Re: WARNING in snd_pcm_hw_params

[Takashi Iwai]

On Mon, 09 Apr 2018 17:25:15 +0200,
Dmitry Vyukov wrote:
> 
> On Mon, Apr 9, 2018 at 5:18 PM, Takashi Iwai <tiwai@xxxxxxx> wrote:
> > On Fri, 06 Apr 2018 02:02:02 +0200,
> > syzbot wrote:
> >>
> >> WARNING: CPU: 0 PID: 4468 at sound/core/pcm_native.c:620
> >> snd_pcm_hw_params_choose sound/core/pcm_native.c:620 [inline]
> >
> > This is a WARN_ON() to catch the inconsistent hw_params call.
> > This shouldn't happen in theory, but the reality looks weirder.
> >
> > The necessary "fix" is just to get rid of snd_BUG_ON(), but still
> > wonder which condition it hits.  If it's an empty parameter, the
> > previous snd_pcm_hw_refine() should have caught and returned the error
> > beforehand...
> 
> Hi Takashi,
> 
> The reproducer for this is trivially simple:
> 
> https://syzkaller.appspot.com/x/repro.syz?id=4913841943085056
> 
> (and I guess perf_event_open is red herring).
> 
> But the really important part there is probably this: Fault:true
> FaultCall:2 FaultNth:18
> And you can see where it actually happened in the raw console log:
> 
> Raw console output: https://syzkaller.appspot.com/x/log.txt?id=6314122077536256
> 
> Does it explain things?

Yes, it does.  So it's the kmalloc error from snd_pcm_hw_refine().
I overlooked this, as this kmalloc is relatively new, and that
possibility wasn't considered in the WARN_ON() in question.

I'll rephrase the changelog text.


Thanks!

Takashi

> 
> > In anyway this can't be a real crash, let's remove the warning to stop
> > the chorus of syzbot birds.
> >
> >
> > thanks,
> >
> > Takashi
> >
> > -- 8< --
> > From: Takashi Iwai <tiwai@xxxxxxx>
> > Subject: [PATCH] ALSA: pcm: Remove WARN_ON() at snd_pcm_hw_params() error
> >
> > snd_pcm_hw_params() may hit WARN_ON() when an inconsistent parameter
> > set is passed.  Although this should have been caught by the former
> > snd_pcm_hw_refine() and returned the error, syzkaller succeeded to
> > pass through and hit the WARN_ON(), which in turn made it panic
> > unnecessarily.
> >
> > For reducing this kind of noise, let's drop snd_BUG_ON() check.
> >
> > Reported-by: syzbot+803e0047ac3a3096bb4f@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
> > ---
> >  sound/core/pcm_native.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> > index b84554893fab..35ffccea94c3 100644
> > --- a/sound/core/pcm_native.c
> > +++ b/sound/core/pcm_native.c
> > @@ -617,7 +617,7 @@ static int snd_pcm_hw_params_choose(struct snd_pcm_substream *pcm,
> >                         changed = snd_pcm_hw_param_first(pcm, params, *v, NULL);
> >                 else
> >                         changed = snd_pcm_hw_param_last(pcm, params, *v, NULL);
> > -               if (snd_BUG_ON(changed < 0))
> > +               if (changed < 0)
> >                         return changed;
> >                 if (changed == 0)
> >                         continue;
> > --
> > 2.16.3
> >
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