linux-kernel - Re: [PATCHv2] perf: Fix race in perf_mmap

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Date:   Wed, 16 Sep 2020 16:38:19 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     peterz@...radead.org
Cc:     Namhyung Kim <namhyung@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Wade Mealing <wmealing@...hat.com>
Subject: Re: [PATCHv2] perf: Fix race in perf_mmap_close function

On Wed, Sep 16, 2020 at 03:54:02PM +0200, peterz@...radead.org wrote:
> On Wed, Sep 16, 2020 at 01:53:11PM +0200, Jiri Olsa wrote:
> > 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.
> > This fixes CVE-2020-14351.
> 
> I'm tempted to remove that line; I can never seem to find anything
> useful in a CVE.

I was asked by security guys to add this, Wade?

> 
> Fixes: ?

right, sry..

Fixes: 9bb5d40cd93c ("perf: Fix mmap() accounting hole");

thanks,
jirka

> 
> > Acked-by: Namhyung Kim <namhyung@...nel.org>
> > Tested-by: Michael Petlan <mpetlan@...hat.com>
> > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
>