[PATCH v4] perf: Fix use-after-free when perf mmap() revival races with the last munmap()

[PATCH v4] perf: Fix use-after-free when perf mmap() revival races with the last munmap()

[Yilin Zhang]

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")
Link: https://lore.kernel.org/linux-perf-users/20260804060931.711308-1-david.lee@trailofbits.com/ [0]
Cc: stable@vger.kernel.org # 6.18+
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>
---
v3: https://lore.kernel.org/linux-perf-users/20260831133152.1231045-1-yilinzhang@moonshot.ai/
v4: restore the ordering comment; no functional change

 kernel/events/core.c | 20 ++++++++++----------
 1 file 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;
 
 	/*
-- 
2.43.0

Re: [PATCH v4] perf: Fix use-after-free when perf mmap() revival races with the last munmap()

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] Preexisting memory accounting leak for locked_vm when mmap() is called multiple times on the same perf event (alias mmaps).
--

commit a96fa5a03867a00fbbf2c3b6dd3bf82931c0fdde
Author: Yilin Zhang <yilinzhang@moonshot.ai>

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

This commit swaps the order of counter updates in perf_mmap_close() to fix
a page-level use-after-free. It drops event->mmap_count first, serializing
its 1 -> 0 transition and ring_buffer_attach() with perf_mmap(), preventing
buffer revival after rb->mmap_count reaches zero.

Link: https://lore.kernel.org/linux-perf-users/20260804060931.711308-1-david.lee@trailofbits.com/

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index a6c8e38a31104..f02780529b439 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
[ ... ]
> @@ -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;

[Severity: High]
This is a pre-existing issue, but does this goto bypass the unaccounting for
all but the final close?

If an application calls mmap() on the same perf event file descriptor multiple
times, perf_mmap_rb() accounts user_extra to current_user()->locked_vm on
every alias mmap:

kernel/events/core.c:perf_mmap_rb() {
    ...
			perf_mmap_account(vma, user_extra, extra);
			refcount_inc(&event->mmap_count);
			return 0;
    ...
}

When munmap() is called for all of them, perf_mmap_close() only calls
perf_mmap_unaccount() when rb->mmap_count drops to zero, which happens only
once.

Could this permanently leak locked_vm charges on every alias munmap,
potentially exhausting the user's RLIMIT_MEMLOCK quota and causing a Denial
of Service for other subsystems (like BPF or io_uring) relying on it?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260831162155.1437652-1-yilinzhang@moonshot.ai?part=1