perf/core: Fix concurrent sys_perf_event_open() vs. 'move_group' race (321027c1) · Commits · 戴 / test

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: ("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: ("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>

kernel/events/core.c

+54 −4

Original line number	Diff line number	Diff line
		@@ -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);

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