From: Jiri Olsa jolsa@redhat.com

mainline inclusion from mainline-v5.10 commit f91072ed1b7283b13ca57fcfbece5a3b92726143 category: bugfix bugzilla: NA CVE: CVE-2020-14351

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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@redhat.com Signed-off-by: Jiri Olsa jolsa@kernel.org Signed-off-by: Ingo Molnar mingo@kernel.org Acked-by: Peter Zijlstra a.p.zijlstra@chello.nl Acked-by: Namhyung Kim namhyung@kernel.org Acked-by: Wade Mealing wmealing@redhat.com Fixes: 9bb5d40cd93c ("perf: Fix mmap() accounting hole"); Link: https://lore.kernel.org/r/20200916115311.GE2301783@krava Signed-off-by: Yang Yingliang yangyingliang@huawei.com Conflicts: kernel/events/core.c [yyl: adjust context] Reviewed-by: Jian Cheng cj.chengjian@huawei.com Reviewed-by: Jason Yan yanaijie@huawei.com Signed-off-by: Yang Yingliang yangyingliang@huawei.com --- kernel/events/core.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c index 30a81d1b7b5c..452775f68129 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -5496,11 +5496,11 @@ static void perf_pmu_output_stop(struct perf_event *event); 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); @@ -5531,7 +5531,8 @@ static void perf_mmap_close(struct vm_area_struct *vma) 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; @@ -5540,7 +5541,7 @@ static void perf_mmap_close(struct vm_area_struct *vma) 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;

/*