It turns out there's a hole in perf_remove_from_context() due to event_function_call() not calling its function when the task associated with the event is already dead.

In this case the event will have been detached from the task, but the grouping will have been retained, such that group operations might still work properly while there are live child events etc.

This does however mean that we can miss a perf_group_detach() call when the group decomposes, this in turn can then lead to use-after-free.

Fix it by explicitly doing the group detach if its still required.

Reported-by Dmitry Vyukov <[email protected]>

Tested-by Dmitry Vyukov <[email protected]>

Signed-off-by Peter Zijlstra (Intel) <[email protected]>

Cc Alexander Shishkin <[email protected]>

Cc Arnaldo Carvalho de Melo <[email protected]>

Cc Jiri Olsa <[email protected]>

Cc Linus Torvalds <[email protected]>

Cc Mathieu Desnoyers <[email protected]>

Cc Peter Zijlstra <[email protected]>

Cc Thomas Gleixner <[email protected]>

Cc syzkaller <[email protected]>

Fixes 63b6da39bb38 ("perf: Fix perf_event_exit_task() race")

Link http://lkml.kernel.org/r/[email protected] open here

Signed-off-by Ingo Molnar <[email protected]>

Signed-off-by Greg Kroah-Hartman <[email protected]>

---
 kernel/events/core.c |   27 +++++++++++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

kernel/events/core.c +25 -2

@@ -1469,7 +1469,6 @@ ctx_group_list(struct perf_event *event,
 static void
 list_add_event(struct perf_event *event, struct perf_event_context *ctx)
 {
-
 	lockdep_assert_held(&ctx->lock);
 
 	WARN_ON_ONCE(event->attach_state & PERF_ATTACH_CONTEXT);

@@ -1624,6 +1623,8 @@ static void perf_group_attach(struct per
 {
 	struct perf_event *group_leader = event->group_leader, *pos;
 
+	lockdep_assert_held(&event->ctx->lock);
+
 	/*
 	 * We can have double attach due to group movement in perf_event_open.
 	 */

@@ -1697,6 +1698,8 @@ static void perf_group_detach(struct per
 	struct perf_event *sibling, *tmp;
 	struct list_head *list = NULL;
 
+	lockdep_assert_held(&event->ctx->lock);
+
 	/*
 	 * We can have double detach due to exit/hot-unplug + close.
 	 */

@@ -1895,9 +1898,29 @@ __perf_remove_from_context(struct perf_e
  */
 static void perf_remove_from_context(struct perf_event *event, unsigned long flags)
 {
-	lockdep_assert_held(&event->ctx->mutex);
+	struct perf_event_context *ctx = event->ctx;
+
+	lockdep_assert_held(&ctx->mutex);
 
 	event_function_call(event, __perf_remove_from_context, (void *)flags);
+
+	/*
+	 * The above event_function_call() can NO-OP when it hits
+	 * TASK_TOMBSTONE. In that case we must already have been detached
+	 * from the context (by perf_event_exit_event()) but the grouping
+	 * might still be in-tact.
+	 */
+	WARN_ON_ONCE(event->attach_state & PERF_ATTACH_CONTEXT);
+	if ((flags & DETACH_GROUP) &&
+	    (event->attach_state & PERF_ATTACH_GROUP)) {
+		/*
+		 * Since in that case we cannot possibly be scheduled, simply
+		 * detach now.
+		 */
+		raw_spin_lock_irq(&ctx->lock);
+		perf_group_detach(event);
+		raw_spin_unlock_irq(&ctx->lock);
+	}
 }
 
 /*

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/perf-core-fix-perf_record_mmap2-prot-flags-for-anonymous-memory.patch queue-4.9/perf-core-fix-use-after-free-bug.patch queue-4.9/x86-efi-always-map-the-first-physical-page-into-the-efi-pagetables.patch