write barrier is in __klp_enable_patch(). > > > > + */ > > > > + smp_rmb(); > > > > > > I was curious why the comment did not mention __klp_disable_patch(). > > > It was related to the hours of thinking. I would like to avoid this > > > in the future and add a comment like. > > > > > > * This barrier probably is not needed when the patch is being > > > * disabled. The patch is removed from the stack in > > > * klp_try_complete_transition() and there we need to call > > > * rcu_synchronize() to prevent seeing the patch on the stack > > > * at all. > > > * > > > * Well, it still might be needed to see func->transition > > > * when the patch is removed and the task is migrated. See > > > * the write barrier in __klp_disable_patch(). > > > > Agreed, though as you mentioned earlier, there's also the implicit > > barrier in klp_update_patch_state(), which would execute first in such a > > scenario. So I think I'll update the barrier comments in > > klp_update_patch_state(). > > You inspired me to a scenario with 3 CPUs: > > CPU0 CPU1 CPU2 > > __klp_disable_patch() > > klp_init_transition() > > func->transition = true > > smp_wmb() > > klp_start_transition() > > set TIF_PATCH_PATCHPENDING > > klp_update_patch_state() > > task->patch_state > = KLP_UNPATCHED > > smp_mb() > > klp_ftrace_handler() > func = list_... > > smp_rmb() /*needed?*/ > > if (func->transition) > I think this isn't possible. Remember the comment I added to klp_update_patch_state(): * NOTE: If task is not 'current', the caller must ensure the task is inactive. * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value. Right now klp_update_patch_state() is only called for current. klp_ftrace_handler() on CPU2 would be running in the context of a different task. > We need to make sure the CPU3 sees func->transition set. Otherwise, > it would wrongly use the function from the patch. > > So, the description might be: > > * Enforce the order of the ops->func_stack and > * func->transition reads when the patch is enabled. > * The corresponding write barrier is in __klp_enable_patch(). > * > * Also make sure that func->transition is visible before > * TIF_PATCH_PENDING_FLAG is set and the task might get > * migrated to KLP_UNPATCHED state. The corresponding > * write barrier is in __klp_disable_patch(). > > > By other words, the read barrier here is needed from the same > reason as the write barrier in __klp_disable_patch(). > > > > +void klp_reverse_transition(void) > > > > +{ > > > > + unsigned int cpu; > > > > + struct task_struct *g, *task; > > > > + > > > > + klp_transition_patch->enabled = !klp_transition_patch->enabled; > > > > + > > > > + klp_target_state = !klp_target_state; > > > > + > > > > + /* > > > > + * Clear all TIF_PATCH_PENDING flags to prevent races caused by > > > > + * klp_update_patch_state() running in parallel with > > > > + * klp_start_transition(). > > > > + */ > > > > + read_lock(&tasklist_lock); > > > > + for_each_process_thread(g, task) > > > > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > > > > + read_unlock(&tasklist_lock); > > > > + > > > > + for_each_possible_cpu(cpu) > > > > + clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING); > > > > + > > > > + /* Let any remaining calls to klp_update_patch_state() complete */ > > > > + synchronize_rcu(); > > > > + > > > > + klp_start_transition(); > > > > > > Hmm, we should not call klp_try_complete_transition() when > > > klp_start_transition() is called from here. I can't find a safe > > > way to cancel klp_transition_work() when we own klp_mutex. > > > It smells with a possible deadlock. > > > > > > I suggest to move move klp_try_complete_transition() outside > > > klp_start_transition() and explicitely call it from > > > __klp_disable_patch() and __klp_enabled_patch(). > > > This would fix also the problem with immediate patches, see > > > klp_start_transition(). > > > > Agreed. I'll fix it as you suggest and I'll put the mod_delayed_work() > > call in klp_reverse_transition() again. > > There is one small catch. The mod_delayed_work() might cause that two > works might be scheduled: > > + one already running that is waiting for the klp_mutex > + another one scheduled by that mod_delayed_work() > > A solution would be to cancel the work from klp_transition_work_fn() > if the transition succeeds. > > Another possibility would be to do nothing here. The work is > scheduled very often anyway. Yes, I think I'll do this, for the sake of simplicity.
> > And finally, the section "Limitations" has this text under the first > > bullet: > > > > + The patch must not change the semantic of the patched functions. > > > > The current implementation guarantees only that either the old > > or the new function is called. The functions are patched one > > by one. It means that the patch must _not_ change the semantic > > of the function. > > > > I think it is confusing. The consistency model allows us to change the > > semantic of a function. To certain degree. Of course, there are cases that > > cannot be patched, or have to be patched carefully. For example if a > > function takes a lock by calling foo_lock(), foo_lock() is not on a stack > > afterwards. Then the locking semantics may be changed with a livepatch. > > One has to make sure to patch also the caller foo_lock() to enforce the > > consistency. And so on... But I do not consider a limitation of livepatch. > > It is a feature of the consistency model, which is weaker than kGraft's or > > kpatch's (or stronger. It depends on your point of view.) > > > > So, I propose to remove this text and better describe the properties of > > the consistency model above in the section 3. Maybe a quote from an old > > mail thread (Nov 2014) would be sufficient. I don't remember what was > > mentioned and what not. > > > > What do you think? > > I'll remove the above limitation. > > I'm not sure how to improve the consistency model section. It already > has at least some mentions of changed function semantics and locking > semantics. I'll leave it alone for now, unless you have a specific > suggestion. Fair enough. Let's see if I can come up with something. > > > diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h > > > index 6602b34..ed90ad1 100644 > > > --- a/include/linux/livepatch.h > > > +++ b/include/linux/livepatch.h > > > @@ -68,7 +92,7 @@ struct klp_func { > > > * @funcs: function entries for functions to be patched in the object > > > * @kobj: kobject for sysfs resources > > > * @mod: kernel module associated with the patched object > > > - * (NULL for vmlinux) > > > + * (NULL for vmlinux) > > > > This looks superfluous. > > This is a minor whitespace fix -- remove a space before tab. I figured > I'd go ahead and fix it since I'm already changing some of the > surrounding code. Ok, no problem. Thanks, Miroslav
On Mon 2017-02-06 13:51:48, Josh Poimboeuf wrote: > On Mon, Feb 06, 2017 at 05:44:31PM +0100, Petr Mladek wrote: > > > > > @@ -347,22 +354,37 @@ static int __klp_enable_patch(struct klp_patch *patch) > > > > > > > > > > pr_notice("enabling patch '%s'\n", patch->mod->name); > > > > > > > > > > + klp_init_transition(patch, KLP_PATCHED); > > > > > + > > > > > + /* > > > > > + * Enforce the order of the func->transition writes in > > > > > + * klp_init_transition() and the ops->func_stack writes in > > > > > + * klp_patch_object(), so that klp_ftrace_handler() will see the > > > > > + * func->transition updates before the handler is registered and the > > > > > + * new funcs become visible to the handler. > > > > > + */ > > > > > + smp_wmb(); > > > > > + > > > > > klp_for_each_object(patch, obj) { > > > > > if (!klp_is_object_loaded(obj)) > > > > > continue; > > > > > > > > > > ret = klp_patch_object(obj); > > > > > - if (ret) > > > > > - goto unregister; > > > > > + if (ret) { > > > > > + pr_warn("failed to enable patch '%s'\n", > > > > > + patch->mod->name); > > > > > + > > > > > + klp_unpatch_objects(patch); > > > > > > > > We should call here synchronize_rcu() here as we do > > > > in klp_try_complete_transition(). Some of the affected > > > > functions might have more versions on the stack and we > > > > need to make sure that klp_ftrace_handler() will _not_ > > > > see the removed patch on the stack. > > > > > > Even if the handler sees the new func on the stack, the > > > task->patch_state is still KLP_UNPATCHED, so it will still choose the > > > previous version of the function. Or did I miss your point? > > > > The barrier is needed from exactly the same reason as the one > > in klp_try_complete_transition() > > > > CPU0 CPU1 > > > > __klp_enable_patch() > > klp_init_transition() > > > > for_each... > > task->patch_state = KLP_UNPATCHED > > > > for_each... > > func->transition = true > > > > klp_for_each_object() > > klp_patch_object() > > list_add_rcu() > > > > klp_ftrace_handler() > > func = list_first_...() > > > > if (func->transition) > > > > > > ret = klp_patch_object() > > /* error */ > > if (ret) { > > klp_unpatch_objects() > > > > list_remove_rcu() > > > > klp_complete_transition() > > > > for_each_.... > > func->transition = true > > > > for_each_.... > > task->patch_state = PATCH_UNDEFINED > > > > patch_state = current->patch_state; > > WARN_ON_ONCE(patch_state > > == > > KLP_UNDEFINED); > > > > BANG: The warning is triggered. > > > > => we need to call rcu_synchronize(). It will make sure that > > no ftrace handled will see the removed func on the stack > > and we could clear all the other values. > > Makes sense. > > Notice in this case that klp_target_state is KLP_PATCHED. Which means > that klp_complete_transition() would not call synchronize_rcu() at the > right time, nor would it call module_put(). It can be fixed with: > > @@ -387,7 +389,7 @@ static int __klp_enable_patch(struct klp_patch *patch) > pr_warn("failed to enable patch '%s'\n", > patch->mod->name); > > - klp_unpatch_objects(patch); > + klp_target_state = KLP_UNPATCHED; > klp_complete_transition(); > > return ret; Great catch! Looks good to me. > This assumes that the 'if (klp_target_state == KLP_UNPATCHED)' clause in > klp_try_complete_transition() gets moved to klp_complete_transition() as > you suggested. > > > > > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c > > > > > index 5efa262..1a77f05 100644 > > > > > --- a/kernel/livepatch/patch.c > > > > > +++ b/kernel/livepatch/patch.c > > > > > @@ -29,6 +29,7 @@ > > > > > #include <linux/bug.h> > > > > > #include <linux/printk.h> > > > > > #include "patch.h" > > > > > +#include "transition.h" > > > > > > > > > > static LIST_HEAD(klp_ops); > > > > > > > > > > @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip, > > > > > { > > > > > struct klp_ops *ops; > > > > > struct klp_func *func; > > > > > + int patch_state; > > > > > > > > > > ops = container_of(fops, struct klp_ops, fops); > > > > > > > > > > rcu_read_lock(); > > > > > + > > > > > func = list_first_or_null_rcu(&ops->func_stack, struct klp_func, > > > > > stack_node); > > > > > + > > > > > + /* > > > > > + * func should never be NULL because preemption should be disabled here > > > > > + * and unregister_ftrace_function() does the equivalent of a > > > > > + * synchronize_sched() before the func_stack removal. > > > > > + */ > > > > > + if (WARN_ON_ONCE(!func)) > > > > > + goto unlock; > > > > > + > > > > > + /* > > > > > + * Enforce the order of the ops->func_stack and func->transition reads. > > > > > + * The corresponding write barrier is in __klp_enable_patch(). > > > > > + */ > > > > > + smp_rmb(); > > > > > > > > I was curious why the comment did not mention __klp_disable_patch(). > > > > It was related to the hours of thinking. I would like to avoid this > > > > in the future and add a comment like. > > > > > > > > * This barrier probably is not needed when the patch is being > > > > * disabled. The patch is removed from the stack in > > > > * klp_try_complete_transition() and there we need to call > > > > * rcu_synchronize() to prevent seeing the patch on the stack > > > > * at all. > > > > * > > > > * Well, it still might be needed to see func->transition > > > > * when the patch is removed and the task is migrated. See > > > > * the write barrier in __klp_disable_patch(). > > > > > > Agreed, though as you mentioned earlier, there's also the implicit > > > barrier in klp_update_patch_state(), which would execute first in such a > > > scenario. So I think I'll update the barrier comments in > > > klp_update_patch_state(). > > > > You inspired me to a scenario with 3 CPUs: > > > > CPU0 CPU1 CPU2 > > > > __klp_disable_patch() > > > > klp_init_transition() > > > > func->transition = true > > > > smp_wmb() > > > > klp_start_transition() > > > > set TIF_PATCH_PATCHPENDING > > > > klp_update_patch_state() > > > > task->patch_state > > = KLP_UNPATCHED > > > > smp_mb() > > > > klp_ftrace_handler() > > func = list_... > > > > smp_rmb() /*needed?*/ > > > > if (func->transition) > > > > I think this isn't possible. Remember the comment I added to > klp_update_patch_state(): > > * NOTE: If task is not 'current', the caller must ensure the task is inactive. > * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value. > > Right now klp_update_patch_state() is only called for current. > klp_ftrace_handler() on CPU2 would be running in the context of a > different task. I agree that it is impossible with the current code. In fact, I cannot imagine a way to migrate the task where the barrier would be needed. The question if we could/should somehow document it. Something like * The barrier is not really needed when the patch is being * disabled. The value of func->transition would change * the result of this handled only after the task is migrated. * But the conditions for the migration are very limited * and practically include a full barrier, see * klp_update_patch_state(). > > We need to make sure the CPU3 sees func->transition set. Otherwise, > > it would wrongly use the function from the patch. > > > > So, the description might be: > > > > * Enforce the order of the ops->func_stack and > > * func->transition reads when the patch is enabled. > > * The corresponding write barrier is in __klp_enable_patch(). > > * > > * Also make sure that func->transition is visible before > > * TIF_PATCH_PENDING_FLAG is set and the task might get > > * migrated to KLP_UNPATCHED state. The corresponding > > * write barrier is in __klp_disable_patch(). > > > > > > By other words, the read barrier here is needed from the same > > reason as the write barrier in __klp_disable_patch(). > > > > > +void klp_reverse_transition(void) > > > > > +{ > > > > > + unsigned int cpu; > > > > > + struct task_struct *g, *task; > > > > > + > > > > > + klp_transition_patch->enabled = !klp_transition_patch->enabled; > > > > > + > > > > > + klp_target_state = !klp_target_state; > > > > > + > > > > > + /* > > > > > + * Clear all TIF_PATCH_PENDING flags to prevent races caused by > > > > > + * klp_update_patch_state() running in parallel with > > > > > + * klp_start_transition(). > > > > > + */ > > > > > + read_lock(&tasklist_lock); > > > > > + for_each_process_thread(g, task) > > > > > + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); > > > > > + read_unlock(&tasklist_lock); > > > > > + > > > > > + for_each_possible_cpu(cpu) > > > > > + clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING); > > > > > + > > > > > + /* Let any remaining calls to klp_update_patch_state() complete */ > > > > > + synchronize_rcu(); > > > > > + > > > > > + klp_start_transition(); > > > > > > > > Hmm, we should not call klp_try_complete_transition() when > > > > klp_start_transition() is called from here. I can't find a safe > > > > way to cancel klp_transition_work() when we own klp_mutex. > > > > It smells with a possible deadlock. > > > > > > > > I suggest to move move klp_try_complete_transition() outside > > > > klp_start_transition() and explicitely call it from > > > > __klp_disable_patch() and __klp_enabled_patch(). > > > > This would fix also the problem with immediate patches, see > > > > klp_start_transition(). > > > > > > Agreed. I'll fix it as you suggest and I'll put the mod_delayed_work() > > > call in klp_reverse_transition() again. > > > > There is one small catch. The mod_delayed_work() might cause that two > > works might be scheduled: > > > > + one already running that is waiting for the klp_mutex > > + another one scheduled by that mod_delayed_work() > > > > A solution would be to cancel the work from klp_transition_work_fn() > > if the transition succeeds. > > > > Another possibility would be to do nothing here. The work is > > scheduled very often anyway. > > Yes, I think I'll do this, for the sake of simplicity. Sounds good to me. I am sorry for the late reply. I am ill and work only limited time. Best Regards, Petr
On Wed, Feb 08, 2017 at 04:47:50PM +0100, Petr Mladek wrote: > > Notice in this case that klp_target_state is KLP_PATCHED. Which means > > that klp_complete_transition() would not call synchronize_rcu() at the > > right time, nor would it call module_put(). It can be fixed with: > > > > @@ -387,7 +389,7 @@ static int __klp_enable_patch(struct klp_patch *patch) > > pr_warn("failed to enable patch '%s'\n", > > patch->mod->name); > > > > - klp_unpatch_objects(patch); > > + klp_target_state = KLP_UNPATCHED; > > klp_complete_transition(); > > > > return ret; > > Great catch! Looks good to me. As it turns out, klp_target_state isn't accessible from this file, so I'll make a klp_cancel_transition() helper function which reverses klp_target_state and calls klp_complete_transition(). > > This assumes that the 'if (klp_target_state == KLP_UNPATCHED)' clause in > > klp_try_complete_transition() gets moved to klp_complete_transition() as > > you suggested. > > > > > > > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c > > > > > > index 5efa262..1a77f05 100644 > > > > > > --- a/kernel/livepatch/patch.c > > > > > > +++ b/kernel/livepatch/patch.c > > > > > > @@ -29,6 +29,7 @@ > > > > > > #include <linux/bug.h> > > > > > > #include <linux/printk.h> > > > > > > #include "patch.h" > > > > > > +#include "transition.h" > > > > > > > > > > > > static LIST_HEAD(klp_ops); > > > > > > > > > > > > @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip, > > > > > > { > > > > > > struct klp_ops *ops; > > > > > > struct klp_func *func; > > > > > > + int patch_state; > > > > > > > > > > > > ops = container_of(fops, struct klp_ops, fops); > > > > > > > > > > > > rcu_read_lock(); > > > > > > + > > > > > > func = list_first_or_null_rcu(&ops->func_stack, struct klp_func, > > > > > > stack_node); > > > > > > + > > > > > > + /* > > > > > > + * func should never be NULL because preemption should be disabled here > > > > > > + * and unregister_ftrace_function() does the equivalent of a > > > > > > + * synchronize_sched() before the func_stack removal. > > > > > > + */ > > > > > > + if (WARN_ON_ONCE(!func)) > > > > > > + goto unlock; > > > > > > + > > > > > > + /* > > > > > > + * Enforce the order of the ops->func_stack and func->transition reads. > > > > > > + * The corresponding write barrier is in __klp_enable_patch(). > > > > > > + */ > > > > > > + smp_rmb(); > > > > > > > > > > I was curious why the comment did not mention __klp_disable_patch(). > > > > > It was related to the hours of thinking. I would like to avoid this > > > > > in the future and add a comment like. > > > > > > > > > > * This barrier probably is not needed when the patch is being > > > > > * disabled. The patch is removed from the stack in > > > > > * klp_try_complete_transition() and there we need to call > > > > > * rcu_synchronize() to prevent seeing the patch on the stack > > > > > * at all. > > > > > * > > > > > * Well, it still might be needed to see func->transition > > > > > * when the patch is removed and the task is migrated. See > > > > > * the write barrier in __klp_disable_patch(). > > > > > > > > Agreed, though as you mentioned earlier, there's also the implicit > > > > barrier in klp_update_patch_state(), which would execute first in such a > > > > scenario. So I think I'll update the barrier comments in > > > > klp_update_patch_state(). > > > > > > You inspired me to a scenario with 3 CPUs: > > > > > > CPU0 CPU1 CPU2 > > > > > > __klp_disable_patch() > > > > > > klp_init_transition() > > > > > > func->transition = true > > > > > > smp_wmb() > > > > > > klp_start_transition() > > > > > > set TIF_PATCH_PATCHPENDING > > > > > > klp_update_patch_state() > > > > > > task->patch_state > > > = KLP_UNPATCHED > > > > > > smp_mb() > > > > > > klp_ftrace_handler() > > > func = list_... > > > > > > smp_rmb() /*needed?*/ > > > > > > if (func->transition) > > > > > > > I think this isn't possible. Remember the comment I added to > > klp_update_patch_state(): > > > > * NOTE: If task is not 'current', the caller must ensure the task is inactive. > > * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value. > > > > Right now klp_update_patch_state() is only called for current. > > klp_ftrace_handler() on CPU2 would be running in the context of a > > different task. > > I agree that it is impossible with the current code. In fact, I cannot > imagine a way to migrate the task where the barrier would be needed. > The question if we could/should somehow document it. Something like > > * The barrier is not really needed when the patch is being > * disabled. The value of func->transition would change > * the result of this handled only after the task is migrated. > * But the conditions for the migration are very limited > * and practically include a full barrier, see > * klp_update_patch_state(). Well, I'd like to avoid over-commenting this issue. For v5 I've added the following comment to klp_update_patch_state() -- see #2: /* * This test_and_clear_tsk_thread_flag() call also serves as a read * barrier (smp_rmb) for two cases: * * 1) Enforce the order of the TIF_PATCH_PENDING read and the * klp_target_state read. The corresponding write barrier is in * klp_init_transition(). * * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read * of func->transition, if klp_ftrace_handler() is called later on * the same CPU. See __klp_disable_patch(). */ Is that sufficient? If not, I could maybe add another related comment in klp_ftrace_handler() which refers to this comment.
On Wed 2017-02-08 10:46:36, Josh Poimboeuf wrote: > On Wed, Feb 08, 2017 at 04:47:50PM +0100, Petr Mladek wrote: > > > Notice in this case that klp_target_state is KLP_PATCHED. Which means > > > that klp_complete_transition() would not call synchronize_rcu() at the > > > right time, nor would it call module_put(). It can be fixed with: > > > > > > @@ -387,7 +389,7 @@ static int __klp_enable_patch(struct klp_patch *patch) > > > pr_warn("failed to enable patch '%s'\n", > > > patch->mod->name); > > > > > > - klp_unpatch_objects(patch); > > > + klp_target_state = KLP_UNPATCHED; > > > klp_complete_transition(); > > > > > > return ret; > > > > Great catch! Looks good to me. > > As it turns out, klp_target_state isn't accessible from this file, so > I'll make a klp_cancel_transition() helper function which reverses > klp_target_state and calls klp_complete_transition(). Sound good to me. > > > This assumes that the 'if (klp_target_state == KLP_UNPATCHED)' clause in > > > klp_try_complete_transition() gets moved to klp_complete_transition() as > > > you suggested. > > > > > > > > > > diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c > > > > > > > index 5efa262..1a77f05 100644 > > > > > > > --- a/kernel/livepatch/patch.c > > > > > > > +++ b/kernel/livepatch/patch.c > > > > > > > @@ -29,6 +29,7 @@ > > > > > > > #include <linux/bug.h> > > > > > > > #include <linux/printk.h> > > > > > > > #include "patch.h" > > > > > > > +#include "transition.h" > > > > > > > > > > > > > > static LIST_HEAD(klp_ops); > > > > > > > > > > > > > > @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip, > > > > > > > { > > > > > > > struct klp_ops *ops; > > > > > > > struct klp_func *func; > > > > > > > + int patch_state; > > > > > > > > > > > > > > ops = container_of(fops, struct klp_ops, fops); > > > > > > > > > > > > > > rcu_read_lock(); > > > > > > > + > > > > > > > func = list_first_or_null_rcu(&ops->func_stack, struct klp_func, > > > > > > > stack_node); > > > > > > > + > > > > > > > + /* > > > > > > > + * func should never be NULL because preemption should be disabled here > > > > > > > + * and unregister_ftrace_function() does the equivalent of a > > > > > > > + * synchronize_sched() before the func_stack removal. > > > > > > > + */ > > > > > > > + if (WARN_ON_ONCE(!func)) > > > > > > > + goto unlock; > > > > > > > + > > > > > > > + /* > > > > > > > + * Enforce the order of the ops->func_stack and func->transition reads. > > > > > > > + * The corresponding write barrier is in __klp_enable_patch(). > > > > > > > + */ > > > > > > > + smp_rmb(); > > > > > > > > > > > > I was curious why the comment did not mention __klp_disable_patch(). > > > > > > It was related to the hours of thinking. I would like to avoid this > > > > > > in the future and add a comment like. > > > > > > > > > > > > * This barrier probably is not needed when the patch is being > > > > > > * disabled. The patch is removed from the stack in > > > > > > * klp_try_complete_transition() and there we need to call > > > > > > * rcu_synchronize() to prevent seeing the patch on the stack > > > > > > * at all. > > > > > > * > > > > > > * Well, it still might be needed to see func->transition > > > > > > * when the patch is removed and the task is migrated. See > > > > > > * the write barrier in __klp_disable_patch(). > > > > > > > > > > Agreed, though as you mentioned earlier, there's also the implicit > > > > > barrier in klp_update_patch_state(), which would execute first in such a > > > > > scenario. So I think I'll update the barrier comments in > > > > > klp_update_patch_state(). > > > > > > > > You inspired me to a scenario with 3 CPUs: > > > > > > > > CPU0 CPU1 CPU2 > > > > > > > > __klp_disable_patch() > > > > > > > > klp_init_transition() > > > > > > > > func->transition = true > > > > > > > > smp_wmb() > > > > > > > > klp_start_transition() > > > > > > > > set TIF_PATCH_PATCHPENDING > > > > > > > > klp_update_patch_state() > > > > > > > > task->patch_state > > > > = KLP_UNPATCHED > > > > > > > > smp_mb() > > > > > > > > klp_ftrace_handler() > > > > func = list_... > > > > > > > > smp_rmb() /*needed?*/ > > > > > > > > if (func->transition) > > > > > > > > > > I think this isn't possible. Remember the comment I added to > > > klp_update_patch_state(): > > > > > > * NOTE: If task is not 'current', the caller must ensure the task is inactive. > > > * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value. > > > > > > Right now klp_update_patch_state() is only called for current. > > > klp_ftrace_handler() on CPU2 would be running in the context of a > > > different task. > > > > I agree that it is impossible with the current code. In fact, I cannot > > imagine a way to migrate the task where the barrier would be needed. > > The question if we could/should somehow document it. Something like > > > > * The barrier is not really needed when the patch is being > > * disabled. The value of func->transition would change > > * the result of this handled only after the task is migrated. > > * But the conditions for the migration are very limited > > * and practically include a full barrier, see > > * klp_update_patch_state(). > > Well, I'd like to avoid over-commenting this issue. For v5 I've added > the following comment to klp_update_patch_state() -- see #2: > > /* > * This test_and_clear_tsk_thread_flag() call also serves as a read > * barrier (smp_rmb) for two cases: > * > * 1) Enforce the order of the TIF_PATCH_PENDING read and the > * klp_target_state read. The corresponding write barrier is in > * klp_init_transition(). > * > * 2) Enforce the order of the TIF_PATCH_PENDING read and a future read > * of func->transition, if klp_ftrace_handler() is called later on > * the same CPU. See __klp_disable_patch(). > */ Sounds good. > Is that sufficient? If not, I could maybe add another related comment > in klp_ftrace_handler() which refers to this comment. It would be nice. I do not want over-commenting either. On the other hand, the code is really complex and it is not easy to understand all the relations. The comments should safe us a lot of pain in the long term. Thanks a lot for working on it Best Regards, Petr
diff --git a/Documentation/ABI/testing/sysfs-kernel-livepatch b/Documentation/ABI/testing/sysfs-kernel-livepatch index da87f43..24b6570 100644 --- a/Documentation/ABI/testing/sysfs-kernel-livepatch +++ b/Documentation/ABI/testing/sysfs-kernel-livepatch @@ -25,6 +25,14 @@ Description: code is currently applied. Writing 0 will disable the patch while writing 1 will re-enable the patch. +What: /sys/kernel/livepatch/<patch>/transition +Date: May 2016 +KernelVersion: 4.11.0 +Contact: live-patching@vger.kernel.org +Description: + An attribute which indicates whether the patch is currently in + transition. + What: /sys/kernel/livepatch/<patch>/<object> Date: Nov 2014 KernelVersion: 3.19.0 diff --git a/Documentation/livepatch/livepatch.txt b/Documentation/livepatch/livepatch.txt index 7f04e13..fb00d66 100644 --- a/Documentation/livepatch/livepatch.txt +++ b/Documentation/livepatch/livepatch.txt @@ -72,7 +72,8 @@ example, they add a NULL pointer or a boundary check, fix a race by adding a missing memory barrier, or add some locking around a critical section. Most of these changes are self contained and the function presents itself the same way to the rest of the system. In this case, the functions might -be updated independently one by one. +be updated independently one by one. (This can be done by setting the +'immediate' flag in the klp_patch struct.) But there are more complex fixes. For example, a patch might change ordering of locking in multiple functions at the same time. Or a patch @@ -86,20 +87,127 @@ or no data are stored in the modified structures at the moment. The theory about how to apply functions a safe way is rather complex. The aim is to define a so-called consistency model. It attempts to define conditions when the new implementation could be used so that the system -stays consistent. The theory is not yet finished. See the discussion at -https://lkml.kernel.org/r/20141107140458.GA21774@suse.cz - -The current consistency model is very simple. It guarantees that either -the old or the new function is called. But various functions get redirected -one by one without any synchronization. - -In other words, the current implementation _never_ modifies the behavior -in the middle of the call. It is because it does _not_ rewrite the entire -function in the memory. Instead, the function gets redirected at the -very beginning. But this redirection is used immediately even when -some other functions from the same patch have not been redirected yet. - -See also the section "Limitations" below. +stays consistent. + +Livepatch has a consistency model which is a hybrid of kGraft and +kpatch: it uses kGraft's per-task consistency and syscall barrier +switching combined with kpatch's stack trace switching. There are also +a number of fallback options which make it quite flexible. + +Patches are applied on a per-task basis, when the task is deemed safe to +switch over. When a patch is enabled, livepatch enters into a +transition state where tasks are converging to the patched state. +Usually this transition state can complete in a few seconds. The same +sequence occurs when a patch is disabled, except the tasks converge from +the patched state to the unpatched state. + +An interrupt handler inherits the patched state of the task it +interrupts. The same is true for forked tasks: the child inherits the +patched state of the parent. + +Livepatch uses several complementary approaches to determine when it's +safe to patch tasks: + +1. The first and most effective approach is stack checking of sleeping + tasks. If no affected functions are on the stack of a given task, + the task is patched. In most cases this will patch most or all of + the tasks on the first try. Otherwise it'll keep trying + periodically. This option is only available if the architecture has + reliable stacks (HAVE_RELIABLE_STACKTRACE). + +2. The second approach, if needed, is kernel exit switching. A + task is switched when it returns to user space from a system call, a + user space IRQ, or a signal. It's useful in the following cases: + + a) Patching I/O-bound user tasks which are sleeping on an affected + function. In this case you have to send SIGSTOP and SIGCONT to + force it to exit the kernel and be patched. + b) Patching CPU-bound user tasks. If the task is highly CPU-bound + then it will get patched the next time it gets interrupted by an + IRQ. + c) In the future it could be useful for applying patches for + architectures which don't yet have HAVE_RELIABLE_STACKTRACE. In + this case you would have to signal most of the tasks on the + system. However this isn't supported yet because there's + currently no way to patch kthreads without + HAVE_RELIABLE_STACKTRACE. + +3. For idle "swapper" tasks, since they don't ever exit the kernel, they + instead have a klp_update_patch_state() call in the idle loop which + allows them to be patched before the CPU enters the idle state. + + (Note there's not yet such an approach for kthreads.) + +All the above approaches may be skipped by setting the 'immediate' flag +in the 'klp_patch' struct, which will disable per-task consistency and +patch all tasks immediately. This can be useful if the patch doesn't +change any function or data semantics. Note that, even with this flag +set, it's possible that some tasks may still be running with an old +version of the function, until that function returns. + +There's also an 'immediate' flag in the 'klp_func' struct which allows +you to specify that certain functions in the patch can be applied +without per-task consistency. This might be useful if you want to patch +a common function like schedule(), and the function change doesn't need +consistency but the rest of the patch does. + +For architectures which don't have HAVE_RELIABLE_STACKTRACE, the user +must set patch->immediate which causes all tasks to be patched +immediately. This option should be used with care, only when the patch +doesn't change any function or data semantics. + +In the future, architectures which don't have HAVE_RELIABLE_STACKTRACE +may be allowed to use per-task consistency if we can come up with +another way to patch kthreads. + +The /sys/kernel/livepatch/<patch>/transition file shows whether a patch +is in transition. Only a single patch (the topmost patch on the stack) +can be in transition at a given time. A patch can remain in transition +indefinitely, if any of the tasks are stuck in the initial patch state. + +A transition can be reversed and effectively canceled by writing the +opposite value to the /sys/kernel/livepatch/<patch>/enabled file while +the transition is in progress. Then all the tasks will attempt to +converge back to the original patch state. + +There's also a /proc/<pid>/patch_state file which can be used to +determine which tasks are blocking completion of a patching operation. +If a patch is in transition, this file shows 0 to indicate the task is +unpatched and 1 to indicate it's patched. Otherwise, if no patch is in +transition, it shows -1. Any tasks which are blocking the transition +can be signaled with SIGSTOP and SIGCONT to force them to change their +patched state. + + +3.1 Adding consistency model support to new architectures +--------------------------------------------------------- + +For adding consistency model support to new architectures, there are a +few options: + +1) Add CONFIG_HAVE_RELIABLE_STACKTRACE. This means porting objtool, and + for non-DWARF unwinders, also making sure there's a way for the stack + tracing code to detect interrupts on the stack. + +2) Alternatively, figure out a way to patch kthreads without stack + checking. If all kthreads sleep in the same place, then we can + designate that place as a patching point. I think Petr M has been + working on that? In that case, arches without + HAVE_RELIABLE_STACKTRACE would still be able to use the + non-stack-checking parts of the consistency model: + + a) patching user tasks when they cross the kernel/user space + boundary; and + + b) patching kthreads and idle tasks at their designated patch points. + + This option isn't as good as option 1 because it requires signaling + most of the tasks to patch them. But it could still be a good backup + option for those architectures which don't have reliable stack traces + yet. + +In the meantime, patches for such architectures can bypass the +consistency model by setting klp_patch.immediate to true. 4. Livepatch module @@ -134,7 +242,7 @@ Documentation/livepatch/module-elf-format.txt for more details. 4.2. Metadata ------------- +------------- The patch is described by several structures that split the information into three levels: @@ -239,9 +347,15 @@ Registered patches might be enabled either by calling klp_enable_patch() or by writing '1' to /sys/kernel/livepatch/<name>/enabled. The system will start using the new implementation of the patched functions at this stage. -In particular, if an original function is patched for the first time, a -function specific struct klp_ops is created and an universal ftrace handler -is registered. +When a patch is enabled, livepatch enters into a transition state where +tasks are converging to the patched state. This is indicated by a value +of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks have +been patched, the 'transition' value changes to '0'. For more +information about this process, see the "Consistency model" section. + +If an original function is patched for the first time, a function +specific struct klp_ops is created and an universal ftrace handler is +registered. Functions might be patched multiple times. The ftrace handler is registered only once for the given function. Further patches just add an entry to the @@ -261,6 +375,12 @@ by writing '0' to /sys/kernel/livepatch/<name>/enabled. At this stage either the code from the previously enabled patch or even the original code gets used. +When a patch is disabled, livepatch enters into a transition state where +tasks are converging to the unpatched state. This is indicated by a +value of '1' in /sys/kernel/livepatch/<name>/transition. Once all tasks +have been unpatched, the 'transition' value changes to '0'. For more +information about this process, see the "Consistency model" section. + Here all the functions (struct klp_func) associated with the to-be-disabled patch are removed from the corresponding struct klp_ops. The ftrace handler is unregistered and the struct klp_ops is freed when the func_stack list diff --git a/include/linux/init_task.h b/include/linux/init_task.h index 325f649..25f0360 100644 --- a/include/linux/init_task.h +++ b/include/linux/init_task.h @@ -14,6 +14,7 @@ #include <linux/rbtree.h> #include <net/net_namespace.h> #include <linux/sched/rt.h> +#include <linux/livepatch.h> #include <asm/thread_info.h> @@ -185,6 +186,13 @@ extern struct task_group root_task_group; # define INIT_KASAN(tsk) #endif +#ifdef CONFIG_LIVEPATCH +# define INIT_LIVEPATCH(tsk) \ + .patch_state = KLP_UNDEFINED, +#else +# define INIT_LIVEPATCH(tsk) +#endif + #ifdef CONFIG_THREAD_INFO_IN_TASK # define INIT_TASK_TI(tsk) \ .thread_info = INIT_THREAD_INFO(tsk), \ @@ -271,6 +279,7 @@ extern struct task_group root_task_group; INIT_VTIME(tsk) \ INIT_NUMA_BALANCING(tsk) \ INIT_KASAN(tsk) \ + INIT_LIVEPATCH(tsk) \ } diff --git a/include/linux/livepatch.h b/include/linux/livepatch.h index 6602b34..ed90ad1 100644 --- a/include/linux/livepatch.h +++ b/include/linux/livepatch.h @@ -28,18 +28,40 @@ #include <asm/livepatch.h> +/* task patch states */ +#define KLP_UNDEFINED -1 +#define KLP_UNPATCHED 0 +#define KLP_PATCHED 1 + /** * struct klp_func - function structure for live patching * @old_name: name of the function to be patched * @new_func: pointer to the patched function code * @old_sympos: a hint indicating which symbol position the old function * can be found (optional) + * @immediate: patch the func immediately, bypassing safety mechanisms * @old_addr: the address of the function being patched * @kobj: kobject for sysfs resources * @stack_node: list node for klp_ops func_stack list * @old_size: size of the old function * @new_size: size of the new function * @patched: the func has been added to the klp_ops list + * @transition: the func is currently being applied or reverted + * + * The patched and transition variables define the func's patching state. When + * patching, a func is always in one of the following states: + * + * patched=0 transition=0: unpatched + * patched=0 transition=1: unpatched, temporary starting state + * patched=1 transition=1: patched, may be visible to some tasks + * patched=1 transition=0: patched, visible to all tasks + * + * And when unpatching, it goes in the reverse order: + * + * patched=1 transition=0: patched, visible to all tasks + * patched=1 transition=1: patched, may be visible to some tasks + * patched=0 transition=1: unpatched, temporary ending state + * patched=0 transition=0: unpatched */ struct klp_func { /* external */ @@ -53,6 +75,7 @@ struct klp_func { * in kallsyms for the given object is used. */ unsigned long old_sympos; + bool immediate; /* internal */ unsigned long old_addr; @@ -60,6 +83,7 @@ struct klp_func { struct list_head stack_node; unsigned long old_size, new_size; bool patched; + bool transition; }; /** @@ -68,7 +92,7 @@ struct klp_func { * @funcs: function entries for functions to be patched in the object * @kobj: kobject for sysfs resources * @mod: kernel module associated with the patched object - * (NULL for vmlinux) + * (NULL for vmlinux) * @patched: the object's funcs have been added to the klp_ops list */ struct klp_object { @@ -86,6 +110,7 @@ struct klp_object { * struct klp_patch - patch structure for live patching * @mod: reference to the live patch module * @objs: object entries for kernel objects to be patched + * @immediate: patch all funcs immediately, bypassing safety mechanisms * @list: list node for global list of registered patches * @kobj: kobject for sysfs resources * @enabled: the patch is enabled (but operation may be incomplete) @@ -94,6 +119,7 @@ struct klp_patch { /* external */ struct module *mod; struct klp_object *objs; + bool immediate; /* internal */ struct list_head list; @@ -121,13 +147,27 @@ void arch_klp_init_object_loaded(struct klp_patch *patch, int klp_module_coming(struct module *mod); void klp_module_going(struct module *mod); +void klp_copy_process(struct task_struct *child); void klp_update_patch_state(struct task_struct *task); +static inline bool klp_patch_pending(struct task_struct *task) +{ + return test_tsk_thread_flag(task, TIF_PATCH_PENDING); +} + +static inline bool klp_have_reliable_stack(void) +{ + return IS_ENABLED(CONFIG_STACKTRACE) && + IS_ENABLED(CONFIG_HAVE_RELIABLE_STACKTRACE); +} + #else /* !CONFIG_LIVEPATCH */ static inline int klp_module_coming(struct module *mod) { return 0; } static inline void klp_module_going(struct module *mod) {} +static inline bool klp_patch_pending(struct task_struct *task) { return false; } static inline void klp_update_patch_state(struct task_struct *task) {} +static inline void klp_copy_process(struct task_struct *child) {} #endif /* CONFIG_LIVEPATCH */ diff --git a/include/linux/sched.h b/include/linux/sched.h index 9df1f42..ccbb1fa 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1973,6 +1973,9 @@ struct task_struct { /* A live task holds one reference. */ atomic_t stack_refcount; #endif +#ifdef CONFIG_LIVEPATCH + int patch_state; +#endif /* CPU-specific state of this task */ struct thread_struct thread; /* diff --git a/kernel/fork.c b/kernel/fork.c index 7da33cb..8c8de80 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -77,6 +77,7 @@ #include <linux/compiler.h> #include <linux/sysctl.h> #include <linux/kcov.h> +#include <linux/livepatch.h> #include <asm/pgtable.h> #include <asm/pgalloc.h> @@ -1765,6 +1766,8 @@ static __latent_entropy struct task_struct *copy_process( p->parent_exec_id = current->self_exec_id; } + klp_copy_process(p); + spin_lock(¤t->sighand->siglock); /* diff --git a/kernel/livepatch/Makefile b/kernel/livepatch/Makefile index e136dad..2b8bdb1 100644 --- a/kernel/livepatch/Makefile +++ b/kernel/livepatch/Makefile @@ -1,3 +1,3 @@ obj-$(CONFIG_LIVEPATCH) += livepatch.o -livepatch-objs := core.o patch.o +livepatch-objs := core.o patch.o transition.o diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c index 10ba3a1..4c5a816 100644 --- a/kernel/livepatch/core.c +++ b/kernel/livepatch/core.c @@ -31,22 +31,22 @@ #include <linux/moduleloader.h> #include <asm/cacheflush.h> #include "patch.h" +#include "transition.h" /* - * The klp_mutex protects the global lists and state transitions of any - * structure reachable from them. References to any structure must be obtained - * under mutex protection (except in klp_ftrace_handler(), which uses RCU to - * ensure it gets consistent data). + * klp_mutex is a coarse lock which serializes access to klp data. All + * accesses to klp-related variables and structures must have mutex protection, + * except within the following functions which carefully avoid the need for it: + * + * - klp_ftrace_handler() + * - klp_update_patch_state() */ -static DEFINE_MUTEX(klp_mutex); +DEFINE_MUTEX(klp_mutex); static LIST_HEAD(klp_patches); static struct kobject *klp_root_kobj; -/* TODO: temporary stub */ -void klp_update_patch_state(struct task_struct *task) {} - static bool klp_is_module(struct klp_object *obj) { return obj->name; @@ -85,7 +85,6 @@ static void klp_find_object_module(struct klp_object *obj) mutex_unlock(&module_mutex); } -/* klp_mutex must be held by caller */ static bool klp_is_patch_registered(struct klp_patch *patch) { struct klp_patch *mypatch; @@ -281,20 +280,25 @@ static int klp_write_object_relocations(struct module *pmod, static int __klp_disable_patch(struct klp_patch *patch) { - struct klp_object *obj; + if (klp_transition_patch) + return -EBUSY; /* enforce stacking: only the last enabled patch can be disabled */ if (!list_is_last(&patch->list, &klp_patches) && list_next_entry(patch, list)->enabled) return -EBUSY; - pr_notice("disabling patch '%s'\n", patch->mod->name); + klp_init_transition(patch, KLP_UNPATCHED); - klp_for_each_object(patch, obj) { - if (obj->patched) - klp_unpatch_object(obj); - } + /* + * Enforce the order of the klp_target_state write in + * klp_init_transition() and the TIF_PATCH_PENDING writes in + * klp_start_transition() to ensure that klp_update_patch_state() + * doesn't set a task->patch_state to KLP_UNDEFINED. + */ + smp_wmb(); + klp_start_transition(); patch->enabled = false; return 0; @@ -337,6 +341,9 @@ static int __klp_enable_patch(struct klp_patch *patch) struct klp_object *obj; int ret; + if (klp_transition_patch) + return -EBUSY; + if (WARN_ON(patch->enabled)) return -EINVAL; @@ -347,22 +354,37 @@ static int __klp_enable_patch(struct klp_patch *patch) pr_notice("enabling patch '%s'\n", patch->mod->name); + klp_init_transition(patch, KLP_PATCHED); + + /* + * Enforce the order of the func->transition writes in + * klp_init_transition() and the ops->func_stack writes in + * klp_patch_object(), so that klp_ftrace_handler() will see the + * func->transition updates before the handler is registered and the + * new funcs become visible to the handler. + */ + smp_wmb(); + klp_for_each_object(patch, obj) { if (!klp_is_object_loaded(obj)) continue; ret = klp_patch_object(obj); - if (ret) - goto unregister; + if (ret) { + pr_warn("failed to enable patch '%s'\n", + patch->mod->name); + + klp_unpatch_objects(patch); + klp_complete_transition(); + + return ret; + } } + klp_start_transition(); patch->enabled = true; return 0; - -unregister: - WARN_ON(__klp_disable_patch(patch)); - return ret; } /** @@ -399,6 +421,7 @@ EXPORT_SYMBOL_GPL(klp_enable_patch); * /sys/kernel/livepatch * /sys/kernel/livepatch/<patch> * /sys/kernel/livepatch/<patch>/enabled + * /sys/kernel/livepatch/<patch>/transition * /sys/kernel/livepatch/<patch>/<object> * /sys/kernel/livepatch/<patch>/<object>/<function,sympos> */ @@ -424,7 +447,9 @@ static ssize_t enabled_store(struct kobject *kobj, struct kobj_attribute *attr, goto err; } - if (enabled) { + if (patch == klp_transition_patch) { + klp_reverse_transition(); + } else if (enabled) { ret = __klp_enable_patch(patch); if (ret) goto err; @@ -452,9 +477,21 @@ static ssize_t enabled_show(struct kobject *kobj, return snprintf(buf, PAGE_SIZE-1, "%d\n", patch->enabled); } +static ssize_t transition_show(struct kobject *kobj, + struct kobj_attribute *attr, char *buf) +{ + struct klp_patch *patch; + + patch = container_of(kobj, struct klp_patch, kobj); + return snprintf(buf, PAGE_SIZE-1, "%d\n", + patch == klp_transition_patch); +} + static struct kobj_attribute enabled_kobj_attr = __ATTR_RW(enabled); +static struct kobj_attribute transition_kobj_attr = __ATTR_RO(transition); static struct attribute *klp_patch_attrs[] = { &enabled_kobj_attr.attr, + &transition_kobj_attr.attr, NULL }; @@ -544,6 +581,7 @@ static int klp_init_func(struct klp_object *obj, struct klp_func *func) INIT_LIST_HEAD(&func->stack_node); func->patched = false; + func->transition = false; /* The format for the sysfs directory is <function,sympos> where sympos * is the nth occurrence of this symbol in kallsyms for the patched @@ -740,6 +778,16 @@ int klp_register_patch(struct klp_patch *patch) return -ENODEV; /* + * Architectures without reliable stack traces have to set + * patch->immediate because there's currently no way to patch kthreads + * with the consistency model. + */ + if (!klp_have_reliable_stack() && !patch->immediate) { + pr_err("This architecture doesn't have support for the livepatch consistency model.\n"); + return -ENOSYS; + } + + /* * A reference is taken on the patch module to prevent it from being * unloaded. Right now, we don't allow patch modules to unload since * there is currently no method to determine if a thread is still @@ -788,7 +836,11 @@ int klp_module_coming(struct module *mod) goto err; } - if (!patch->enabled) + /* + * Only patch the module if the patch is enabled or is + * in transition. + */ + if (!patch->enabled && patch != klp_transition_patch) break; pr_notice("applying patch '%s' to loading module '%s'\n", diff --git a/kernel/livepatch/patch.c b/kernel/livepatch/patch.c index 5efa262..1a77f05 100644 --- a/kernel/livepatch/patch.c +++ b/kernel/livepatch/patch.c @@ -29,6 +29,7 @@ #include <linux/bug.h> #include <linux/printk.h> #include "patch.h" +#include "transition.h" static LIST_HEAD(klp_ops); @@ -54,15 +55,58 @@ static void notrace klp_ftrace_handler(unsigned long ip, { struct klp_ops *ops; struct klp_func *func; + int patch_state; ops = container_of(fops, struct klp_ops, fops); rcu_read_lock(); + func = list_first_or_null_rcu(&ops->func_stack, struct klp_func, stack_node); + + /* + * func should never be NULL because preemption should be disabled here + * and unregister_ftrace_function() does the equivalent of a + * synchronize_sched() before the func_stack removal. + */ if (WARN_ON_ONCE(!func)) goto unlock; + /* + * Enforce the order of the ops->func_stack and func->transition reads. + * The corresponding write barrier is in __klp_enable_patch(). + */ + smp_rmb(); + + if (unlikely(func->transition)) { + + /* + * Enforce the order of the func->transition and + * current->patch_state reads. Otherwise we could read an + * out-of-date task state and pick the wrong function. The + * corresponding write barriers are in klp_init_transition() + * and __klp_disable_patch(). + */ + smp_rmb(); + + patch_state = current->patch_state; + + WARN_ON_ONCE(patch_state == KLP_UNDEFINED); + + if (patch_state == KLP_UNPATCHED) { + /* + * Use the previously patched version of the function. + * If no previous patches exist, continue with the + * original function. + */ + func = list_entry_rcu(func->stack_node.next, + struct klp_func, stack_node); + + if (&func->stack_node == &ops->func_stack) + goto unlock; + } + } + klp_arch_set_pc(regs, (unsigned long)func->new_func); unlock: rcu_read_unlock(); @@ -211,3 +255,12 @@ int klp_patch_object(struct klp_object *obj) return 0; } + +void klp_unpatch_objects(struct klp_patch *patch) +{ + struct klp_object *obj; + + klp_for_each_object(patch, obj) + if (obj->patched) + klp_unpatch_object(obj); +} diff --git a/kernel/livepatch/patch.h b/kernel/livepatch/patch.h index 2d0cce0..0db2271 100644 --- a/kernel/livepatch/patch.h +++ b/kernel/livepatch/patch.h @@ -28,5 +28,6 @@ struct klp_ops *klp_find_ops(unsigned long old_addr); int klp_patch_object(struct klp_object *obj); void klp_unpatch_object(struct klp_object *obj); +void klp_unpatch_objects(struct klp_patch *patch); #endif /* _LIVEPATCH_PATCH_H */ diff --git a/kernel/livepatch/transition.c b/kernel/livepatch/transition.c new file mode 100644 index 0000000..2b87ff9 --- /dev/null +++ b/kernel/livepatch/transition.c @@ -0,0 +1,525 @@ +/* + * transition.c - Kernel Live Patching transition functions + * + * Copyright (C) 2015-2016 Josh Poimboeuf <jpoimboe@redhat.com> + * + * This program is free software; you can redistribute it and/or + * modify it under the terms of the GNU General Public License + * as published by the Free Software Foundation; either version 2 + * of the License, or (at your option) any later version. + * + * This program is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License + * along with this program; if not, see <http://www.gnu.org/licenses/>. + */ + +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt + +#include <linux/cpu.h> +#include <linux/stacktrace.h> +#include "patch.h" +#include "transition.h" +#include "../sched/sched.h" + +#define MAX_STACK_ENTRIES 100 +#define STACK_ERR_BUF_SIZE 128 + +extern struct mutex klp_mutex; + +struct klp_patch *klp_transition_patch; + +static int klp_target_state = KLP_UNDEFINED; + +/* + * This work can be performed periodically to finish patching or unpatching any + * "straggler" tasks which failed to transition in the first attempt. + */ +static void klp_try_complete_transition(void); +static void klp_transition_work_fn(struct work_struct *work) +{ + mutex_lock(&klp_mutex); + + if (klp_transition_patch) + klp_try_complete_transition(); + + mutex_unlock(&klp_mutex); +} +static DECLARE_DELAYED_WORK(klp_transition_work, klp_transition_work_fn); + +/* + * The transition to the target patch state is complete. Clean up the data + * structures. + */ +void klp_complete_transition(void) +{ + struct klp_object *obj; + struct klp_func *func; + struct task_struct *g, *task; + unsigned int cpu; + + if (klp_transition_patch->immediate) + goto done; + + klp_for_each_object(klp_transition_patch, obj) + klp_for_each_func(obj, func) + func->transition = false; + + /* Prevent klp_ftrace_handler() from seeing KLP_UNDEFINED state */ + if (klp_target_state == KLP_PATCHED) + synchronize_rcu(); + + read_lock(&tasklist_lock); + for_each_process_thread(g, task) { + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); + task->patch_state = KLP_UNDEFINED; + } + read_unlock(&tasklist_lock); + + for_each_possible_cpu(cpu) { + task = idle_task(cpu); + WARN_ON_ONCE(test_tsk_thread_flag(task, TIF_PATCH_PENDING)); + task->patch_state = KLP_UNDEFINED; + } + +done: + klp_target_state = KLP_UNDEFINED; + klp_transition_patch = NULL; +} + +/* + * Switch the patched state of the task to the set of functions in the target + * patch state. + * + * NOTE: If task is not 'current', the caller must ensure the task is inactive. + * Otherwise klp_ftrace_handler() might read the wrong 'patch_state' value. + */ +void klp_update_patch_state(struct task_struct *task) +{ + rcu_read_lock(); + + /* + * This test_and_clear_tsk_thread_flag() call also serves as a read + * barrier to enforce the order of the TIF_PATCH_PENDING and + * klp_target_state reads. The corresponding write barrier is in + * __klp_disable_patch(). + */ + if (test_and_clear_tsk_thread_flag(task, TIF_PATCH_PENDING)) + task->patch_state = READ_ONCE(klp_target_state); + + rcu_read_unlock(); +} + +/* + * Determine whether the given stack trace includes any references to a + * to-be-patched or to-be-unpatched function. + */ +static int klp_check_stack_func(struct klp_func *func, + struct stack_trace *trace) +{ + unsigned long func_addr, func_size, address; + struct klp_ops *ops; + int i; + + if (func->immediate) + return 0; + + for (i = 0; i < trace->nr_entries; i++) { + address = trace->entries[i]; + + if (klp_target_state == KLP_UNPATCHED) { + /* + * Check for the to-be-unpatched function + * (the func itself). + */ + func_addr = (unsigned long)func->new_func; + func_size = func->new_size; + } else { + /* + * Check for the to-be-patched function + * (the previous func). + */ + ops = klp_find_ops(func->old_addr); + + if (list_is_singular(&ops->func_stack)) { + /* original function */ + func_addr = func->old_addr; + func_size = func->old_size; + } else { + /* previously patched function */ + struct klp_func *prev; + + prev = list_next_entry(func, stack_node); + func_addr = (unsigned long)prev->new_func; + func_size = prev->new_size; + } + } + + if (address >= func_addr && address < func_addr + func_size) + return -EAGAIN; + } + + return 0; +} + +/* + * Determine whether it's safe to transition the task to the target patch state + * by looking for any to-be-patched or to-be-unpatched functions on its stack. + */ +static int klp_check_stack(struct task_struct *task, char *err_buf) +{ + static unsigned long entries[MAX_STACK_ENTRIES]; + struct stack_trace trace; + struct klp_object *obj; + struct klp_func *func; + int ret; + + trace.skip = 0; + trace.nr_entries = 0; + trace.max_entries = MAX_STACK_ENTRIES; + trace.entries = entries; + ret = save_stack_trace_tsk_reliable(task, &trace); + WARN_ON_ONCE(ret == -ENOSYS); + if (ret) { + snprintf(err_buf, STACK_ERR_BUF_SIZE, + "%s: %s:%d has an unreliable stack\n", + __func__, task->comm, task->pid); + return ret; + } + + klp_for_each_object(klp_transition_patch, obj) { + if (!obj->patched) + continue; + klp_for_each_func(obj, func) { + ret = klp_check_stack_func(func, &trace); + if (ret) { + snprintf(err_buf, STACK_ERR_BUF_SIZE, + "%s: %s:%d is sleeping on function %s\n", + __func__, task->comm, task->pid, + func->old_name); + return ret; + } + } + } + + return 0; +} + +/* + * Try to safely switch a task to the target patch state. If it's currently + * running, or it's sleeping on a to-be-patched or to-be-unpatched function, or + * if the stack is unreliable, return false. + */ +static bool klp_try_switch_task(struct task_struct *task) +{ + struct rq *rq; + struct rq_flags flags; + int ret; + bool success = false; + char err_buf[STACK_ERR_BUF_SIZE]; + + err_buf[0] = '\0'; + + /* check if this task has already switched over */ + if (task->patch_state == klp_target_state) + return true; + + /* + * For arches which don't have reliable stack traces, we have to rely + * on other methods (e.g., switching tasks at kernel exit). + */ + if (!klp_have_reliable_stack()) + return false; + + /* + * Now try to check the stack for any to-be-patched or to-be-unpatched + * functions. If all goes well, switch the task to the target patch + * state. + */ + rq = task_rq_lock(task, &flags); + + if (task_running(rq, task) && task != current) { + snprintf(err_buf, STACK_ERR_BUF_SIZE, + "%s: %s:%d is running\n", __func__, task->comm, + task->pid); + goto done; + } + + ret = klp_check_stack(task, err_buf); + if (ret) + goto done; + + success = true; + + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); + task->patch_state = klp_target_state; + +done: + task_rq_unlock(rq, task, &flags); + + /* + * Due to console deadlock issues, pr_debug() can't be used while + * holding the task rq lock. Instead we have to use a temporary buffer + * and print the debug message after releasing the lock. + */ + if (err_buf[0] != '\0') + pr_debug("%s", err_buf); + + return success; + +} + +/* + * Try to switch all remaining tasks to the target patch state by walking the + * stacks of sleeping tasks and looking for any to-be-patched or + * to-be-unpatched functions. If such functions are found, the task can't be + * switched yet. + * + * If any tasks are still stuck in the initial patch state, schedule a retry. + */ +static void klp_try_complete_transition(void) +{ + unsigned int cpu; + struct task_struct *g, *task; + bool complete = true; + + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED); + + /* + * If the patch can be applied or reverted immediately, skip the + * per-task transitions. + */ + if (klp_transition_patch->immediate) + goto success; + + /* + * Try to switch the tasks to the target patch state by walking their + * stacks and looking for any to-be-patched or to-be-unpatched + * functions. If such functions are found on a stack, or if the stack + * is deemed unreliable, the task can't be switched yet. + * + * Usually this will transition most (or all) of the tasks on a system + * unless the patch includes changes to a very common function. + */ + read_lock(&tasklist_lock); + for_each_process_thread(g, task) + if (!klp_try_switch_task(task)) + complete = false; + read_unlock(&tasklist_lock); + + /* + * Ditto for the idle "swapper" tasks. + */ + get_online_cpus(); + for_each_possible_cpu(cpu) { + task = idle_task(cpu); + if (cpu_online(cpu)) { + if (!klp_try_switch_task(task)) + complete = false; + } else if (task->patch_state != klp_target_state) { + /* offline idle tasks can be switched immediately */ + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); + task->patch_state = klp_target_state; + } + } + put_online_cpus(); + + /* + * Some tasks weren't able to be switched over. Try again later and/or + * wait for other methods like kernel exit switching. + */ + if (!complete) { + schedule_delayed_work(&klp_transition_work, + round_jiffies_relative(HZ)); + return; + } + +success: + + if (klp_target_state == KLP_UNPATCHED) { + /* + * All tasks have transitioned to KLP_UNPATCHED so we can now + * remove the new functions from the func_stack. + */ + klp_unpatch_objects(klp_transition_patch); + + /* + * Make sure klp_ftrace_handler() sees that the functions have + * been removed from ops->func_stack before we clear + * func->transition. Otherwise it may pick the wrong + * transition. + */ + synchronize_rcu(); + } + + pr_notice("'%s': %s complete\n", klp_transition_patch->mod->name, + klp_target_state == KLP_PATCHED ? "patching" : "unpatching"); + + /* we're done, now cleanup the data structures */ + klp_complete_transition(); +} + +/* + * Start the transition to the specified target patch state so tasks can begin + * switching to it. + */ +void klp_start_transition(void) +{ + struct task_struct *g, *task; + unsigned int cpu; + + WARN_ON_ONCE(klp_target_state == KLP_UNDEFINED); + + pr_notice("'%s': %s...\n", klp_transition_patch->mod->name, + klp_target_state == KLP_PATCHED ? "patching" : "unpatching"); + + /* + * If the patch can be applied or reverted immediately, skip the + * per-task transitions. + */ + if (klp_transition_patch->immediate) + return; + + /* + * Mark all normal tasks as needing a patch state update. They'll + * switch either in klp_try_complete_transition() or as they exit the + * kernel. + */ + read_lock(&tasklist_lock); + for_each_process_thread(g, task) + if (task->patch_state != klp_target_state) + set_tsk_thread_flag(task, TIF_PATCH_PENDING); + read_unlock(&tasklist_lock); + + /* + * Mark all idle tasks as needing a patch state update. They'll switch + * either in klp_try_complete_transition() or at the idle loop switch + * point. + */ + for_each_possible_cpu(cpu) { + task = idle_task(cpu); + if (task->patch_state != klp_target_state) + set_tsk_thread_flag(task, TIF_PATCH_PENDING); + } + + klp_try_complete_transition(); +} + +/* + * Initialize the global target patch state and all tasks to the initial patch + * state, and initialize all function transition states to true in preparation + * for patching or unpatching. + */ +void klp_init_transition(struct klp_patch *patch, int state) +{ + struct task_struct *g, *task; + unsigned int cpu; + struct klp_object *obj; + struct klp_func *func; + int initial_state = !state; + + WARN_ON_ONCE(klp_target_state != KLP_UNDEFINED); + + klp_transition_patch = patch; + + /* + * Set the global target patch state which tasks will switch to. This + * has no effect until the TIF_PATCH_PENDING flags get set later. + */ + klp_target_state = state; + + /* + * If the patch can be applied or reverted immediately, skip the + * per-task transitions. + */ + if (patch->immediate) + return; + + /* + * Initialize all tasks to the initial patch state to prepare them for + * switching to the target state. + */ + read_lock(&tasklist_lock); + for_each_process_thread(g, task) { + WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED); + task->patch_state = initial_state; + } + read_unlock(&tasklist_lock); + + /* + * Ditto for the idle "swapper" tasks. + */ + for_each_possible_cpu(cpu) { + task = idle_task(cpu); + WARN_ON_ONCE(task->patch_state != KLP_UNDEFINED); + task->patch_state = initial_state; + } + + /* + * Enforce the order of the task->patch_state initializations and the + * func->transition updates to ensure that, in the enable path, + * klp_ftrace_handler() doesn't see a func in transition with a + * task->patch_state of KLP_UNDEFINED. + */ + smp_wmb(); + + /* + * Set the func transition states so klp_ftrace_handler() will know to + * switch to the transition logic. + * + * When patching, the funcs aren't yet in the func_stack and will be + * made visible to the ftrace handler shortly by the calls to + * klp_patch_object(). + * + * When unpatching, the funcs are already in the func_stack and so are + * already visible to the ftrace handler. + */ + klp_for_each_object(patch, obj) + klp_for_each_func(obj, func) + func->transition = true; +} + +/* + * This function can be called in the middle of an existing transition to + * reverse the direction of the target patch state. This can be done to + * effectively cancel an existing enable or disable operation if there are any + * tasks which are stuck in the initial patch state. + */ +void klp_reverse_transition(void) +{ + unsigned int cpu; + struct task_struct *g, *task; + + klp_transition_patch->enabled = !klp_transition_patch->enabled; + + klp_target_state = !klp_target_state; + + /* + * Clear all TIF_PATCH_PENDING flags to prevent races caused by + * klp_update_patch_state() running in parallel with + * klp_start_transition(). + */ + read_lock(&tasklist_lock); + for_each_process_thread(g, task) + clear_tsk_thread_flag(task, TIF_PATCH_PENDING); + read_unlock(&tasklist_lock); + + for_each_possible_cpu(cpu) + clear_tsk_thread_flag(idle_task(cpu), TIF_PATCH_PENDING); + + /* Let any remaining calls to klp_update_patch_state() complete */ + synchronize_rcu(); + + klp_start_transition(); +} + +/* Called from copy_process() during fork */ +void klp_copy_process(struct task_struct *child) +{ + child->patch_state = current->patch_state; + + /* TIF_PATCH_PENDING gets copied in setup_thread_stack() */ +} diff --git a/kernel/livepatch/transition.h b/kernel/livepatch/transition.h new file mode 100644 index 0000000..3ee625f --- /dev/null +++ b/kernel/livepatch/transition.h @@ -0,0 +1,13 @@ +#ifndef _LIVEPATCH_TRANSITION_H +#define _LIVEPATCH_TRANSITION_H + +#include <linux/livepatch.h> + +extern struct klp_patch *klp_transition_patch; + +void klp_init_transition(struct klp_patch *patch, int state); +void klp_start_transition(void); +void klp_reverse_transition(void); +void klp_complete_transition(void); + +#endif /* _LIVEPATCH_TRANSITION_H */ diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c index 6a4bae0..a8b3f1a 100644 --- a/kernel/sched/idle.c +++ b/kernel/sched/idle.c @@ -9,6 +9,7 @@ #include <linux/mm.h> #include <linux/stackprotector.h> #include <linux/suspend.h> +#include <linux/livepatch.h> #include <asm/tlb.h> @@ -264,6 +265,9 @@ static void do_idle(void) sched_ttwu_pending(); schedule_preempt_disabled(); + + if (unlikely(klp_patch_pending(current))) + klp_update_patch_state(current); } bool cpu_in_idle(unsigned long pc) diff --git a/samples/livepatch/livepatch-sample.c b/samples/livepatch/livepatch-sample.c index e34f871..629e0dc 100644 --- a/samples/livepatch/livepatch-sample.c +++ b/samples/livepatch/livepatch-sample.c @@ -17,6 +17,8 @@ * along with this program; if not, see <http://www.gnu.org/licenses/>. */ +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt + #include <linux/module.h> #include <linux/kernel.h> #include <linux/livepatch.h> @@ -69,6 +71,21 @@ static int livepatch_init(void) { int ret; + if (!klp_have_reliable_stack() && !patch.immediate) { + /* + * WARNING: Be very careful when using 'patch.immediate' in + * your patches. It's ok to use it for simple patches like + * this, but for more complex patches which change function + * semantics, locking semantics, or data structures, it may not + * be safe. Use of this option will also prevent removal of + * the patch. + * + * See Documentation/livepatch/livepatch.txt for more details. + */ + patch.immediate = true; + pr_notice("The consistency model isn't supported for your architecture. Bypassing safety mechanisms and applying the patch immediately.\n"); + } + ret = klp_register_patch(&patch); if (ret) return ret;
Change livepatch to use a basic per-task consistency model. This is the foundation which will eventually enable us to patch those ~10% of security patches which change function or data semantics. This is the biggest remaining piece needed to make livepatch more generally useful. This code stems from the design proposal made by Vojtech [1] in November 2014. It's a hybrid of kGraft and kpatch: it uses kGraft's per-task consistency and syscall barrier switching combined with kpatch's stack trace switching. There are also a number of fallback options which make it quite flexible. Patches are applied on a per-task basis, when the task is deemed safe to switch over. When a patch is enabled, livepatch enters into a transition state where tasks are converging to the patched state. Usually this transition state can complete in a few seconds. The same sequence occurs when a patch is disabled, except the tasks converge from the patched state to the unpatched state. An interrupt handler inherits the patched state of the task it interrupts. The same is true for forked tasks: the child inherits the patched state of the parent. Livepatch uses several complementary approaches to determine when it's safe to patch tasks: 1. The first and most effective approach is stack checking of sleeping tasks. If no affected functions are on the stack of a given task, the task is patched. In most cases this will patch most or all of the tasks on the first try. Otherwise it'll keep trying periodically. This option is only available if the architecture has reliable stacks (HAVE_RELIABLE_STACKTRACE). 2. The second approach, if needed, is kernel exit switching. A task is switched when it returns to user space from a system call, a user space IRQ, or a signal. It's useful in the following cases: a) Patching I/O-bound user tasks which are sleeping on an affected function. In this case you have to send SIGSTOP and SIGCONT to force it to exit the kernel and be patched. b) Patching CPU-bound user tasks. If the task is highly CPU-bound then it will get patched the next time it gets interrupted by an IRQ. c) In the future it could be useful for applying patches for architectures which don't yet have HAVE_RELIABLE_STACKTRACE. In this case you would have to signal most of the tasks on the system. However this isn't supported yet because there's currently no way to patch kthreads without HAVE_RELIABLE_STACKTRACE. 3. For idle "swapper" tasks, since they don't ever exit the kernel, they instead have a klp_update_patch_state() call in the idle loop which allows them to be patched before the CPU enters the idle state. (Note there's not yet such an approach for kthreads.) All the above approaches may be skipped by setting the 'immediate' flag in the 'klp_patch' struct, which will disable per-task consistency and patch all tasks immediately. This can be useful if the patch doesn't change any function or data semantics. Note that, even with this flag set, it's possible that some tasks may still be running with an old version of the function, until that function returns. There's also an 'immediate' flag in the 'klp_func' struct which allows you to specify that certain functions in the patch can be applied without per-task consistency. This might be useful if you want to patch a common function like schedule(), and the function change doesn't need consistency but the rest of the patch does. For architectures which don't have HAVE_RELIABLE_STACKTRACE, the user must set patch->immediate which causes all tasks to be patched immediately. This option should be used with care, only when the patch doesn't change any function or data semantics. In the future, architectures which don't have HAVE_RELIABLE_STACKTRACE may be allowed to use per-task consistency if we can come up with another way to patch kthreads. The /sys/kernel/livepatch/<patch>/transition file shows whether a patch is in transition. Only a single patch (the topmost patch on the stack) can be in transition at a given time. A patch can remain in transition indefinitely, if any of the tasks are stuck in the initial patch state. A transition can be reversed and effectively canceled by writing the opposite value to the /sys/kernel/livepatch/<patch>/enabled file while the transition is in progress. Then all the tasks will attempt to converge back to the original patch state. [1] https://lkml.kernel.org/r/20141107140458.GA21774@suse.cz Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com> --- Documentation/ABI/testing/sysfs-kernel-livepatch | 8 + Documentation/livepatch/livepatch.txt | 158 ++++++- include/linux/init_task.h | 9 + include/linux/livepatch.h | 42 +- include/linux/sched.h | 3 + kernel/fork.c | 3 + kernel/livepatch/Makefile | 2 +- kernel/livepatch/core.c | 98 ++++- kernel/livepatch/patch.c | 53 +++ kernel/livepatch/patch.h | 1 + kernel/livepatch/transition.c | 525 +++++++++++++++++++++++ kernel/livepatch/transition.h | 13 + kernel/sched/idle.c | 4 + samples/livepatch/livepatch-sample.c | 17 + 14 files changed, 892 insertions(+), 44 deletions(-) create mode 100644 kernel/livepatch/transition.c create mode 100644 kernel/livepatch/transition.h