This is a note to let you know that I've just added the patch titled perf/core: Fix race in the perf_mmap_close() function to the 4.14-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: perf-core-fix-race-in-the-perf_mmap_close-function.patch and it can be found in the queue-4.14 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From f91072ed1b7283b13ca57fcfbece5a3b92726143 Mon Sep 17 00:00:00 2001 From: Jiri Olsa <jolsa@xxxxxxxxxx> Date: Wed, 16 Sep 2020 13:53:11 +0200 Subject: perf/core: Fix race in the perf_mmap_close() function From: Jiri Olsa <jolsa@xxxxxxxxxx> commit f91072ed1b7283b13ca57fcfbece5a3b92726143 upstream. There's a possible race in perf_mmap_close() when checking ring buffer's mmap_count refcount value. The problem is that the mmap_count check is not atomic because we call atomic_dec() and atomic_read() separately. perf_mmap_close: ... atomic_dec(&rb->mmap_count); ... if (atomic_read(&rb->mmap_count)) goto out_put; <ring buffer detach> free_uid out_put: ring_buffer_put(rb); /* could be last */ The race can happen when we have two (or more) events sharing same ring buffer and they go through atomic_dec() and then they both see 0 as refcount value later in atomic_read(). Then both will go on and execute code which is meant to be run just once. The code that detaches ring buffer is probably fine to be executed more than once, but the problem is in calling free_uid(), which will later on demonstrate in related crashes and refcount warnings, like: refcount_t: addition on 0; use-after-free. ... RIP: 0010:refcount_warn_saturate+0x6d/0xf ... Call Trace: prepare_creds+0x190/0x1e0 copy_creds+0x35/0x172 copy_process+0x471/0x1a80 _do_fork+0x83/0x3a0 __do_sys_wait4+0x83/0x90 __do_sys_clone+0x85/0xa0 do_syscall_64+0x5b/0x1e0 entry_SYSCALL_64_after_hwframe+0x44/0xa9 Using atomic decrease and check instead of separated calls. Tested-by: Michael Petlan <mpetlan@xxxxxxxxxx> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> Acked-by: Namhyung Kim <namhyung@xxxxxxxxxx> Acked-by: Wade Mealing <wmealing@xxxxxxxxxx> Fixes: 9bb5d40cd93c ("perf: Fix mmap() accounting hole"); Link: https://lore.kernel.org/r/20200916115311.GE2301783@krava [sudip: used ring_buffer] Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- kernel/events/core.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -5207,11 +5207,11 @@ static void perf_pmu_output_stop(struct static void perf_mmap_close(struct vm_area_struct *vma) { struct perf_event *event = vma->vm_file->private_data; - struct ring_buffer *rb = ring_buffer_get(event); struct user_struct *mmap_user = rb->mmap_user; int mmap_locked = rb->mmap_locked; unsigned long size = perf_data_size(rb); + bool detach_rest = false; if (event->pmu->event_unmapped) event->pmu->event_unmapped(event, vma->vm_mm); @@ -5242,7 +5242,8 @@ static void perf_mmap_close(struct vm_ar mutex_unlock(&event->mmap_mutex); } - atomic_dec(&rb->mmap_count); + if (atomic_dec_and_test(&rb->mmap_count)) + detach_rest = true; if (!atomic_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex)) goto out_put; @@ -5251,7 +5252,7 @@ static void perf_mmap_close(struct vm_ar mutex_unlock(&event->mmap_mutex); /* If there's still other mmap()s of this buffer, we're done. */ - if (atomic_read(&rb->mmap_count)) + if (!detach_rest) goto out_put; /* Patches currently in stable-queue which might be from jolsa@xxxxxxxxxx are queue-4.14/perf-core-fix-race-in-the-perf_mmap_close-function.patch queue-4.14/perf-core-fix-bad-use-of-igrab.patch queue-4.14/perf-core-fix-crash-when-using-hw-tracing-kernel-filters.patch