Commit d06bf78e authored Jan 19, 2026 by Will Rosenberg Committed by Peter Zijlstra Jan 21, 2026

perf: Fix refcount warning on event->mmap_count increment



When calling refcount_inc(&event->mmap_count) inside perf_mmap_rb(), the
following warning is triggered:

        refcount_t: addition on 0; use-after-free.
        WARNING: lib/refcount.c:25

PoC:

    struct perf_event_attr attr = {0};
    int fd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0);
    mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
    int victim = syscall(__NR_perf_event_open, &attr, 0, -1, fd,
                         PERF_FLAG_FD_OUTPUT);
    mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED, victim, 0);

This occurs when creating a group member event with the flag
PERF_FLAG_FD_OUTPUT. The group leader should be mmap-ed and then mmap-ing
the event triggers the warning.

Since the event has copied the output_event in perf_event_set_output(),
event->rb is set. As a result, perf_mmap_rb() calls
refcount_inc(&event->mmap_count) when event->mmap_count = 0.

Disallow the case when event->mmap_count = 0. This also prevents two
events from updating the same user_page.

Fixes: 448f97fb ("perf: Convert mmap() refcounts to refcount_t")
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Rosenberg <whrosenb@asu.edu>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260119184956.801238-1-whrosenb@asu.edu

parent 24d479d2

kernel/events/core.c

+9 −0

Original line number	Diff line number	Diff line
		@@ -6997,6 +6997,15 @@ static int perf_mmap_rb(struct vm_area_struct vma, struct perf_event event,
		if (data_page_nr(event->rb) != nr_pages)
		return -EINVAL;

		/*
		* If this event doesn't have mmap_count, we're attempting to
		* create an alias of another event's mmap(); this would mean
		* both events will end up scribbling the same user_page;
		* which makes no sense.
		*/
		if (!refcount_read(&event->mmap_count))
		return -EBUSY;

		if (refcount_inc_not_zero(&event->rb->mmap_count)) {
		/*
		* Success -- managed to mmap() the same buffer