Re: INFO: rcu detected stall in snd_pcm_oss_write3 (2)

[Takashi Iwai]

On Fri, 06 Apr 2018 23:14:01 +0200,
syzbot wrote:
> 
> syzbot has found reproducer for the following crash on upstream commit
> 38c23685b273cfb4ccf31a199feccce3bdcb5d83 (Fri Apr 6 04:29:35 2018 +0000)
> Merge tag 'armsoc-drivers' of
> git://git.kernel.org/pub/scm/linux/kernel/git/arm/arm-soc
> syzbot dashboard link:
> https://syzkaller.appspot.com/bug?extid=150189c103427d31a053
> 
> So far this crash happened 15 times on upstream.
> C reproducer: https://syzkaller.appspot.com/x/repro.c?id=5405588854931456
> syzkaller reproducer:
> https://syzkaller.appspot.com/x/repro.syz?id=5561439796330496
> Raw console output:
> https://syzkaller.appspot.com/x/log.txt?id=5697900571000832
> Kernel config:
> https://syzkaller.appspot.com/x/.config?id=-5813481738265533882
> compiler: gcc (GCC) 8.0.1 20180301 (experimental)

Thanks to the reproducer, I could spot out now.
Below is the patch (which was submitted as well).


thanks,

Takashi

-- 8< --
From: Takashi Iwai <tiwai@xxxxxxx>
Subject: [PATCH] ALSA: pcm: Fix endless loop for XRUN recovery in OSS
 emulation

The commit 02a5d6925cd3 ("ALSA: pcm: Avoid potential races between OSS
ioctls and read/write") split the PCM preparation code to a locked
version, and it added a sanity check of runtime->oss.prepare flag
along with the change.  This leaded to an endless loop when the stream
gets XRUN: namely, snd_pcm_oss_write3() and co call
snd_pcm_oss_prepare() without setting runtime->oss.prepare flag and
the loop continues until the PCM state reaches to another one.

As the function is supposed to execute the preparation
unconditionally, drop the invalid state check there.

The bug was triggered by syzkaller.

Fixes: 02a5d6925cd3 ("ALSA: pcm: Avoid potential races between OSS ioctls and read/write")
Reported-by: syzbot+7e3f31a52646f939c052@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: syzbot+4f2016cf5185da7759dc@xxxxxxxxxxxxxxxxxxxxxxxxx
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
---
 sound/core/oss/pcm_oss.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/sound/core/oss/pcm_oss.c b/sound/core/oss/pcm_oss.c
index 481ab0e94ffa..727647755aab 100644
--- a/sound/core/oss/pcm_oss.c
+++ b/sound/core/oss/pcm_oss.c
@@ -1128,13 +1128,12 @@ static int snd_pcm_oss_get_active_substream(struct snd_pcm_oss_file *pcm_oss_fil
 }
 
 /* call with params_lock held */
+/* NOTE: this doesn't care whether runtime->oss.prepare is set or not */
 static int snd_pcm_oss_prepare(struct snd_pcm_substream *substream)
 {
 	int err;
 	struct snd_pcm_runtime *runtime = substream->runtime;
 
-	if (!runtime->oss.prepare)
-		return 0;
 	err = snd_pcm_kernel_ioctl(substream, SNDRV_PCM_IOCTL_PREPARE, NULL);
 	if (err < 0) {
 		pcm_dbg(substream->pcm,
-- 
2.16.3