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 Signed-off-by: Takashi Iwai tiwai@suse.de --- include/sound/pcm.h | 1 + sound/core/pcm.c | 1 + sound/core/pcm_lib.c | 9 +++++---- sound/core/pcm_native.c | 39 ++++++++++++++++++++++++++++++++------- 4 files changed, 39 insertions(+), 11 deletions(-)

diff --git a/include/sound/pcm.h b/include/sound/pcm.h index 314f2779cab5..6b99310b5b88 100644 --- a/include/sound/pcm.h +++ b/include/sound/pcm.h @@ -402,6 +402,7 @@ struct snd_pcm_runtime { struct fasync_struct *fasync; bool stop_operating; /* sync_stop will be called */ struct mutex buffer_mutex; /* protect for buffer changes */ + atomic_t buffer_accessing; /* >0: in r/w operation, <0: blocked */

/* -- private section -- */ void *private_data; diff --git a/sound/core/pcm.c b/sound/core/pcm.c index edd9849210f2..977d54320a5c 100644 --- a/sound/core/pcm.c +++ b/sound/core/pcm.c @@ -970,6 +970,7 @@ int snd_pcm_attach_substream(struct snd_pcm *pcm, int stream,

runtime->status->state = SNDRV_PCM_STATE_OPEN; mutex_init(&runtime->buffer_mutex); + atomic_set(&runtime->buffer_accessing, 0);

substream->runtime = runtime; substream->private_data = pcm->private_data; 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); diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c index 704fdc9ebf91..4adaee62ef33 100644 --- a/sound/core/pcm_native.c +++ b/sound/core/pcm_native.c @@ -685,6 +685,24 @@ static int snd_pcm_hw_params_choose(struct snd_pcm_substream *pcm, return 0; }

+/* acquire buffer_mutex; if it's in r/w operation, return -EBUSY, otherwise + * block the further r/w operations + */ +static int snd_pcm_buffer_access_lock(struct snd_pcm_runtime *runtime) +{ + if (!atomic_dec_unless_positive(&runtime->buffer_accessing)) + return -EBUSY; + mutex_lock(&runtime->buffer_mutex); + return 0; /* keep buffer_mutex, unlocked by below */ +} + +/* release buffer_mutex and clear r/w access flag */ +static void snd_pcm_buffer_access_unlock(struct snd_pcm_runtime *runtime) +{ + mutex_unlock(&runtime->buffer_mutex); + atomic_inc(&runtime->buffer_accessing); +} + #if IS_ENABLED(CONFIG_SND_PCM_OSS) #define is_oss_stream(substream) ((substream)->oss.oss) #else @@ -695,14 +713,16 @@ static int snd_pcm_hw_params(struct snd_pcm_substream *substream, struct snd_pcm_hw_params *params) { struct snd_pcm_runtime *runtime; - int err = 0, usecs; + int err, usecs; unsigned int bits; snd_pcm_uframes_t frames;

if (PCM_RUNTIME_CHECK(substream)) return -ENXIO; runtime = substream->runtime; - mutex_lock(&runtime->buffer_mutex); + err = snd_pcm_buffer_access_lock(runtime); + if (err < 0) + return err; snd_pcm_stream_lock_irq(substream); switch (runtime->status->state) { case SNDRV_PCM_STATE_OPEN: @@ -820,7 +840,7 @@ static int snd_pcm_hw_params(struct snd_pcm_substream *substream, snd_pcm_lib_free_pages(substream); } unlock: - mutex_unlock(&runtime->buffer_mutex); + snd_pcm_buffer_access_unlock(runtime); return err; }

@@ -865,7 +885,9 @@ static int snd_pcm_hw_free(struct snd_pcm_substream *substream) if (PCM_RUNTIME_CHECK(substream)) return -ENXIO; runtime = substream->runtime; - mutex_lock(&runtime->buffer_mutex); + result = snd_pcm_buffer_access_lock(runtime); + if (result < 0) + return result; snd_pcm_stream_lock_irq(substream); switch (runtime->status->state) { case SNDRV_PCM_STATE_SETUP: @@ -884,7 +906,7 @@ static int snd_pcm_hw_free(struct snd_pcm_substream *substream) snd_pcm_set_state(substream, SNDRV_PCM_STATE_OPEN); cpu_latency_qos_remove_request(&substream->latency_pm_qos_req); unlock: - mutex_unlock(&runtime->buffer_mutex); + snd_pcm_buffer_access_unlock(runtime); return result; }

@@ -1369,12 +1391,15 @@ static int snd_pcm_action_nonatomic(const struct action_ops *ops,

/* Guarantee the group members won't change during non-atomic action */ down_read(&snd_pcm_link_rwsem); - mutex_lock(&substream->runtime->buffer_mutex); + res = snd_pcm_buffer_access_lock(substream->runtime); + if (res < 0) + goto unlock; if (snd_pcm_stream_linked(substream)) res = snd_pcm_action_group(ops, substream, state, false); else res = snd_pcm_action_single(ops, substream, state); - mutex_unlock(&substream->runtime->buffer_mutex); + snd_pcm_buffer_access_unlock(substream->runtime); + unlock: up_read(&snd_pcm_link_rwsem); return res; }