| Message ID | 20210713053113.4632-8-cmr@linux.ibm.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | Use per-CPU temporary mappings for patching on Radix MMU | expand |
| Related | show |
Le 13/07/2021 à 07:31, Christopher M. Riedl a écrit : > When code patching a STRICT_KERNEL_RWX kernel the page containing the > address to be patched is temporarily mapped as writeable. Currently, a > per-cpu vmalloc patch area is used for this purpose. While the patch > area is per-cpu, the temporary page mapping is inserted into the kernel > page tables for the duration of patching. The mapping is exposed to CPUs > other than the patching CPU - this is undesirable from a hardening > perspective. Use a temporary mm instead which keeps the mapping local to > the CPU doing the patching. > > Use the `poking_init` init hook to prepare a temporary mm and patching > address. Initialize the temporary mm by copying the init mm. Choose a > randomized patching address inside the temporary mm userspace address > space. The patching address is randomized between PAGE_SIZE and > DEFAULT_MAP_WINDOW-PAGE_SIZE. > > Bits of entropy with 64K page size on BOOK3S_64: > > bits of entropy = log2(DEFAULT_MAP_WINDOW_USER64 / PAGE_SIZE) > > PAGE_SIZE=64K, DEFAULT_MAP_WINDOW_USER64=128TB > bits of entropy = log2(128TB / 64K) > bits of entropy = 31 > > The upper limit is DEFAULT_MAP_WINDOW due to how the Book3s64 Hash MMU > operates - by default the space above DEFAULT_MAP_WINDOW is not > available. Currently the Hash MMU does not use a temporary mm so > technically this upper limit isn't necessary; however, a larger > randomization range does not further "harden" this overall approach and > future work may introduce patching with a temporary mm on Hash as well. > > Randomization occurs only once during initialization at boot for each > possible CPU in the system. > > Introduce two new functions, map_patch() and unmap_patch(), to > respectively create and remove the temporary mapping with write > permissions at patching_addr. Map the page with PAGE_KERNEL to set > EAA[0] for the PTE which ignores the AMR (so no need to unlock/lock > KUAP) according to PowerISA v3.0b Figure 35 on Radix. > > Based on x86 implementation: > > commit 4fc19708b165 > ("x86/alternatives: Initialize temporary mm for patching") > > and: > > commit b3fd8e83ada0 > ("x86/alternatives: Use temporary mm for text poking") > > Signed-off-by: Christopher M. Riedl <cmr@linux.ibm.com> > > --- > > v5: * Only support Book3s64 Radix MMU for now. > * Use a per-cpu datastructure to hold the patching_addr and > patching_mm to avoid the need for a synchronization lock/mutex. > > v4: * In the previous series this was two separate patches: one to init > the temporary mm in poking_init() (unused in powerpc at the time) > and the other to use it for patching (which removed all the > per-cpu vmalloc code). Now that we use poking_init() in the > existing per-cpu vmalloc approach, that separation doesn't work > as nicely anymore so I just merged the two patches into one. > * Preload the SLB entry and hash the page for the patching_addr > when using Hash on book3s64 to avoid taking an SLB and Hash fault > during patching. The previous implementation was a hack which > changed current->mm to allow the SLB and Hash fault handlers to > work with the temporary mm since both of those code-paths always > assume mm == current->mm. > * Also (hmm - seeing a trend here) with the book3s64 Hash MMU we > have to manage the mm->context.active_cpus counter and mm cpumask > since they determine (via mm_is_thread_local()) if the TLB flush > in pte_clear() is local or not - it should always be local when > we're using the temporary mm. On book3s64's Radix MMU we can > just call local_flush_tlb_mm(). > * Use HPTE_USE_KERNEL_KEY on Hash to avoid costly lock/unlock of > KUAP. > --- > arch/powerpc/lib/code-patching.c | 132 +++++++++++++++++++++++++++++-- > 1 file changed, 125 insertions(+), 7 deletions(-) > > diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c > index 9f2eba9b70ee4..027dabd42b8dd 100644 > --- a/arch/powerpc/lib/code-patching.c > +++ b/arch/powerpc/lib/code-patching.c > @@ -11,6 +11,7 @@ > #include <linux/cpuhotplug.h> > #include <linux/slab.h> > #include <linux/uaccess.h> > +#include <linux/random.h> > > #include <asm/tlbflush.h> > #include <asm/page.h> > @@ -103,6 +104,7 @@ static inline void unuse_temporary_mm(struct temp_mm *temp_mm) > > static DEFINE_PER_CPU(struct vm_struct *, text_poke_area); > static DEFINE_PER_CPU(unsigned long, cpu_patching_addr); > +static DEFINE_PER_CPU(struct mm_struct *, cpu_patching_mm); > > #if IS_BUILTIN(CONFIG_LKDTM) > unsigned long read_cpu_patching_addr(unsigned int cpu) > @@ -133,6 +135,51 @@ static int text_area_cpu_down(unsigned int cpu) > return 0; > } > > +static __always_inline void __poking_init_temp_mm(void) > +{ > + int cpu; > + spinlock_t *ptl; /* for protecting pte table */ > + pte_t *ptep; > + struct mm_struct *patching_mm; > + unsigned long patching_addr; > + > + for_each_possible_cpu(cpu) { > + /* > + * Some parts of the kernel (static keys for example) depend on > + * successful code patching. Code patching under > + * STRICT_KERNEL_RWX requires this setup - otherwise we cannot > + * patch at all. We use BUG_ON() here and later since an early > + * failure is preferred to buggy behavior and/or strange > + * crashes later. > + */ > + patching_mm = copy_init_mm(); > + BUG_ON(!patching_mm); Read https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on and https://github.com/linuxppc/issues/issues/88 Avoid BUG_ON()s thanks. > + > + per_cpu(cpu_patching_mm, cpu) = patching_mm; > + > + /* > + * Choose a randomized, page-aligned address from the range: > + * [PAGE_SIZE, DEFAULT_MAP_WINDOW - PAGE_SIZE] The lower > + * address bound is PAGE_SIZE to avoid the zero-page. The > + * upper address bound is DEFAULT_MAP_WINDOW - PAGE_SIZE to > + * stay under DEFAULT_MAP_WINDOW with the Book3s64 Hash MMU. > + */ > + patching_addr = PAGE_SIZE + ((get_random_long() & PAGE_MASK) > + % (DEFAULT_MAP_WINDOW - 2 * PAGE_SIZE)); % should be at the end of first line and the second line alignment should match open parenthesis in first line. > + > + per_cpu(cpu_patching_addr, cpu) = patching_addr; > + > + /* > + * PTE allocation uses GFP_KERNEL which means we need to > + * pre-allocate the PTE here because we cannot do the > + * allocation during patching when IRQs are disabled. > + */ > + ptep = get_locked_pte(patching_mm, patching_addr, &ptl); > + BUG_ON(!ptep); Avoid BUG_ON() please > + pte_unmap_unlock(ptep, ptl); > + } > +} > + > /* > * Although BUG_ON() is rude, in this case it should only happen if ENOMEM, and > * we judge it as being preferable to a kernel that will crash later when > @@ -140,6 +187,11 @@ static int text_area_cpu_down(unsigned int cpu) > */ > void __init poking_init(void) > { > + if (radix_enabled()) { > + __poking_init_temp_mm(); > + return; > + } > + > BUG_ON(!cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, > "powerpc/text_poke:online", text_area_cpu_up, > text_area_cpu_down)); > @@ -213,30 +265,96 @@ static inline int unmap_patch_area(void) > return -EINVAL; > } > > +struct patch_mapping { > + spinlock_t *ptl; /* for protecting pte table */ > + pte_t *ptep; > + struct temp_mm temp_mm; > +}; > + > +/* > + * This can be called for kernel text or a module. > + */ > +static int map_patch(const void *addr, struct patch_mapping *patch_mapping) > +{ > + struct page *page; > + pte_t pte; > + pgprot_t pgprot; > + struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm); > + unsigned long patching_addr = __this_cpu_read(cpu_patching_addr); > + > + if (is_vmalloc_or_module_addr(addr)) > + page = vmalloc_to_page(addr); > + else > + page = virt_to_page(addr); > + > + patch_mapping->ptep = get_locked_pte(patching_mm, patching_addr, > + &patch_mapping->ptl); Not sure you need to split this line, checkpatch now allows 100 chars per line. > + if (unlikely(!patch_mapping->ptep)) { > + pr_warn("map patch: failed to allocate pte for patching\n"); That's a lot better than all above BUG_ONs > + return -1; > + } > + > + pgprot = PAGE_KERNEL; > + pte = mk_pte(page, pgprot); > + pte = pte_mkdirty(pte); I'm sure you can do pte = pte_mkdirty(mk_pte(page, PAGE_KERNEL)); And indeed PAGE_KERNEL already includes _PAGE_DIRTY, so all you should need is pte = mk_pte(page, PAGE_KERNEL); Or even just set_pte_at(patching_mm, patching_addr, patch_mapping->ptep, mk_pte(page, PAGE_KERNEL)); > + set_pte_at(patching_mm, patching_addr, patch_mapping->ptep, pte); > + > + init_temp_mm(&patch_mapping->temp_mm, patching_mm); > + use_temporary_mm(&patch_mapping->temp_mm); > + > + return 0; > +} > + > +static void unmap_patch(struct patch_mapping *patch_mapping) > +{ > + struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm); > + unsigned long patching_addr = __this_cpu_read(cpu_patching_addr); > + > + pte_clear(patching_mm, patching_addr, patch_mapping->ptep); > + > + local_flush_tlb_mm(patching_mm); > + > + pte_unmap_unlock(patch_mapping->ptep, patch_mapping->ptl); > + > + unuse_temporary_mm(&patch_mapping->temp_mm); Shouldn't you stop using it before unmapping/unlocking it ? > +} > + > static int do_patch_instruction(u32 *addr, struct ppc_inst instr) > { > int err, rc = 0; > u32 *patch_addr = NULL; > unsigned long flags; > + struct patch_mapping patch_mapping; > > /* > - * During early early boot patch_instruction is called > - * when text_poke_area is not ready, but we still need > - * to allow patching. We just do the plain old patching > + * During early early boot patch_instruction is called when the > + * patching_mm/text_poke_area is not ready, but we still need to allow > + * patching. We just do the plain old patching. > */ > - if (!this_cpu_read(text_poke_area)) > - return raw_patch_instruction(addr, instr); > + if (radix_enabled()) { > + if (!this_cpu_read(cpu_patching_mm)) > + return raw_patch_instruction(addr, instr); > + } else { > + if (!this_cpu_read(text_poke_area)) > + return raw_patch_instruction(addr, instr); > + } > > local_irq_save(flags); > > - err = map_patch_area(addr); > + if (radix_enabled()) > + err = map_patch(addr, &patch_mapping); Maybe call it map_patch_mm() or map_patch_mapping() ? > + else > + err = map_patch_area(addr); > if (err) > goto out; > > patch_addr = (u32 *)(__this_cpu_read(cpu_patching_addr) | offset_in_page(addr)); > rc = __patch_instruction(addr, instr, patch_addr); > > - err = unmap_patch_area(); > + if (radix_enabled()) > + unmap_patch(&patch_mapping); No err ? Would be better if it could return something, allthough always 0. And same comment about naming. > + else > + err = unmap_patch_area(); > > out: > local_irq_restore(flags); >
On Thu Aug 5, 2021 at 4:48 AM CDT, Christophe Leroy wrote: > > > Le 13/07/2021 à 07:31, Christopher M. Riedl a écrit : > > When code patching a STRICT_KERNEL_RWX kernel the page containing the > > address to be patched is temporarily mapped as writeable. Currently, a > > per-cpu vmalloc patch area is used for this purpose. While the patch > > area is per-cpu, the temporary page mapping is inserted into the kernel > > page tables for the duration of patching. The mapping is exposed to CPUs > > other than the patching CPU - this is undesirable from a hardening > > perspective. Use a temporary mm instead which keeps the mapping local to > > the CPU doing the patching. > > > > Use the `poking_init` init hook to prepare a temporary mm and patching > > address. Initialize the temporary mm by copying the init mm. Choose a > > randomized patching address inside the temporary mm userspace address > > space. The patching address is randomized between PAGE_SIZE and > > DEFAULT_MAP_WINDOW-PAGE_SIZE. > > > > Bits of entropy with 64K page size on BOOK3S_64: > > > > bits of entropy = log2(DEFAULT_MAP_WINDOW_USER64 / PAGE_SIZE) > > > > PAGE_SIZE=64K, DEFAULT_MAP_WINDOW_USER64=128TB > > bits of entropy = log2(128TB / 64K) > > bits of entropy = 31 > > > > The upper limit is DEFAULT_MAP_WINDOW due to how the Book3s64 Hash MMU > > operates - by default the space above DEFAULT_MAP_WINDOW is not > > available. Currently the Hash MMU does not use a temporary mm so > > technically this upper limit isn't necessary; however, a larger > > randomization range does not further "harden" this overall approach and > > future work may introduce patching with a temporary mm on Hash as well. > > > > Randomization occurs only once during initialization at boot for each > > possible CPU in the system. > > > > Introduce two new functions, map_patch() and unmap_patch(), to > > respectively create and remove the temporary mapping with write > > permissions at patching_addr. Map the page with PAGE_KERNEL to set > > EAA[0] for the PTE which ignores the AMR (so no need to unlock/lock > > KUAP) according to PowerISA v3.0b Figure 35 on Radix. > > > > Based on x86 implementation: > > > > commit 4fc19708b165 > > ("x86/alternatives: Initialize temporary mm for patching") > > > > and: > > > > commit b3fd8e83ada0 > > ("x86/alternatives: Use temporary mm for text poking") > > > > Signed-off-by: Christopher M. Riedl <cmr@linux.ibm.com> > > > > --- > > > > v5: * Only support Book3s64 Radix MMU for now. > > * Use a per-cpu datastructure to hold the patching_addr and > > patching_mm to avoid the need for a synchronization lock/mutex. > > > > v4: * In the previous series this was two separate patches: one to init > > the temporary mm in poking_init() (unused in powerpc at the time) > > and the other to use it for patching (which removed all the > > per-cpu vmalloc code). Now that we use poking_init() in the > > existing per-cpu vmalloc approach, that separation doesn't work > > as nicely anymore so I just merged the two patches into one. > > * Preload the SLB entry and hash the page for the patching_addr > > when using Hash on book3s64 to avoid taking an SLB and Hash fault > > during patching. The previous implementation was a hack which > > changed current->mm to allow the SLB and Hash fault handlers to > > work with the temporary mm since both of those code-paths always > > assume mm == current->mm. > > * Also (hmm - seeing a trend here) with the book3s64 Hash MMU we > > have to manage the mm->context.active_cpus counter and mm cpumask > > since they determine (via mm_is_thread_local()) if the TLB flush > > in pte_clear() is local or not - it should always be local when > > we're using the temporary mm. On book3s64's Radix MMU we can > > just call local_flush_tlb_mm(). > > * Use HPTE_USE_KERNEL_KEY on Hash to avoid costly lock/unlock of > > KUAP. > > --- > > arch/powerpc/lib/code-patching.c | 132 +++++++++++++++++++++++++++++-- > > 1 file changed, 125 insertions(+), 7 deletions(-) > > > > diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c > > index 9f2eba9b70ee4..027dabd42b8dd 100644 > > --- a/arch/powerpc/lib/code-patching.c > > +++ b/arch/powerpc/lib/code-patching.c > > @@ -11,6 +11,7 @@ > > #include <linux/cpuhotplug.h> > > #include <linux/slab.h> > > #include <linux/uaccess.h> > > +#include <linux/random.h> > > > > #include <asm/tlbflush.h> > > #include <asm/page.h> > > @@ -103,6 +104,7 @@ static inline void unuse_temporary_mm(struct temp_mm *temp_mm) > > > > static DEFINE_PER_CPU(struct vm_struct *, text_poke_area); > > static DEFINE_PER_CPU(unsigned long, cpu_patching_addr); > > +static DEFINE_PER_CPU(struct mm_struct *, cpu_patching_mm); > > > > #if IS_BUILTIN(CONFIG_LKDTM) > > unsigned long read_cpu_patching_addr(unsigned int cpu) > > @@ -133,6 +135,51 @@ static int text_area_cpu_down(unsigned int cpu) > > return 0; > > } > > > > +static __always_inline void __poking_init_temp_mm(void) > > +{ > > + int cpu; > > + spinlock_t *ptl; /* for protecting pte table */ > > + pte_t *ptep; > > + struct mm_struct *patching_mm; > > + unsigned long patching_addr; > > + > > + for_each_possible_cpu(cpu) { > > + /* > > + * Some parts of the kernel (static keys for example) depend on > > + * successful code patching. Code patching under > > + * STRICT_KERNEL_RWX requires this setup - otherwise we cannot > > + * patch at all. We use BUG_ON() here and later since an early > > + * failure is preferred to buggy behavior and/or strange > > + * crashes later. > > + */ > > + patching_mm = copy_init_mm(); > > + BUG_ON(!patching_mm); > > Read > https://www.kernel.org/doc/html/latest/process/deprecated.html#bug-and-bug-on > and > https://github.com/linuxppc/issues/issues/88 > > Avoid BUG_ON()s thanks. > Fine, @mpe's reply on the GH issue says the check is probably redundant: "In general we don't need to BUG_ON(!ptr), the MMU will catch NULL pointer dereferences for us." > > + > > + per_cpu(cpu_patching_mm, cpu) = patching_mm; > > + > > + /* > > + * Choose a randomized, page-aligned address from the range: > > + * [PAGE_SIZE, DEFAULT_MAP_WINDOW - PAGE_SIZE] The lower > > + * address bound is PAGE_SIZE to avoid the zero-page. The > > + * upper address bound is DEFAULT_MAP_WINDOW - PAGE_SIZE to > > + * stay under DEFAULT_MAP_WINDOW with the Book3s64 Hash MMU. > > + */ > > + patching_addr = PAGE_SIZE + ((get_random_long() & PAGE_MASK) > > + % (DEFAULT_MAP_WINDOW - 2 * PAGE_SIZE)); > > % should be at the end of first line and the second line alignment > should match open parenthesis in > first line. Ok - thanks! > > > + > > + per_cpu(cpu_patching_addr, cpu) = patching_addr; > > + > > + /* > > + * PTE allocation uses GFP_KERNEL which means we need to > > + * pre-allocate the PTE here because we cannot do the > > + * allocation during patching when IRQs are disabled. > > + */ > > + ptep = get_locked_pte(patching_mm, patching_addr, &ptl); > > + BUG_ON(!ptep); > > Avoid BUG_ON() please > Yup, I'll remove these in the next spin. > > > + pte_unmap_unlock(ptep, ptl); > > + } > > +} > > + > > /* > > * Although BUG_ON() is rude, in this case it should only happen if ENOMEM, and > > * we judge it as being preferable to a kernel that will crash later when > > @@ -140,6 +187,11 @@ static int text_area_cpu_down(unsigned int cpu) > > */ > > void __init poking_init(void) > > { > > + if (radix_enabled()) { > > + __poking_init_temp_mm(); > > + return; > > + } > > + > > BUG_ON(!cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, > > "powerpc/text_poke:online", text_area_cpu_up, > > text_area_cpu_down)); > > @@ -213,30 +265,96 @@ static inline int unmap_patch_area(void) > > return -EINVAL; > > } > > > > +struct patch_mapping { > > + spinlock_t *ptl; /* for protecting pte table */ > > + pte_t *ptep; > > + struct temp_mm temp_mm; > > +}; > > + > > +/* > > + * This can be called for kernel text or a module. > > + */ > > +static int map_patch(const void *addr, struct patch_mapping *patch_mapping) > > +{ > > + struct page *page; > > + pte_t pte; > > + pgprot_t pgprot; > > + struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm); > > + unsigned long patching_addr = __this_cpu_read(cpu_patching_addr); > > + > > + if (is_vmalloc_or_module_addr(addr)) > > + page = vmalloc_to_page(addr); > > + else > > + page = virt_to_page(addr); > > + > > + patch_mapping->ptep = get_locked_pte(patching_mm, patching_addr, > > + &patch_mapping->ptl); > > Not sure you need to split this line, checkpatch now allows 100 chars > per line. > I prefer sticking to 80 columns unless readability *really* improves by going over that limit. > > > + if (unlikely(!patch_mapping->ptep)) { > > + pr_warn("map patch: failed to allocate pte for patching\n"); > > That's a lot better than all above BUG_ONs > > > > + return -1; > > + } > > + > > + pgprot = PAGE_KERNEL; > > + pte = mk_pte(page, pgprot); > > + pte = pte_mkdirty(pte); > > I'm sure you can do > > pte = pte_mkdirty(mk_pte(page, PAGE_KERNEL)); > > And indeed PAGE_KERNEL already includes _PAGE_DIRTY, so all you should > need is > > pte = mk_pte(page, PAGE_KERNEL); > > Or even just > > set_pte_at(patching_mm, patching_addr, patch_mapping->ptep, mk_pte(page, > PAGE_KERNEL)); > Ok, I'll consolidate this in the next spin. Thanks! > > > + set_pte_at(patching_mm, patching_addr, patch_mapping->ptep, pte); > > + > > + init_temp_mm(&patch_mapping->temp_mm, patching_mm); > > + use_temporary_mm(&patch_mapping->temp_mm); > > + > > + return 0; > > +} > > + > > +static void unmap_patch(struct patch_mapping *patch_mapping) > > +{ > > + struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm); > > + unsigned long patching_addr = __this_cpu_read(cpu_patching_addr); > > + > > + pte_clear(patching_mm, patching_addr, patch_mapping->ptep); > > + > > + local_flush_tlb_mm(patching_mm); > > + > > + pte_unmap_unlock(patch_mapping->ptep, patch_mapping->ptl); > > + > > + unuse_temporary_mm(&patch_mapping->temp_mm); > > Shouldn't you stop using it before unmapping/unlocking it ? > Yes I think you're right - IIRC I had to do this for the Hash MMU (which we don't support w/ this verion of the series anymore anyways). I'll revisit this for the next spin. > > > +} > > + > > static int do_patch_instruction(u32 *addr, struct ppc_inst instr) > > { > > int err, rc = 0; > > u32 *patch_addr = NULL; > > unsigned long flags; > > + struct patch_mapping patch_mapping; > > > > /* > > - * During early early boot patch_instruction is called > > - * when text_poke_area is not ready, but we still need > > - * to allow patching. We just do the plain old patching > > + * During early early boot patch_instruction is called when the > > + * patching_mm/text_poke_area is not ready, but we still need to allow > > + * patching. We just do the plain old patching. > > */ > > - if (!this_cpu_read(text_poke_area)) > > - return raw_patch_instruction(addr, instr); > > + if (radix_enabled()) { > > + if (!this_cpu_read(cpu_patching_mm)) > > + return raw_patch_instruction(addr, instr); > > + } else { > > + if (!this_cpu_read(text_poke_area)) > > + return raw_patch_instruction(addr, instr); > > + } > > > > local_irq_save(flags); > > > > - err = map_patch_area(addr); > > + if (radix_enabled()) > > + err = map_patch(addr, &patch_mapping); > > Maybe call it map_patch_mm() or map_patch_mapping() ? Yes that does sound better. > > > + else > > + err = map_patch_area(addr); > > if (err) > > goto out; > > > > patch_addr = (u32 *)(__this_cpu_read(cpu_patching_addr) | offset_in_page(addr)); > > rc = __patch_instruction(addr, instr, patch_addr); > > > > - err = unmap_patch_area(); > > + if (radix_enabled()) > > + unmap_patch(&patch_mapping); > > No err ? Would be better if it could return something, allthough always > 0. Ok I'll do that. > > And same comment about naming. > Yes I'll use your suggested names. > > + else > > + err = unmap_patch_area(); > > > > out: > > local_irq_restore(flags); > >
diff --git a/arch/powerpc/lib/code-patching.c b/arch/powerpc/lib/code-patching.c index 9f2eba9b70ee4..027dabd42b8dd 100644 --- a/arch/powerpc/lib/code-patching.c +++ b/arch/powerpc/lib/code-patching.c @@ -11,6 +11,7 @@ #include <linux/cpuhotplug.h> #include <linux/slab.h> #include <linux/uaccess.h> +#include <linux/random.h> #include <asm/tlbflush.h> #include <asm/page.h> @@ -103,6 +104,7 @@ static inline void unuse_temporary_mm(struct temp_mm *temp_mm) static DEFINE_PER_CPU(struct vm_struct *, text_poke_area); static DEFINE_PER_CPU(unsigned long, cpu_patching_addr); +static DEFINE_PER_CPU(struct mm_struct *, cpu_patching_mm); #if IS_BUILTIN(CONFIG_LKDTM) unsigned long read_cpu_patching_addr(unsigned int cpu) @@ -133,6 +135,51 @@ static int text_area_cpu_down(unsigned int cpu) return 0; } +static __always_inline void __poking_init_temp_mm(void) +{ + int cpu; + spinlock_t *ptl; /* for protecting pte table */ + pte_t *ptep; + struct mm_struct *patching_mm; + unsigned long patching_addr; + + for_each_possible_cpu(cpu) { + /* + * Some parts of the kernel (static keys for example) depend on + * successful code patching. Code patching under + * STRICT_KERNEL_RWX requires this setup - otherwise we cannot + * patch at all. We use BUG_ON() here and later since an early + * failure is preferred to buggy behavior and/or strange + * crashes later. + */ + patching_mm = copy_init_mm(); + BUG_ON(!patching_mm); + + per_cpu(cpu_patching_mm, cpu) = patching_mm; + + /* + * Choose a randomized, page-aligned address from the range: + * [PAGE_SIZE, DEFAULT_MAP_WINDOW - PAGE_SIZE] The lower + * address bound is PAGE_SIZE to avoid the zero-page. The + * upper address bound is DEFAULT_MAP_WINDOW - PAGE_SIZE to + * stay under DEFAULT_MAP_WINDOW with the Book3s64 Hash MMU. + */ + patching_addr = PAGE_SIZE + ((get_random_long() & PAGE_MASK) + % (DEFAULT_MAP_WINDOW - 2 * PAGE_SIZE)); + + per_cpu(cpu_patching_addr, cpu) = patching_addr; + + /* + * PTE allocation uses GFP_KERNEL which means we need to + * pre-allocate the PTE here because we cannot do the + * allocation during patching when IRQs are disabled. + */ + ptep = get_locked_pte(patching_mm, patching_addr, &ptl); + BUG_ON(!ptep); + pte_unmap_unlock(ptep, ptl); + } +} + /* * Although BUG_ON() is rude, in this case it should only happen if ENOMEM, and * we judge it as being preferable to a kernel that will crash later when @@ -140,6 +187,11 @@ static int text_area_cpu_down(unsigned int cpu) */ void __init poking_init(void) { + if (radix_enabled()) { + __poking_init_temp_mm(); + return; + } + BUG_ON(!cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "powerpc/text_poke:online", text_area_cpu_up, text_area_cpu_down)); @@ -213,30 +265,96 @@ static inline int unmap_patch_area(void) return -EINVAL; } +struct patch_mapping { + spinlock_t *ptl; /* for protecting pte table */ + pte_t *ptep; + struct temp_mm temp_mm; +}; + +/* + * This can be called for kernel text or a module. + */ +static int map_patch(const void *addr, struct patch_mapping *patch_mapping) +{ + struct page *page; + pte_t pte; + pgprot_t pgprot; + struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm); + unsigned long patching_addr = __this_cpu_read(cpu_patching_addr); + + if (is_vmalloc_or_module_addr(addr)) + page = vmalloc_to_page(addr); + else + page = virt_to_page(addr); + + patch_mapping->ptep = get_locked_pte(patching_mm, patching_addr, + &patch_mapping->ptl); + if (unlikely(!patch_mapping->ptep)) { + pr_warn("map patch: failed to allocate pte for patching\n"); + return -1; + } + + pgprot = PAGE_KERNEL; + pte = mk_pte(page, pgprot); + pte = pte_mkdirty(pte); + set_pte_at(patching_mm, patching_addr, patch_mapping->ptep, pte); + + init_temp_mm(&patch_mapping->temp_mm, patching_mm); + use_temporary_mm(&patch_mapping->temp_mm); + + return 0; +} + +static void unmap_patch(struct patch_mapping *patch_mapping) +{ + struct mm_struct *patching_mm = __this_cpu_read(cpu_patching_mm); + unsigned long patching_addr = __this_cpu_read(cpu_patching_addr); + + pte_clear(patching_mm, patching_addr, patch_mapping->ptep); + + local_flush_tlb_mm(patching_mm); + + pte_unmap_unlock(patch_mapping->ptep, patch_mapping->ptl); + + unuse_temporary_mm(&patch_mapping->temp_mm); +} + static int do_patch_instruction(u32 *addr, struct ppc_inst instr) { int err, rc = 0; u32 *patch_addr = NULL; unsigned long flags; + struct patch_mapping patch_mapping; /* - * During early early boot patch_instruction is called - * when text_poke_area is not ready, but we still need - * to allow patching. We just do the plain old patching + * During early early boot patch_instruction is called when the + * patching_mm/text_poke_area is not ready, but we still need to allow + * patching. We just do the plain old patching. */ - if (!this_cpu_read(text_poke_area)) - return raw_patch_instruction(addr, instr); + if (radix_enabled()) { + if (!this_cpu_read(cpu_patching_mm)) + return raw_patch_instruction(addr, instr); + } else { + if (!this_cpu_read(text_poke_area)) + return raw_patch_instruction(addr, instr); + } local_irq_save(flags); - err = map_patch_area(addr); + if (radix_enabled()) + err = map_patch(addr, &patch_mapping); + else + err = map_patch_area(addr); if (err) goto out; patch_addr = (u32 *)(__this_cpu_read(cpu_patching_addr) | offset_in_page(addr)); rc = __patch_instruction(addr, instr, patch_addr); - err = unmap_patch_area(); + if (radix_enabled()) + unmap_patch(&patch_mapping); + else + err = unmap_patch_area(); out: local_irq_restore(flags);
When code patching a STRICT_KERNEL_RWX kernel the page containing the address to be patched is temporarily mapped as writeable. Currently, a per-cpu vmalloc patch area is used for this purpose. While the patch area is per-cpu, the temporary page mapping is inserted into the kernel page tables for the duration of patching. The mapping is exposed to CPUs other than the patching CPU - this is undesirable from a hardening perspective. Use a temporary mm instead which keeps the mapping local to the CPU doing the patching. Use the `poking_init` init hook to prepare a temporary mm and patching address. Initialize the temporary mm by copying the init mm. Choose a randomized patching address inside the temporary mm userspace address space. The patching address is randomized between PAGE_SIZE and DEFAULT_MAP_WINDOW-PAGE_SIZE. Bits of entropy with 64K page size on BOOK3S_64: bits of entropy = log2(DEFAULT_MAP_WINDOW_USER64 / PAGE_SIZE) PAGE_SIZE=64K, DEFAULT_MAP_WINDOW_USER64=128TB bits of entropy = log2(128TB / 64K) bits of entropy = 31 The upper limit is DEFAULT_MAP_WINDOW due to how the Book3s64 Hash MMU operates - by default the space above DEFAULT_MAP_WINDOW is not available. Currently the Hash MMU does not use a temporary mm so technically this upper limit isn't necessary; however, a larger randomization range does not further "harden" this overall approach and future work may introduce patching with a temporary mm on Hash as well. Randomization occurs only once during initialization at boot for each possible CPU in the system. Introduce two new functions, map_patch() and unmap_patch(), to respectively create and remove the temporary mapping with write permissions at patching_addr. Map the page with PAGE_KERNEL to set EAA[0] for the PTE which ignores the AMR (so no need to unlock/lock KUAP) according to PowerISA v3.0b Figure 35 on Radix. Based on x86 implementation: commit 4fc19708b165 ("x86/alternatives: Initialize temporary mm for patching") and: commit b3fd8e83ada0 ("x86/alternatives: Use temporary mm for text poking") Signed-off-by: Christopher M. Riedl <cmr@linux.ibm.com> --- v5: * Only support Book3s64 Radix MMU for now. * Use a per-cpu datastructure to hold the patching_addr and patching_mm to avoid the need for a synchronization lock/mutex. v4: * In the previous series this was two separate patches: one to init the temporary mm in poking_init() (unused in powerpc at the time) and the other to use it for patching (which removed all the per-cpu vmalloc code). Now that we use poking_init() in the existing per-cpu vmalloc approach, that separation doesn't work as nicely anymore so I just merged the two patches into one. * Preload the SLB entry and hash the page for the patching_addr when using Hash on book3s64 to avoid taking an SLB and Hash fault during patching. The previous implementation was a hack which changed current->mm to allow the SLB and Hash fault handlers to work with the temporary mm since both of those code-paths always assume mm == current->mm. * Also (hmm - seeing a trend here) with the book3s64 Hash MMU we have to manage the mm->context.active_cpus counter and mm cpumask since they determine (via mm_is_thread_local()) if the TLB flush in pte_clear() is local or not - it should always be local when we're using the temporary mm. On book3s64's Radix MMU we can just call local_flush_tlb_mm(). * Use HPTE_USE_KERNEL_KEY on Hash to avoid costly lock/unlock of KUAP. --- arch/powerpc/lib/code-patching.c | 132 +++++++++++++++++++++++++++++-- 1 file changed, 125 insertions(+), 7 deletions(-)