diff mbox series

[v2] tracepoint: Do not fail unregistering a probe due to memory allocation

Message ID 20201117211836.54acaef2@oasis.local.home
State Superseded
Headers show
Series [v2] tracepoint: Do not fail unregistering a probe due to memory allocation | expand

Commit Message

Steven Rostedt Nov. 18, 2020, 2:18 a.m. UTC 
From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

The list of tracepoint callbacks is managed by an array that is protected
by RCU. To update this array, a new array is allocated, the updates are
copied over to the new array, and then the list of functions for the
tracepoint is switched over to the new array. After a completion of an RCU
grace period, the old array is freed.

This process happens for both adding a callback as well as removing one.
But on removing a callback, if the new array fails to be allocated, the
callback is not removed, and may be used after it is freed by the clients
of the tracepoint.

There's really no reason to fail if the allocation for a new array fails
when removing a function. Instead, the function can simply be replaced by a
stub that will be ignored in the callback loop, and it will be cleaned up
on the next modification of the array.

Link: https://lore.kernel.org/r/20201115055256.65625-1-mmullins@mmlx.us
Link: https://lkml.kernel.org/r/20201116175107.02db396d@gandalf.local.home

Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Andrii Nakryiko <andriin@fb.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@chromium.org>
Cc: netdev <netdev@vger.kernel.org>
Cc: bpf <bpf@vger.kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
Fixes: 97e1c18e8d17b ("tracing: Kernel Tracepoints")
Reported-by: syzbot+83aa762ef23b6f0d1991@syzkaller.appspotmail.com
Reported-by: syzbot+d29e58bb557324e55e5e@syzkaller.appspotmail.com
Reported-by: Matt Mullins <mmullins@mmlx.us>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
Changes since v1:
   Use 1L value for stub function, and ignore calling it.

 include/linux/tracepoint.h |  9 ++++-
 kernel/tracepoint.c        | 80 +++++++++++++++++++++++++++++---------
 2 files changed, 69 insertions(+), 20 deletions(-)

Comments

Alexei Starovoitov Nov. 18, 2020, 4:54 a.m. UTC | #1 
On Tue, Nov 17, 2020 at 6:18 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
>
> The list of tracepoint callbacks is managed by an array that is protected
> by RCU. To update this array, a new array is allocated, the updates are
> copied over to the new array, and then the list of functions for the
> tracepoint is switched over to the new array. After a completion of an RCU
> grace period, the old array is freed.
>
> This process happens for both adding a callback as well as removing one.
> But on removing a callback, if the new array fails to be allocated, the
> callback is not removed, and may be used after it is freed by the clients
> of the tracepoint.
>
> There's really no reason to fail if the allocation for a new array fails
> when removing a function. Instead, the function can simply be replaced by a
> stub that will be ignored in the callback loop, and it will be cleaned up
> on the next modification of the array.
>
> Link: https://lore.kernel.org/r/20201115055256.65625-1-mmullins@mmlx.us
> Link: https://lkml.kernel.org/r/20201116175107.02db396d@gandalf.local.home
>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Martin KaFai Lau <kafai@fb.com>
> Cc: Song Liu <songliubraving@fb.com>
> Cc: Yonghong Song <yhs@fb.com>
> Cc: Andrii Nakryiko <andriin@fb.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: KP Singh <kpsingh@chromium.org>
> Cc: netdev <netdev@vger.kernel.org>
> Cc: bpf <bpf@vger.kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: stable@vger.kernel.org
> Fixes: 97e1c18e8d17b ("tracing: Kernel Tracepoints")
> Reported-by: syzbot+83aa762ef23b6f0d1991@syzkaller.appspotmail.com
> Reported-by: syzbot+d29e58bb557324e55e5e@syzkaller.appspotmail.com
> Reported-by: Matt Mullins <mmullins@mmlx.us>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
> Changes since v1:
>    Use 1L value for stub function, and ignore calling it.
>
>  include/linux/tracepoint.h |  9 ++++-
>  kernel/tracepoint.c        | 80 +++++++++++++++++++++++++++++---------
>  2 files changed, 69 insertions(+), 20 deletions(-)
>
> diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
> index 0f21617f1a66..2e06e05b9d2a 100644
> --- a/include/linux/tracepoint.h
> +++ b/include/linux/tracepoint.h
> @@ -33,6 +33,8 @@ struct trace_eval_map {
>
>  #define TRACEPOINT_DEFAULT_PRIO        10
>
> +#define TRACEPOINT_STUB                ((void *)0x1L)
> +
>  extern struct srcu_struct tracepoint_srcu;
>
>  extern int
> @@ -310,7 +312,12 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
>                 do {                                                    \
>                         it_func = (it_func_ptr)->func;                  \
>                         __data = (it_func_ptr)->data;                   \
> -                       ((void(*)(void *, proto))(it_func))(__data, args); \
> +                       /*                                              \
> +                        * Removed functions that couldn't be allocated \
> +                        * are replaced with TRACEPOINT_STUB.           \
> +                        */                                             \
> +                       if (likely(it_func != TRACEPOINT_STUB))         \
> +                               ((void(*)(void *, proto))(it_func))(__data, args); \

I think you're overreacting to the problem.
Adding run-time check to extremely unlikely problem seems wasteful.
99.9% of the time allocate_probes() will do kmalloc from slab of small
objects.
If that slab is out of memory it means it cannot allocate a single page.
In such case so many things will be failing to alloc that system
is unlikely operational. oom should have triggered long ago.
Imo Matt's approach to add __GFP_NOFAIL to allocate_probes()
when it's called from func_remove() is much better.
The error was reported by syzbot that was using
memory fault injections. ENOMEM in allocate_probes() was
never seen in real life and highly unlikely will ever be seen.
Steven Rostedt Nov. 18, 2020, 12:46 p.m. UTC | #2 
On Tue, 17 Nov 2020 20:54:24 -0800
Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:

> >  extern int
> > @@ -310,7 +312,12 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
> >                 do {                                                    \
> >                         it_func = (it_func_ptr)->func;                  \
> >                         __data = (it_func_ptr)->data;                   \
> > -                       ((void(*)(void *, proto))(it_func))(__data, args); \
> > +                       /*                                              \
> > +                        * Removed functions that couldn't be allocated \
> > +                        * are replaced with TRACEPOINT_STUB.           \
> > +                        */                                             \
> > +                       if (likely(it_func != TRACEPOINT_STUB))         \
> > +                               ((void(*)(void *, proto))(it_func))(__data, args); \  
> 
> I think you're overreacting to the problem.

I will disagree with that.

> Adding run-time check to extremely unlikely problem seems wasteful.

Show me a real benchmark that you can notice a problem here. I bet that
check is even within the noise of calling an indirect function. Especially
on a machine with retpolines.

> 99.9% of the time allocate_probes() will do kmalloc from slab of small
> objects.
> If that slab is out of memory it means it cannot allocate a single page.
> In such case so many things will be failing to alloc that system
> is unlikely operational. oom should have triggered long ago.
> Imo Matt's approach to add __GFP_NOFAIL to allocate_probes()

Looking at the GFP_NOFAIL comment:

 * %__GFP_NOFAIL: The VM implementation _must_ retry infinitely: the caller
 * cannot handle allocation failures. The allocation could block
 * indefinitely but will never return with failure. Testing for
 * failure is pointless.
 * New users should be evaluated carefully (and the flag should be
 * used only when there is no reasonable failure policy) but it is
 * definitely preferable to use the flag rather than opencode endless
 * loop around allocator.

I realized I made a mistake in my patch for using it, as my patch is a
failure policy. It looks like something we want to avoid in general.

Thanks, I'll send a v3 that removes it.

> when it's called from func_remove() is much better.
> The error was reported by syzbot that was using
> memory fault injections. ENOMEM in allocate_probes() was
> never seen in real life and highly unlikely will ever be seen.

And the biggest thing you are missing here, is that if you are running on a
machine that has static calls, this code is never hit unless you have more
than one callback on a single tracepoint. That's because when there's only
one callback, it gets called directly, and this loop is not involved.

-- Steve
diff mbox series

Patch

diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 0f21617f1a66..2e06e05b9d2a 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -33,6 +33,8 @@  struct trace_eval_map {
 
 #define TRACEPOINT_DEFAULT_PRIO	10
 
+#define TRACEPOINT_STUB		((void *)0x1L)
+
 extern struct srcu_struct tracepoint_srcu;
 
 extern int
@@ -310,7 +312,12 @@  static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
 		do {							\
 			it_func = (it_func_ptr)->func;			\
 			__data = (it_func_ptr)->data;			\
-			((void(*)(void *, proto))(it_func))(__data, args); \
+			/*						\
+			 * Removed functions that couldn't be allocated \
+			 * are replaced with TRACEPOINT_STUB.		\
+			 */						\
+			if (likely(it_func != TRACEPOINT_STUB))		\
+				((void(*)(void *, proto))(it_func))(__data, args); \
 		} while ((++it_func_ptr)->func);			\
 		return 0;						\
 	}								\
diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index 3f659f855074..20ce78b3f578 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -53,10 +53,10 @@  struct tp_probes {
 	struct tracepoint_func probes[];
 };
 
-static inline void *allocate_probes(int count)
+static inline void *allocate_probes(int count, gfp_t extra_flags)
 {
 	struct tp_probes *p  = kmalloc(struct_size(p, probes, count),
-				       GFP_KERNEL);
+				       GFP_KERNEL | extra_flags);
 	return p == NULL ? NULL : p->probes;
 }
 
@@ -131,6 +131,7 @@  func_add(struct tracepoint_func **funcs, struct tracepoint_func *tp_func,
 {
 	struct tracepoint_func *old, *new;
 	int nr_probes = 0;
+	int stub_funcs = 0;
 	int pos = -1;
 
 	if (WARN_ON(!tp_func->func))
@@ -147,14 +148,34 @@  func_add(struct tracepoint_func **funcs, struct tracepoint_func *tp_func,
 			if (old[nr_probes].func == tp_func->func &&
 			    old[nr_probes].data == tp_func->data)
 				return ERR_PTR(-EEXIST);
+			if (old[nr_probes].func == TRACEPOINT_STUB)
+				stub_funcs++;
 		}
 	}
-	/* + 2 : one for new probe, one for NULL func */
-	new = allocate_probes(nr_probes + 2);
+	/* + 2 : one for new probe, one for NULL func - stub functions */
+	new = allocate_probes(nr_probes + 2 - stub_funcs, 0);
 	if (new == NULL)
 		return ERR_PTR(-ENOMEM);
 	if (old) {
-		if (pos < 0) {
+		if (stub_funcs) {
+			/* Need to copy one at a time to remove stubs */
+			int probes = 0;
+
+			pos = -1;
+			for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
+				if (old[nr_probes].func == TRACEPOINT_STUB)
+					continue;
+				if (pos < 0 && old[nr_probes].prio < prio)
+					pos = probes++;
+				new[probes++] = old[nr_probes];
+			}
+			nr_probes = probes;
+			if (pos < 0)
+				pos = probes;
+			else
+				nr_probes--; /* Account for insertion */
+
+		} else if (pos < 0) {
 			pos = nr_probes;
 			memcpy(new, old, nr_probes * sizeof(struct tracepoint_func));
 		} else {
@@ -188,8 +209,9 @@  static void *func_remove(struct tracepoint_func **funcs,
 	/* (N -> M), (N > 1, M >= 0) probes */
 	if (tp_func->func) {
 		for (nr_probes = 0; old[nr_probes].func; nr_probes++) {
-			if (old[nr_probes].func == tp_func->func &&
-			     old[nr_probes].data == tp_func->data)
+			if ((old[nr_probes].func == tp_func->func &&
+			     old[nr_probes].data == tp_func->data) ||
+			    old[nr_probes].func == TRACEPOINT_STUB)
 				nr_del++;
 		}
 	}
@@ -207,15 +229,33 @@  static void *func_remove(struct tracepoint_func **funcs,
 		int j = 0;
 		/* N -> M, (N > 1, M > 0) */
 		/* + 1 for NULL */
-		new = allocate_probes(nr_probes - nr_del + 1);
-		if (new == NULL)
-			return ERR_PTR(-ENOMEM);
-		for (i = 0; old[i].func; i++)
-			if (old[i].func != tp_func->func
-					|| old[i].data != tp_func->data)
-				new[j++] = old[i];
-		new[nr_probes - nr_del].func = NULL;
-		*funcs = new;
+		new = allocate_probes(nr_probes - nr_del + 1, __GFP_NOFAIL);
+		if (new) {
+			for (i = 0; old[i].func; i++)
+				if ((old[i].func != tp_func->func
+				     || old[i].data != tp_func->data)
+				    && old[i].func != TRACEPOINT_STUB)
+					new[j++] = old[i];
+			new[nr_probes - nr_del].func = NULL;
+			*funcs = new;
+		} else {
+			/*
+			 * Failed to allocate, replace the old function
+			 * with TRACEPOINT_STUB.
+			 */
+			for (i = 0; old[i].func; i++)
+				if (old[i].func == tp_func->func &&
+				    old[i].data == tp_func->data) {
+					old[i].func = TRACEPOINT_STUB;
+					/* Set the prio to the next event. */
+					if (old[i + 1].func)
+						old[i].prio =
+							old[i + 1].prio;
+					else
+						old[i].prio = -1;
+				}
+			*funcs = old;
+		}
 	}
 	debug_print_probes(*funcs);
 	return old;
@@ -295,10 +335,12 @@  static int tracepoint_remove_func(struct tracepoint *tp,
 	tp_funcs = rcu_dereference_protected(tp->funcs,
 			lockdep_is_held(&tracepoints_mutex));
 	old = func_remove(&tp_funcs, func);
-	if (IS_ERR(old)) {
-		WARN_ON_ONCE(PTR_ERR(old) != -ENOMEM);
+	if (WARN_ON_ONCE(IS_ERR(old)))
 		return PTR_ERR(old);
-	}
+
+	if (tp_funcs == old)
+		/* Failed allocating new tp_funcs, replaced func with stub */
+		return 0;
 
 	if (!tp_funcs) {
 		/* Removed last function */