Re: [PATCH v3] perf: Fix use-after-free when perf mmap() revival races with the last munmap()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Aug 31, 2026 at 09:31:52PM +0800, Yilin Zhang wrote:
> 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>
> Tested-by: Weiming Shi <shiweiming@moonshot.ai>
> Signed-off-by: Yilin Zhang <yilinzhang@moonshot.ai>
> ---
> v2: https://lore.kernel.org/linux-perf-users/91825d0335d2f7cc017ee23886ffeef44c1cc633.f061f277.0045.493a.937e.53242f19a7de@feishu.cn/T/#u
> v3: swap the counter drop order instead of serializing both under
>     mmap_mutex; keeps the lockless fast path, drops detach_rest, and
>     references the earlier independent fix [0]

I presume this means you and your AI agree with my pre-wakeup-juice
morning musings?

Also, you seem to have lost the 'helpful' comments that I drafted :-(


>  kernel/events/core.c | 16 ++++++----------
>  1 file changed, 6 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index a6c8e38a3110..f56f9d9e4f01 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 +7059,14 @@ 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);
> +	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