[PATCH] ALSA: pcm: Fix race between non-atomic ops and trigger-start

We protect the races of the concurrent state transitions between
atomic PCM ops, but the checks between the non-atomic ops (hw_params,
hw_free and prepare) and the atomic ops aren't perfect; there is a
check of the conflicting PCM state at the beginning of hw_params & co,
but the atomic PCM ops can be still issued during the non-atomic PCM
operations.  An example such scenario is that a thread A re-issues the
PREPARE or HW_PARAMS for the already prepared stream, while another
thread B triggers the PCM start in the middle of the prepare
operation.  Although this usually doesn't lead to much serious issues,
it can give some inconsistency as reported by syzkaller (such as
ODEBUG warning).

There are various atomic PCM ops, and basically the only problem is
the PCM start as it operates from the PREPARED state.  Other trigger
commands (stop, etc) are for the running or the other special state,
hence they are filtered as pre-condition.

This patch is for preventing the PCM trigger-start during the non-
atomic operations in order to address the problems above.
Fortunately, the hw_params, hw_free and prepare operations call
snd_pcm_buffer_access_lock(), and this can be used for checking the
concurrent operations at the PCM trigger -- which sets the
runtime->buffer_accessing to a negative (if possible), so the PCM
trigger just needs to check the runtime->buffer_accessing value; if
it's negative, it means the concurrent non-atomic PCM ops is running.

Reported-by: [email protected]
Closes: https://lore.kernel.org/[email protected]
Cc: <[email protected]>
Signed-off-by: Takashi Iwai <[email protected]>
---
 sound/core/pcm_native.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 4a5057e7629d..20ae67949e05 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -1468,6 +1468,8 @@ static int snd_pcm_pre_start(struct snd_pcm_substream *substream,
 	struct snd_pcm_runtime *runtime = substream->runtime;
 	if (runtime->state != SNDRV_PCM_STATE_PREPARED)
 		return -EBADFD;
+	if (atomic_read(&runtime->buffer_accessing) < 0)
+		return -EBADFD; /* during hw_params, hw_free or prepare */
 	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK &&
 	    !snd_pcm_playback_data(substream))
 		return -EPIPE;
-- 
2.55.0