perf: Fix use-after-free when perf mmap() revival races with the last munmap()

perf_mmap_close() drops rb->mmap_count *without* holding
event->mmap_mutex (the refcount_dec_and_test() right before the
refcount_dec_and_mutex_lock() of event->mmap_count). A concurrent
perf_mmap_rb() can slot its entire "revival" path into that window
(perf_mmap holds event->mmap_mutex for its whole duration, including
rb_alloc):

  munmap side (perf_mmap_close)          mmap side (perf_mmap_rb)
  -----------------------------------    --------------------------------
  rb->mmap_count 1 -> 0   (no lock)      (holds event->mmap_mutex)
                                         inc_not_zero(rb->mmap_count) fails
                                         ring_buffer_attach(event, NULL)
                                         rb_alloc() + attach new rb
                                         refcount_set(&event->mmap_count, 1)
  lock; event->mmap_count 1 -> 0
  ring_buffer_attach(event, NULL)
  ring_buffer_put() -> frees the *new* rb

The revival's refcount_set(&event->mmap_count, 1) is an invisible
1 -> 1 write: the close frees the just-revived buffer although the
other process still has it mapped -- a page-level use-after-free
allowing local privilege escalation to root by any unprivileged user
(default kernel.perf_event_paranoid=2).

Swap the order of the two counter updates: event->mmap_count is
dropped first via refcount_dec_and_mutex_lock(), so its 1 -> 0
transition and the ring_buffer_attach() stay serialized with
perf_mmap(). rb->mmap_count == 0 then implies every event using the
buffer is detached already, so the result of the rb->mmap_count drop
can gate the remaining teardown directly and detach_rest is no longer
needed.

An earlier fix for this race from Kyle Zeng and David Lee takes
event->mmap_mutex around both counter updates [0]; here the not-last
close stays lockless.

Fixes: 59741451b49c ("perf: Identify the 0->1 transition for event::mmap_count")
Reported-by: Kimi Security Team <bug-report@moonshot.ai>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Co-developed-by: Weiming Shi <shiweiming@moonshot.ai>
Signed-off-by: Weiming Shi <shiweiming@moonshot.ai>
Signed-off-by: Yilin Zhang <yilinzhang@moonshot.ai>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/linux-perf-users/20260804060931.711308-1-david.lee@trailofbits.com/ [0]
Cc: <stable@vger.kernel.org>
Cc: stable@vger.kernel.org # 6.18+
Link: https://patch.msgid.link/20260831162155.1437652-1-yilinzhang@moonshot.ai

1 files changed, 10 insertions, 10 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index a6c8e38a3110..f02780529b43 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7029,7 +7029,6 @@ static void perf_mmap_close(struct vm_area_struct *vma)
 	mapped_f unmapped = get_mapped(event, event_unmapped);
 	struct perf_buffer *rb = ring_buffer_get(event);
 	struct user_struct *mmap_user = rb->mmap_user;
-	bool detach_rest = false;
 
 	/* FIXIES vs perf_pmu_unregister() */
 	if (unmapped)
@@ -7060,17 +7059,18 @@ static void perf_mmap_close(struct vm_area_struct *vma)
 		mutex_unlock(&rb->aux_mutex);
 	}
 
-	if (refcount_dec_and_test(&rb->mmap_count))
-		detach_rest = true;
-
-	if (!refcount_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex))
-		goto out_put;
-
-	ring_buffer_attach(event, NULL);
-	mutex_unlock(&event->mmap_mutex);
+	/*
+	 * Drop references in reverse order of perf_mmap() to prevent
+	 * rb revival after rb->mmap_count reaches zero.
+	 */
+	if (refcount_dec_and_mutex_lock(&event->mmap_count,
+					&event->mmap_mutex)) {
+		ring_buffer_attach(event, NULL);
+		mutex_unlock(&event->mmap_mutex);
+	}
 
 	/* If there's still other mmap()s of this buffer, we're done. */
-	if (!detach_rest)
+	if (!refcount_dec_and_test(&rb->mmap_count))
 		goto out_put;
 
 	/*