ALSA: pcm: Fix endless loop for XRUN recovery in OSS emulation

[ Upstream commit e15dc99dbb9cf99f6432e8e3c0b3a8f7a3403a86 ]

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+150189c103427d31a053@syzkaller.appspotmail.com
Reported-by: syzbot+7e3f31a52646f939c052@syzkaller.appspotmail.com
Reported-by: syzbot+4f2016cf5185da7759dc@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>

diff --git a/sound/core/oss/pcm_oss.c b/sound/core/oss/pcm_oss.c
index 6b7f7da3f3946d3ce590b7ac4c3058e871da6fa4..7b2719acbeba3a24bc6900962c3577ddb9b6cc53 100644 (file)
--- a/sound/core/oss/pcm_oss.c
+++ b/sound/core/oss/pcm_oss.c
@@ -1138,13 +1138,14 @@ static int snd_pcm_oss_get_active_substream(struct snd_pcm_oss_file *pcm_oss_fil
 }
 
 /* call with params_lock held */
+/* NOTE: this always call PREPARE unconditionally no matter 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,