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@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: dca947d4d26d ("ALSA: pcm: Fix races among concurrent read/write and buffer changes")
Cc: <stable@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/000000000000381a0d05db622a81@xxxxxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
---
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;
}
--
2.31.1
