[PATCH] ALSA: pcm: Fix tight loop of OSS capture stream

[PATCH] ALSA: pcm: Fix tight loop of OSS capture stream

[Takashi Iwai]

When the trigger=off is passed for a PCM OSS stream, it sets the
start_threshold of the substream to the boundary.  This can be
problematic for a capture stream, unfortunately, like the following,
as detected by syzkaller:
- In __snd_pcm_lib_xfer() that is invoked from snd_pcm_oss_read()
  loop, we have a check whether the stream was already started or the
  stream can be auto-started.
- With trigger=0, we explicitly disable the auto-start, and the
  function returns 0 at this check.
- The loop continues tightly, which may lead to an RCU stall.

This patch fixes the bug by simply allowing the wait for non-started
stream in the case of OSS capture.  For native usages, it's supposed
to be done by the caller side (which is user-space), hence it returns
zero like before.

(In theory, this could wait there, but I'd like to stay in a safer
 side for not breaking the existing stuff.)

Reported-by: syzbot+fbe0496f92a0ce7b786c@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 sound/core/pcm_lib.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
index 40013b26f671..6c99fa8ac5fa 100644
--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -2112,6 +2112,13 @@ int pcm_lib_apply_appl_ptr(struct snd_pcm_substream *substream,
 	return 0;
 }
 
+/* allow waiting for a capture stream that hasn't been started */
+#if IS_ENABLED(CONFIG_SND_PCM_OSS)
+#define wait_capture_start(substream)	((substream)->oss.oss)
+#else
+#define wait_capture_start(substream)	false
+#endif
+
 /* the common loop for read/write data */
 snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
 				     void *data, bool interleaved,
@@ -2182,7 +2189,7 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
 			err = snd_pcm_start(substream);
 			if (err < 0)
 				goto _end_unlock;
-		} else {
+		} else if (!wait_capture_start(substream)) {
 			/* nothing to do */
 			err = 0;
 			goto _end_unlock;
-- 
2.16.4