linux-kernel - Re: sound: out-of-bounds write in snd_rawmidi_kernel

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Date:	Wed, 03 Feb 2016 10:41:14 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	"Dmitry Vyukov" <dvyukov@...gle.com>
Cc:	<alsa-devel@...a-project.org>, "Jaroslav Kysela" <perex@...ex.cz>,
	"LKML" <linux-kernel@...r.kernel.org>,
	"Alexander Potapenko" <glider@...gle.com>,
	"Kostya Serebryany" <kcc@...gle.com>,
	"syzkaller" <syzkaller@...glegroups.com>,
	"Sasha Levin" <sasha.levin@...cle.com>
Subject: Re: sound: out-of-bounds write in snd_rawmidi_kernel_write1

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.


thanks,

Takashi

---
diff --git a/sound/core/rawmidi.c b/sound/core/rawmidi.c
index 26ca02248885..2fef77d9de50 100644
--- a/sound/core/rawmidi.c
+++ b/sound/core/rawmidi.c
@@ -1239,6 +1239,8 @@ static long snd_rawmidi_kernel_write1(struct snd_rawmidi_substream *substream,
 	}
 	while (count > 0 && runtime->avail > 0) {
 		count1 = runtime->buffer_size - runtime->appl_ptr;
+		if (count1 <= 0)
+			break;
 		if (count1 > count)
 			count1 = count;
 		if (count1 > (long)runtime->avail)