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Subject[PATCH 4.14 13/14] ALSA: pcm: Fix potential AB/BA lock with buffer_mutex and mmap_lock
Date
From: Takashi Iwai <tiwai@suse.de>

commit bc55cfd5718c7c23e5524582e9fa70b4d10f2433 upstream.

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>
[OP: backport to 4.14: adjusted context]
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
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(-)

--- a/include/sound/pcm.h
+++ b/include/sound/pcm.h
@@ -397,6 +397,7 @@ struct snd_pcm_runtime {
wait_queue_head_t tsleep; /* transfer sleep */
struct fasync_struct *fasync;
struct mutex buffer_mutex; /* protect for buffer changes */
+ atomic_t buffer_accessing; /* >0: in r/w operation, <0: blocked */

/* -- private section -- */
void *private_data;
--- a/sound/core/pcm.c
+++ b/sound/core/pcm.c
@@ -1033,6 +1033,7 @@ int snd_pcm_attach_substream(struct snd_

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;
--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -1878,11 +1878,9 @@ static int wait_for_avail(struct snd_pcm
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) {
@@ -2176,7 +2174,6 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(str

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)
@@ -2229,10 +2226,15 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(str
snd_pcm_stream_unlock_irq(substream);
return -EINVAL;
}
+ if (!atomic_inc_unless_negative(&runtime->buffer_accessing)) {
+ err = -EBUSY;
+ goto _end_unlock;
+ }
snd_pcm_stream_unlock_irq(substream);
err = writer(substream, appl_ofs, data, offset, frames,
transfer);
snd_pcm_stream_lock_irq(substream);
+ atomic_dec(&runtime->buffer_accessing);
if (err < 0)
goto _end_unlock;
err = pcm_accessible_state(runtime);
@@ -2262,7 +2264,6 @@ snd_pcm_sframes_t __snd_pcm_lib_xfer(str
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);
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -634,6 +634,24 @@ static int snd_pcm_hw_params_choose(stru
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
@@ -644,14 +662,16 @@ static int snd_pcm_hw_params(struct snd_
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:
@@ -756,7 +776,7 @@ static int snd_pcm_hw_params(struct snd_
substream->ops->hw_free(substream);
}
unlock:
- mutex_unlock(&runtime->buffer_mutex);
+ snd_pcm_buffer_access_unlock(runtime);
return err;
}

@@ -789,7 +809,9 @@ static int snd_pcm_hw_free(struct snd_pc
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:
@@ -809,7 +831,7 @@ static int snd_pcm_hw_free(struct snd_pc
snd_pcm_set_state(substream, SNDRV_PCM_STATE_OPEN);
pm_qos_remove_request(&substream->latency_pm_qos_req);
unlock:
- mutex_unlock(&runtime->buffer_mutex);
+ snd_pcm_buffer_access_unlock(runtime);
return result;
}

@@ -1176,12 +1198,15 @@ static int snd_pcm_action_nonatomic(cons
int res;

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, 0);
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;
}

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