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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Fri, 11 Sep 2020 12:05:10 +0900
Message-ID: <CAM9d7ciEAA_3Quo1-q7hU=Te+hBgJ2wYAjbDazXd7yS70HrhPA@mail.gmail.com>
Subject: Re: [PATCH] perf: Fix race in perf_mmap_close function
To:     Jiri Olsa <jolsa@redhat.com>
Cc:     Jiri Olsa <jolsa@kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Michael Petlan <mpetlan@redhat.com>,
        Wade Mealing <wmealing@redhat.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Hi Jiri,

On Thu, Sep 10, 2020 at 11:50 PM Jiri Olsa <jolsa@redhat.com> wrote:
>
> 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

Maybe I'm missing something.

But as far as I can see, perf_mmap_close() always decremented both
rb->mmap_count and event->mmap_count.  But with this change,
it seems not decrement rb->mmap_count when event->mmap_count
doesn't go to zero, right?

Thanks
Namhyung