ALSA: pcm: Fix potential AB/BA lock with buffer_mutex and mmap_lock

syzbot caught a potential deadlock between the PCM
runtime->buffer_mutex and the mm->mmap_lock.  It was brought by the
recent fix to cover the racy read/write and other ioctls, and in that
commit, I overlooked a (hopefully only) corner case that may take the
revert lock, namely, the OSS mmap.  The OSS mmap operation
exceptionally allows to re-configure the parameters inside the OSS
mmap syscall, where mm->mmap_mutex is already held.  Meanwhile, the
copy_from/to_user calls at read/write operations also take the
mm->mmap_lock internally, hence it may lead to a AB/BA deadlock.

A similar problem was already seen in the past and we fixed it with a
refcount (in commit b248371628aa).  The former fix covered only the
call paths with OSS read/write and OSS ioctls, while we need to cover
the concurrent access via both ALSA and OSS APIs now.

This patch addresses the problem above by replacing the buffer_mutex
lock in the read/write operations with a refcount similar as we've
used for OSS.  The new field, runtime->buffer_accessing, keeps the
number of concurrent read/write operations.  Unlike the former
buffer_mutex protection, this protects only around the
copy_from/to_user() calls; the other codes are basically protected by
the PCM stream lock.  The refcount can be a negative, meaning blocked
by the ioctls.  If a negative value is seen, the read/write aborts
with -EBUSY.  In the ioctl side, OTOH, they check this refcount, too,
and set to a negative value for blocking unless it's already being
accessed.

Reported-by: syzbot+6e5c88838328e99c7e1c@syzkaller.appspotmail.com
Fixes: dca947d4d26d ("ALSA: pcm: Fix races among concurrent read/write and buffer changes")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/000000000000381a0d05db622a81@google.com
Link: https://lore.kernel.org/r/20220330120903.4738-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 5 insertions, 4 deletions

diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
index a40a35e51fad..1fc7c50ffa62 100644
--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -1906,11 +1906,9 @@ static int wait_for_avail(struct snd_pcm_substream *substream,
 		if (avail >= runtime->twake)
 			break;
 		snd_pcm_stream_unlock_irq(substream);
-		mutex_unlock(&runtime->buffer_mutex);
 
 		tout = schedule_timeout(wait_time);
 
-		mutex_lock(&runtime->buffer_mutex);
 		snd_pcm_stream_lock_irq(substream);
 		set_current_state(TASK_INTERRUPTIBLE);
 		switch (runtime->status->state) {
@@ -2221,7 +2219,6 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
 
 	nonblock = !!(substream->f_flags & O_NONBLOCK);
 
-	mutex_lock(&runtime->buffer_mutex);
 	snd_pcm_stream_lock_irq(substream);
 	err = pcm_accessible_state(runtime);
 	if (err < 0)
@@ -2276,6 +2273,10 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
 			err = -EINVAL;
 			goto _end_unlock;
 		}
+		if (!atomic_inc_unless_negative(&runtime->buffer_accessing)) {
+			err = -EBUSY;
+			goto _end_unlock;
+		}
 		snd_pcm_stream_unlock_irq(substream);
 		if (!is_playback)
 			snd_pcm_dma_buffer_sync(substream, SNDRV_DMA_SYNC_CPU);
@@ -2284,6 +2285,7 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
 		if (is_playback)
 			snd_pcm_dma_buffer_sync(substream, SNDRV_DMA_SYNC_DEVICE);
 		snd_pcm_stream_lock_irq(substream);
+		atomic_dec(&runtime->buffer_accessing);
 		if (err < 0)
 			goto _end_unlock;
 		err = pcm_accessible_state(runtime);
@@ -2313,7 +2315,6 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(struct snd_pcm_substream *substream,
 	if (xfer > 0 && err >= 0)
 		snd_pcm_update_state(substream, runtime);
 	snd_pcm_stream_unlock_irq(substream);
-	mutex_unlock(&runtime->buffer_mutex);
 	return xfer > 0 ? (snd_pcm_sframes_t)xfer : err;
 }
 EXPORT_SYMBOL(__snd_pcm_lib_xfer);