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Date: Wed, 03 Feb 2016 12:39:31 +0100
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From: Takashi Iwai <tiwai@suse.de>
To: "Dmitry Vyukov" <dvyukov@google.com>
Cc: <alsa-devel@alsa-project.org>, "Jaroslav Kysela" <perex@perex.cz>,
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        "Kostya Serebryany" <kcc@google.com>,
        "syzkaller" <syzkaller@googlegroups.com>,
        "Sasha Levin" <sasha.levin@oracle.com>
Subject: Re: sound: out-of-bounds write in snd_rawmidi_kernel_write1
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On Wed, 03 Feb 2016 10:41:14 +0100,
Takashi Iwai wrote:
> 
> On Wed, 03 Feb 2016 10:35:14 +0100,
> Takashi Iwai wrote:
> > 
> > On Wed, 03 Feb 2016 09:57:50 +0100,
> > Dmitry Vyukov wrote:
> > > 
> > > Hello,
> > > 
> > > The following program triggers an out-of-bounds write in
> > > snd_rawmidi_kernel_write1 (run in parallel loop). It seems to try to
> > > copy -1 bytes (aka 4GB) from user space into kernel smashing all on
> > > its way.
> > 
> > What card is /dev/midi3?  Please check /proc/asound/cards.
> > Is it MTPAV?
> 
> In anyway the patch below should paper over it.  But it's still
> strange that it gets a negative value there.  Could you put
> 
>    WARN_ON(count1 < 0)
> 
> before the newly added check?
> 
> I tried it locally with virmidi but it didn't appear, so far.  Maybe
> my setup is too slow and has fewer CPUs than yours.

Scratch my previous patch, I could reproduce the issue on a faster
machine in my office now :)  Will work on it.


Takashi