From: Peter Zijlstra peterz@infradead.org

commit 321027c1fe77f892f4ea07846aeae08cefbbb290 upstream.

Di Shen reported a race between two concurrent sys_perf_event_open() calls where both try and move the same pre-existing software group into a hardware context.

The problem is exactly that described in commit:

f63a8daa5812 ("perf: Fix event->ctx locking")

... where, while we wait for a ctx->mutex acquisition, the event->ctx relation can have changed under us.

That very same commit failed to recognise sys_perf_event_context() as an external access vector to the events and thereby didn't apply the established locking rules correctly.

So while one sys_perf_event_open() call is stuck waiting on mutex_lock_double(), the other (which owns said locks) moves the group about. So by the time the former sys_perf_event_open() acquires the locks, the context we've acquired is stale (and possibly dead).

Apply the established locking rules as per perf_event_ctx_lock_nested() to the mutex_lock_double() for the 'move_group' case. This obviously means we need to validate state after we acquire the locks.

Reported-by: Di Shen (Keen Lab) Tested-by: John Dias joaodias@google.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Cc: Alexander Shishkin alexander.shishkin@linux.intel.com Cc: Arnaldo Carvalho de Melo acme@kernel.org Cc: Arnaldo Carvalho de Melo acme@redhat.com Cc: Jiri Olsa jolsa@redhat.com Cc: Kees Cook keescook@chromium.org Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Min Chong mchong@google.com Cc: Peter Zijlstra peterz@infradead.org Cc: Stephane Eranian eranian@google.com Cc: Thomas Gleixner tglx@linutronix.de Cc: Vince Weaver vincent.weaver@maine.edu Fixes: f63a8daa5812 ("perf: Fix event->ctx locking") Link: http://lkml.kernel.org/r/20170106131444.GZ3174@twins.programming.kicks-ass.n... Signed-off-by: Ingo Molnar mingo@kernel.org [bwh: Backported to 3.16: - Use ACCESS_ONCE() instead of READ_ONCE() - Test perf_event::group_flags instead of group_caps - Add the err_locked cleanup block, which we didn't need before - Adjust context] Signed-off-by: Ben Hutchings ben@decadent.org.uk Signed-off-by: Suren Baghdasaryan surenb@google.com Signed-off-by: Amit Pundir amit.pundir@linaro.org --- This upstream patch is featured in recent Android Security bulletin. Picked up this backported patch from android-3.18. Build tested on 3.18.91

kernel/events/core.c | 61 ++++++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 57 insertions(+), 4 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c index 9b12efcefdf7..de3303aab7d6 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -7414,6 +7414,37 @@ static void mutex_lock_double(struct mutex *a, struct mutex *b) mutex_lock_nested(b, SINGLE_DEPTH_NESTING); }

+/* + * Variation on perf_event_ctx_lock_nested(), except we take two context + * mutexes. + */ +static struct perf_event_context * +__perf_event_ctx_lock_double(struct perf_event *group_leader, + struct perf_event_context *ctx) +{ + struct perf_event_context *gctx; + +again: + rcu_read_lock(); + gctx = ACCESS_ONCE(group_leader->ctx); + if (!atomic_inc_not_zero(&gctx->refcount)) { + rcu_read_unlock(); + goto again; + } + rcu_read_unlock(); + + mutex_lock_double(&gctx->mutex, &ctx->mutex); + + if (group_leader->ctx != gctx) { + mutex_unlock(&ctx->mutex); + mutex_unlock(&gctx->mutex); + put_ctx(gctx); + goto again; + } + + return gctx; +} + /** * sys_perf_event_open - open a performance event, associate it to a task/cpu * @@ -7626,14 +7657,31 @@ SYSCALL_DEFINE5(perf_event_open, }

if (move_group) { - gctx = group_leader->ctx; + gctx = __perf_event_ctx_lock_double(group_leader, ctx); + + /* + * Check if we raced against another sys_perf_event_open() call + * moving the software group underneath us. + */ + if (!(group_leader->group_flags & PERF_GROUP_SOFTWARE)) { + /* + * If someone moved the group out from under us, check + * if this new event wound up on the same ctx, if so + * its the regular !move_group case, otherwise fail. + */ + if (gctx != ctx) { + err = -EINVAL; + goto err_locked; + } else { + perf_event_ctx_unlock(group_leader, gctx); + move_group = 0; + } + }

/* * See perf_event_ctx_lock() for comments on the details * of swizzling perf_event::ctx. */ - mutex_lock_double(&gctx->mutex, &ctx->mutex); - perf_remove_from_context(group_leader, false);

/* @@ -7674,7 +7722,7 @@ SYSCALL_DEFINE5(perf_event_open, perf_unpin_context(ctx);

if (move_group) { - mutex_unlock(&gctx->mutex); + perf_event_ctx_unlock(group_leader, gctx); put_ctx(gctx); } mutex_unlock(&ctx->mutex); @@ -7703,6 +7751,11 @@ SYSCALL_DEFINE5(perf_event_open, fd_install(event_fd, event_file); return event_fd;

+err_locked: + if (move_group) + perf_event_ctx_unlock(group_leader, gctx); + mutex_unlock(&ctx->mutex); + fput(event_file); err_context: perf_unpin_context(ctx); put_ctx(ctx);