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[72.200.102.19]) by smtp.googlemail.com with ESMTPSA id 5a478bee46e88-2b16f18e0f2sm213853eec.12.2026.01.05.08.54.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Jan 2026 08:54:36 -0800 (PST) From: Will Rosenberg To: Cc: yi1.lai@linux.intel.com, Will Rosenberg , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Namhyung Kim , Mark Rutland , Alexander Shishkin , Jiri Olsa , Ian Rogers , Adrian Hunter , James Clark , Lorenzo Stoakes , Thomas Gleixner , linux-perf-users@vger.kernel.org (open list:PERFORMANCE EVENTS SUBSYSTEM), linux-kernel@vger.kernel.org (open list:PERFORMANCE EVENTS SUBSYSTEM) Subject: [PATCH v2] perf: Fix refcount warning on event->mmap_count increment Date: Mon, 5 Jan 2026 09:51:49 -0700 Message-Id: <20260105165149.30200-1-whrosenb@asu.edu> X-Mailer: git-send-email 2.34.1 In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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 =3D {0}; int fd =3D syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0); mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); int victim =3D 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 =3D 0. Account for the case when event->mmap_count =3D 0. This patch goes against the design philosophy of the refcount library by re-enabling an empty refcount, but the patch remains inline with the current treatment of mmap_count. Fixes: 448f97fba901 ("perf: Convert mmap() refcounts to refcount_t") Signed-off-by: Will Rosenberg --- Notes: v1 -> v2: Add Fixes tag =20 I also have a related concern about code that handles the mmap_count. In perf_mmap_close(), if refcount_dec_and_mutex_lock() decrements event->mmap_count to zero, then event->rb is set to NULL. This effectively undos our ring buffer copy. However, is this desired behavior? Should event->rb remain unchanged since it may still be mmap-ed by other events and can still be used? kernel/events/core.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/events/core.c b/kernel/events/core.c index 376fb07d869b..49709b627b1f 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -7279,7 +7279,8 @@ static int perf_mmap_rb(struct vm_area_struct *vma, s= truct perf_event *event, * multiple times. */ perf_mmap_account(vma, user_extra, extra); - refcount_inc(&event->mmap_count); + if (!refcount_inc_not_zero(&event->mmap_count)) + refcount_set(&event->mmap_count, 1); return 0; } =20 base-commit: 538254cd98afb31b09c4cc58219217d8127c79be --=20 2.34.1