From d06bf78e55d5159c1b00072e606ab924ffbbad35 Mon Sep 17 00:00:00 2001 From: Will Rosenberg Date: Mon, 19 Jan 2026 11:49:56 -0700 Subject: [PATCH] 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 Signed-off-by: Will Rosenberg Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260119184956.801238-1-whrosenb@asu.edu --- kernel/events/core.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/kernel/events/core.c b/kernel/events/core.c index 5b5cb620499e..a0fa488bce84 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 -- 2.47.3