[tip: perf/urgent] perf: Fix refcount warning on event->mmap_count increment

[tip-bot2 for Will Rosenberg]

The following commit has been merged into the perf/urgent branch of tip:

Commit-ID:     d06bf78e55d5159c1b00072e606ab924ffbbad35
Gitweb:        https://git.kernel.org/tip/d06bf78e55d5159c1b00072e606ab924ffbbad35
Author:        Will Rosenberg <whrosenb@xxxxxxx>
AuthorDate:    Mon, 19 Jan 2026 11:49:56 -07:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Wed, 21 Jan 2026 16:28:58 +01:00

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: 448f97fba901 ("perf: Convert mmap() refcounts to refcount_t")
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Will Rosenberg <whrosenb@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://patch.msgid.link/20260119184956.801238-1-whrosenb@xxxxxxx
---
 kernel/events/core.c |  9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 5b5cb62..a0fa488 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -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