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SubjectRe: [PATCH] perf: Fix race in perf_mmap_close function
On Thu, Sep 10, 2020 at 10:48:02PM +0900, Namhyung Kim wrote:

SNIP

> > _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.
> > This fixes CVE-2020-14351.
> >
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> > kernel/events/core.c | 4 +---
> > 1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 7ed5248f0445..29313cc54d9e 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -5903,8 +5903,6 @@ static void perf_mmap_close(struct vm_area_struct *vma)
> > mutex_unlock(&event->mmap_mutex);
> > }
> >
> > - atomic_dec(&rb->mmap_count);
> > -
> > if (!atomic_dec_and_mutex_lock(&event->mmap_count, &event->mmap_mutex))
> > goto out_put;
>
> But when it takes the goto, rb->mmap_count won't decrement anymore..

event->mmap_count is per event, so if we have have race in here,
2 threads can go through with each event->mmap_count reaching zero

jirka

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