linux-kernel - Re: sound: use-after-free in hrtimer

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Date:	Mon, 06 Jun 2016 18:39:01 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	Jaroslav Kysela <perex@...ex.cz>, alsa-devel@...a-project.org,
	LKML <linux-kernel@...r.kernel.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Kostya Serebryany <kcc@...gle.com>,
	syzkaller <syzkaller@...glegroups.com>
Subject: Re: sound: use-after-free in hrtimer_cancel

On Mon, 06 Jun 2016 18:29:25 +0200,
Dmitry Vyukov wrote:
> 
> On Mon, Jun 6, 2016 at 4:11 PM, Takashi Iwai <tiwai@...e.de> wrote:
> > On Sat, 04 Jun 2016 20:27:50 +0200,
> > Dmitry Vyukov wrote:
> >>
> >> On Sat, Jun 4, 2016 at 8:00 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
> >> > Hello,
> >> >
> >> > The following program triggers use-after-free:
> >>
> >> Forget to mention that you need to run it in a tight parallel loop. It
> >> takes around 5 minutes to reproduce for me.
> >
> > Hmm, this again is a bug that is difficult to trigger...  At least, I
> > couldn't reproduce locally.  How many processes are you running with
> > stress program?
> 
> I use a VM with 4 cores and use 20 parallel test processes.
> 
> > It seems that there is nothing more than opening /dev/audio and does
> > some mmap in the job.  Is there any other relevant thing there?
> 
> 
> I think poll with timeout is related. It is poll who sets hrtimer, right?

If it's about snd-dummy driver, hrtimer is created at open, and
started/stopped at PCM trigger, and removed at close.

Is there any good way to decode which syscalls are executed in the
test code?


Takashi

> > Also, this assumes that the first sound card is Dummy driver, right?
> > Check /proc/asound/cards.
> 
> # cat /proc/asound/cards
>  0 [Dummy          ]: Dummy - Dummy
>                       Dummy 1
>  1 [Loopback       ]: Loopback - Loopback
>                       Loopback 1
>  2 [VirMIDI        ]: VirMIDI - VirMIDI
>                       Virtual MIDI Card 1
>  3 [pcsp           ]: PC-Speaker - pcsp
>                       Internal PC-Speaker at port 0x61
>  4 [Intel          ]: HDA-Intel - HDA Intel
>                       HDA Intel at 0xfebf0000 irq 24
> 
> 
> 
> > If it's about snd-dummy driver, one blind shot would be a patch like
> > below.  But even if it would fix, it doesn't explain why it's
> > triggered in that way...
> >
> >
> > thanks,
> >
> > Takashi
> >
> > ---
> > diff --git a/sound/drivers/dummy.c b/sound/drivers/dummy.c
> > index c0f8f613f1f1..172dacd925f5 100644
> > --- a/sound/drivers/dummy.c
> > +++ b/sound/drivers/dummy.c
> > @@ -420,6 +420,7 @@ static int dummy_hrtimer_stop(struct snd_pcm_substream *substream)
> >
> >  static inline void dummy_hrtimer_sync(struct dummy_hrtimer_pcm *dpcm)
> >  {
> > +       hrtimer_cancel(&dpcm->timer);
> >         tasklet_kill(&dpcm->tasklet);
> >  }
> >
>