[alsa-devel] BUG: unable to handle kernel paging request in snd_seq_oss_readq_puts

[Takashi Iwai]

On Wed, 01 Nov 2017 19:39:46 +0100,
Dmitry Vyukov wrote:
> 
> On Wed, Nov 1, 2017 at 9:38 PM, syzbot
> <bot+31681772ec7a18dde8d3f8caf475f361a89b9514 at syzkaller.appspotmail.com>
> wrote:
> > Hello,
> >
> > syzkaller hit the following crash on
> > fc2e8b1a47c14b22c33eb087fca0db58e1f4ed0e
> > git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master
> > compiler: gcc (GCC) 7.1.1 20170620
> > .config is attached
> > Raw console output is attached.
> > C reproducer is attached
> > syzkaller reproducer is attached. See https://goo.gl/kgGztJ
> > for information about syzkaller reproducers
> 
> This also happened on more recent commits, including upstream
> 9c323bff13f92832e03657cabdd70d731408d621 (Oct 20):

Could you try the patch below with CONFIG_SND_DEBUG=y and see whether
it catches any bad calls?  It's already in for-next branch for 4.15.


thanks,

Takashi

-- 8< --
From: Takashi Iwai <tiwai at suse.de>
Subject: [PATCH] ALSA: seq: Add sanity check for user-space pointer delivery

The sequencer event may contain a user-space pointer with its
SNDRV_SEQ_EXT_USRPTR bit, and we assure that its delivery is limited
with non-atomic mode.  Otherwise the copy_from_user() may hit the
fault and cause a problem.  Although the core code doesn't set such a
flag (only set at snd_seq_write()), any wild driver may set it
mistakenly and lead to an unexpected crash.

This patch adds a sanity check of such events at the delivery core
code to filter out the invalid invocation in the atomic mode.

Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 sound/core/seq/seq_clientmgr.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index ea2d0ae85bd3..f2343f63ba26 100644
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -802,6 +802,10 @@ static int snd_seq_deliver_event(struct snd_seq_client *client, struct snd_seq_e
 		return -EMLINK;
 	}
 
+	if (snd_seq_ev_is_variable(event) &&
+	    snd_BUG_ON(atomic && (event->data.ext.len & SNDRV_SEQ_EXT_USRPTR)))
+		return -EINVAL;
+
 	if (event->queue == SNDRV_SEQ_ADDRESS_SUBSCRIBERS ||
 	    event->dest.client == SNDRV_SEQ_ADDRESS_SUBSCRIBERS)
 		result = deliver_to_subscribers(client, event, atomic, hop);
-- 
2.14.2