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authorPeter Zijlstra <peterz@infradead.org>2017-01-11 21:09:50 +0100
committerIngo Molnar <mingo@kernel.org>2017-01-14 10:56:11 +0100
commit321027c1fe77f892f4ea07846aeae08cefbbb290 (patch)
treee90493b64144be2e6e9564dca1afe870d393765c
parent63cae12bce9861cec309798d34701cf3da20bc71 (diff)
perf/core: Fix concurrent sys_perf_event_open() vs. 'move_group' race
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.net Signed-off-by: Ingo Molnar <mingo@kernel.org>
-rw-r--r--kernel/events/core.c58
1 files changed, 54 insertions, 4 deletions
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 72ce7d63e561..cbc5937265da 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -9529,6 +9529,37 @@ static int perf_event_set_clock(struct perf_event *event, clockid_t clk_id)
return 0;
}
+/*
+ * 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 = READ_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
*
@@ -9772,12 +9803,31 @@ SYSCALL_DEFINE5(perf_event_open,
}
if (move_group) {
- gctx = group_leader->ctx;
- mutex_lock_double(&gctx->mutex, &ctx->mutex);
+ gctx = __perf_event_ctx_lock_double(group_leader, ctx);
+
if (gctx->task == TASK_TOMBSTONE) {
err = -ESRCH;
goto err_locked;
}
+
+ /*
+ * Check if we raced against another sys_perf_event_open() call
+ * moving the software group underneath us.
+ */
+ if (!(group_leader->group_caps & PERF_EV_CAP_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;
+ }
+ }
} else {
mutex_lock(&ctx->mutex);
}
@@ -9879,7 +9929,7 @@ SYSCALL_DEFINE5(perf_event_open,
perf_unpin_context(ctx);
if (move_group)
- mutex_unlock(&gctx->mutex);
+ perf_event_ctx_unlock(group_leader, gctx);
mutex_unlock(&ctx->mutex);
if (task) {
@@ -9905,7 +9955,7 @@ SYSCALL_DEFINE5(perf_event_open,
err_locked:
if (move_group)
- mutex_unlock(&gctx->mutex);
+ perf_event_ctx_unlock(group_leader, gctx);
mutex_unlock(&ctx->mutex);
/* err_file: */
fput(event_file);