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

0 files changed, 0 insertions, 0 deletions