[PATCH 0/4] ALSA: pcm: Fix ioctl races

[PATCH 0/4] ALSA: pcm: Fix ioctl races

[Takashi Iwai]

Hi,

this is a patch set to address the recently reported bug for the racy
PCM ioctls.  In short, the current ALSA PCM core doesn't take enough
care of concurrent ioctl calls, and the concurrent calls may result in
a use-after-free.  The reported problem was the concurrent hw_free
calls, but there can be similar cases with other code paths like
hw_params, prepare, etc, too.

The patch set introduces the new runtime->buffer_mutex for protecting
those.  The first patch is the fix for the reported issue (the races
with hw_free), while the rest three are more hardening for the other
similar executions.

[ Note that the patch 3 was slightly modified from the version I sent
  to distros list, as I noticed possible lockdep (false-positive)
  warnings.  The behavior is almost same, just written differently. ]


thanks,

Takashi

===

Takashi Iwai (4):
  ALSA: pcm: Fix races among concurrent hw_params and hw_free calls
  ALSA: pcm: Fix races among concurrent read/write and buffer changes
  ALSA: pcm: Fix races among concurrent prepare and hw_params/hw_free
    calls
  ALSA: pcm: Fix races among concurrent prealloc proc writes

 include/sound/pcm.h     |  1 +
 sound/core/pcm.c        |  2 +
 sound/core/pcm_lib.c    |  4 ++
 sound/core/pcm_memory.c | 11 +++--
 sound/core/pcm_native.c | 93 +++++++++++++++++++++++++----------------
 5 files changed, 71 insertions(+), 40 deletions(-)

-- 
2.31.1

[PATCH 1/4] ALSA: pcm: Fix races among concurrent hw_params and hw_free calls

[Takashi Iwai]

Currently we have neither proper check nor protection against the
concurrent calls of PCM hw_params and hw_free ioctls, which may result
in a UAF.  Since the existing PCM stream lock can't be used for
protecting the whole ioctl operations, we need a new mutex to protect
those racy calls.

This patch introduced a new mutex, runtime->buffer_mutex, and applies
it to both hw_params and hw_free ioctl code paths.  Along with it, the
both functions are slightly modified (the mmap_count check is moved
into the state-check block) for code simplicity.

Reported-by: Hu Jiahui <kirin.say@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 include/sound/pcm.h     |  1 +
 sound/core/pcm.c        |  2 ++
 sound/core/pcm_native.c | 61 ++++++++++++++++++++++++++---------------
 3 files changed, 42 insertions(+), 22 deletions(-)

diff --git a/include/sound/pcm.h b/include/sound/pcm.h
index 36da42cd0774..314f2779cab5 100644
--- a/include/sound/pcm.h
+++ b/include/sound/pcm.h
@@ -401,6 +401,7 @@ struct snd_pcm_runtime {
 	wait_queue_head_t tsleep;	/* transfer sleep */
 	struct fasync_struct *fasync;
 	bool stop_operating;		/* sync_stop will be called */
+	struct mutex buffer_mutex;	/* protect for buffer changes */
 
 	/* -- private section -- */
 	void *private_data;
diff --git a/sound/core/pcm.c b/sound/core/pcm.c
index ba4a987ed1c6..edd9849210f2 100644
--- a/sound/core/pcm.c
+++ b/sound/core/pcm.c
@@ -969,6 +969,7 @@ int snd_pcm_attach_substream(struct snd_pcm *pcm, int stream,
 	init_waitqueue_head(&runtime->tsleep);
 
 	runtime->status->state = SNDRV_PCM_STATE_OPEN;
+	mutex_init(&runtime->buffer_mutex);
 
 	substream->runtime = runtime;
 	substream->private_data = pcm->private_data;
@@ -1002,6 +1003,7 @@ void snd_pcm_detach_substream(struct snd_pcm_substream *substream)
 	} else {
 		substream->runtime = NULL;
 	}
+	mutex_destroy(&runtime->buffer_mutex);
 	kfree(runtime);
 	put_pid(substream->pid);
 	substream->pid = NULL;
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index a056b3ef3c84..266895374b83 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -685,33 +685,40 @@ static int snd_pcm_hw_params_choose(struct snd_pcm_substream *pcm,
 	return 0;
 }
 
+#if IS_ENABLED(CONFIG_SND_PCM_OSS)
+#define is_oss_stream(substream)	((substream)->oss.oss)
+#else
+#define is_oss_stream(substream)	false
+#endif
+
 static int snd_pcm_hw_params(struct snd_pcm_substream *substream,
 			     struct snd_pcm_hw_params *params)
 {
 	struct snd_pcm_runtime *runtime;
-	int err, usecs;
+	int err = 0, usecs;
 	unsigned int bits;
 	snd_pcm_uframes_t frames;
 
 	if (PCM_RUNTIME_CHECK(substream))
 		return -ENXIO;
 	runtime = substream->runtime;
+	mutex_lock(&runtime->buffer_mutex);
 	snd_pcm_stream_lock_irq(substream);
 	switch (runtime->status->state) {
 	case SNDRV_PCM_STATE_OPEN:
 	case SNDRV_PCM_STATE_SETUP:
 	case SNDRV_PCM_STATE_PREPARED:
+		if (!is_oss_stream(substream) &&
+		    atomic_read(&substream->mmap_count))
+			err = -EBADFD;
 		break;
 	default:
-		snd_pcm_stream_unlock_irq(substream);
-		return -EBADFD;
+		err = -EBADFD;
+		break;
 	}
 	snd_pcm_stream_unlock_irq(substream);
-#if IS_ENABLED(CONFIG_SND_PCM_OSS)
-	if (!substream->oss.oss)
-#endif
-		if (atomic_read(&substream->mmap_count))
-			return -EBADFD;
+	if (err)
+		goto unlock;
 
 	snd_pcm_sync_stop(substream, true);
 
@@ -799,16 +806,21 @@ static int snd_pcm_hw_params(struct snd_pcm_substream *substream,
 	if (usecs >= 0)
 		cpu_latency_qos_add_request(&substream->latency_pm_qos_req,
 					    usecs);
-	return 0;
+	err = 0;
  _error:
-	/* hardware might be unusable from this time,
-	   so we force application to retry to set
-	   the correct hardware parameter settings */
-	snd_pcm_set_state(substream, SNDRV_PCM_STATE_OPEN);
-	if (substream->ops->hw_free != NULL)
-		substream->ops->hw_free(substream);
-	if (substream->managed_buffer_alloc)
-		snd_pcm_lib_free_pages(substream);
+	if (err) {
+		/* hardware might be unusable from this time,
+		 * so we force application to retry to set
+		 * the correct hardware parameter settings
+		 */
+		snd_pcm_set_state(substream, SNDRV_PCM_STATE_OPEN);
+		if (substream->ops->hw_free != NULL)
+			substream->ops->hw_free(substream);
+		if (substream->managed_buffer_alloc)
+			snd_pcm_lib_free_pages(substream);
+	}
+ unlock:
+	mutex_unlock(&runtime->buffer_mutex);
 	return err;
 }
 
@@ -848,26 +860,31 @@ static int do_hw_free(struct snd_pcm_substream *substream)
 static int snd_pcm_hw_free(struct snd_pcm_substream *substream)
 {
 	struct snd_pcm_runtime *runtime;
-	int result;
+	int result = 0;
 
 	if (PCM_RUNTIME_CHECK(substream))
 		return -ENXIO;
 	runtime = substream->runtime;
+	mutex_lock(&runtime->buffer_mutex);
 	snd_pcm_stream_lock_irq(substream);
 	switch (runtime->status->state) {
 	case SNDRV_PCM_STATE_SETUP:
 	case SNDRV_PCM_STATE_PREPARED:
+		if (atomic_read(&substream->mmap_count))
+			result = -EBADFD;
 		break;
 	default:
-		snd_pcm_stream_unlock_irq(substream);
-		return -EBADFD;
+		result = -EBADFD;
+		break;
 	}
 	snd_pcm_stream_unlock_irq(substream);
-	if (atomic_read(&substream->mmap_count))
-		return -EBADFD;
+	if (result)
+		goto unlock;
 	result = do_hw_free(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);
 	return result;
 }
 
-- 
2.31.1

[PATCH 2/4] ALSA: pcm: Fix races among concurrent read/write and buffer changes

[Takashi Iwai]

In the current PCM design, the read/write syscalls (as well as the
equivalent ioctls) are allowed before the PCM stream is running, that
is, at PCM PREPARED state.  Meanwhile, we also allow to re-issue
hw_params and hw_free ioctl calls at the PREPARED state that may
change or free the buffers, too.  The problem is that there is no
protection against those mix-ups.

This patch applies the previously introduced runtime->buffer_mutex to
the read/write operations so that the concurrent hw_params or hw_free
call can no longer interfere during the operation.  The mutex is
unlocked before scheduling, so we don't take it too long.

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 sound/core/pcm_lib.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/sound/core/pcm_lib.c b/sound/core/pcm_lib.c
index f2090025236b..a40a35e51fad 100644
--- a/sound/core/pcm_lib.c
+++ b/sound/core/pcm_lib.c
@@ -1906,9 +1906,11 @@ 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) {
@@ -2219,6 +2221,7 @@ 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)
@@ -2310,6 +2313,7 @@ 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);
-- 
2.31.1

[PATCH 3/4] ALSA: pcm: Fix races among concurrent prepare and hw_params/hw_free calls

[Takashi Iwai]

Like the previous fixes to hw_params and hw_free ioctl races, we need
to paper over the concurrent prepare ioctl calls against hw_params and
hw_free, too.

This patch implements the locking with the existing
runtime->buffer_mutex for prepare ioctls.  Unlike the previous case
for snd_pcm_hw_hw_params() and snd_pcm_hw_free(), snd_pcm_prepare() is
performed to the linked streams, hence the lock can't be applied
simply on the top.  For tracking the lock in each linked substream, we
modify snd_pcm_action_group() slightly and apply the buffer_mutex for
the case stream_lock=false (formerly there was no lock applied)
there.

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 sound/core/pcm_native.c | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
index 266895374b83..0e4fbf5fd87b 100644
--- a/sound/core/pcm_native.c
+++ b/sound/core/pcm_native.c
@@ -1190,15 +1190,17 @@ struct action_ops {
 static int snd_pcm_action_group(const struct action_ops *ops,
 				struct snd_pcm_substream *substream,
 				snd_pcm_state_t state,
-				bool do_lock)
+				bool stream_lock)
 {
 	struct snd_pcm_substream *s = NULL;
 	struct snd_pcm_substream *s1;
 	int res = 0, depth = 1;
 
 	snd_pcm_group_for_each_entry(s, substream) {
-		if (do_lock && s != substream) {
-			if (s->pcm->nonatomic)
+		if (s != substream) {
+			if (!stream_lock)
+				mutex_lock_nested(&s->runtime->buffer_mutex, depth);
+			else if (s->pcm->nonatomic)
 				mutex_lock_nested(&s->self_group.mutex, depth);
 			else
 				spin_lock_nested(&s->self_group.lock, depth);
@@ -1226,18 +1228,18 @@ static int snd_pcm_action_group(const struct action_ops *ops,
 		ops->post_action(s, state);
 	}
  _unlock:
-	if (do_lock) {
-		/* unlock streams */
-		snd_pcm_group_for_each_entry(s1, substream) {
-			if (s1 != substream) {
-				if (s1->pcm->nonatomic)
-					mutex_unlock(&s1->self_group.mutex);
-				else
-					spin_unlock(&s1->self_group.lock);
-			}
-			if (s1 == s)	/* end */
-				break;
+	/* unlock streams */
+	snd_pcm_group_for_each_entry(s1, substream) {
+		if (s1 != substream) {
+			if (!stream_lock)
+				mutex_unlock(&s1->runtime->buffer_mutex);
+			else if (s1->pcm->nonatomic)
+				mutex_unlock(&s1->self_group.mutex);
+			else
+				spin_unlock(&s1->self_group.lock);
 		}
+		if (s1 == s)	/* end */
+			break;
 	}
 	return res;
 }
@@ -1367,10 +1369,12 @@ 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);
 	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);
 	up_read(&snd_pcm_link_rwsem);
 	return res;
 }
-- 
2.31.1

Re: [PATCH 3/4] ALSA: pcm: Fix races among concurrent prepare and hw_params/hw_free calls

[Amadeusz Sławiński]

On 3/22/2022 6:07 PM, Takashi Iwai wrote:
> Like the previous fixes to hw_params and hw_free ioctl races, we need
> to paper over the concurrent prepare ioctl calls against hw_params and
> hw_free, too.
> 
> This patch implements the locking with the existing
> runtime->buffer_mutex for prepare ioctls.  Unlike the previous case
> for snd_pcm_hw_hw_params() and snd_pcm_hw_free(), snd_pcm_prepare() is
> performed to the linked streams, hence the lock can't be applied
> simply on the top.  For tracking the lock in each linked substream, we
> modify snd_pcm_action_group() slightly and apply the buffer_mutex for
> the case stream_lock=false (formerly there was no lock applied)
> there.
> 
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Takashi Iwai <tiwai@suse.de>
> ---
>   sound/core/pcm_native.c | 32 ++++++++++++++++++--------------
>   1 file changed, 18 insertions(+), 14 deletions(-)
> 
> diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> index 266895374b83..0e4fbf5fd87b 100644
> --- a/sound/core/pcm_native.c
> +++ b/sound/core/pcm_native.c
> @@ -1190,15 +1190,17 @@ struct action_ops {
>   static int snd_pcm_action_group(const struct action_ops *ops,
>   				struct snd_pcm_substream *substream,
>   				snd_pcm_state_t state,
> -				bool do_lock)
> +				bool stream_lock)
>   {
>   	struct snd_pcm_substream *s = NULL;
>   	struct snd_pcm_substream *s1;
>   	int res = 0, depth = 1;
>   
>   	snd_pcm_group_for_each_entry(s, substream) {
> -		if (do_lock && s != substream) {
> -			if (s->pcm->nonatomic)
> +		if (s != substream) {
> +			if (!stream_lock)
> +				mutex_lock_nested(&s->runtime->buffer_mutex, depth);
> +			else if (s->pcm->nonatomic)
>   				mutex_lock_nested(&s->self_group.mutex, depth);
>   			else
>   				spin_lock_nested(&s->self_group.lock, depth);

Maybe
	if (!stream_lock)
		mutex_lock_nested(&s->runtime->buffer_mutex, depth);
	else
		snd_pcm_group_lock(&s->self_group, s->pcm->nonatomic);
?

> @@ -1226,18 +1228,18 @@ static int snd_pcm_action_group(const struct action_ops *ops,
>   		ops->post_action(s, state);
>   	}
>    _unlock:
> -	if (do_lock) {
> -		/* unlock streams */
> -		snd_pcm_group_for_each_entry(s1, substream) {
> -			if (s1 != substream) {
> -				if (s1->pcm->nonatomic)
> -					mutex_unlock(&s1->self_group.mutex);
> -				else
> -					spin_unlock(&s1->self_group.lock);
> -			}
> -			if (s1 == s)	/* end */
> -				break;
> +	/* unlock streams */
> +	snd_pcm_group_for_each_entry(s1, substream) {
> +		if (s1 != substream) {
> +			if (!stream_lock)
> +				mutex_unlock(&s1->runtime->buffer_mutex);
> +			else if (s1->pcm->nonatomic)
> +				mutex_unlock(&s1->self_group.mutex);
> +			else
> +				spin_unlock(&s1->self_group.lock);

And similarly to above, use snd_pcm_group_unlock() here?

>   		}
> +		if (s1 == s)	/* end */
> +			break;
>   	}
>   	return res;
>   }
> @@ -1367,10 +1369,12 @@ 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);
>   	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);
>   	up_read(&snd_pcm_link_rwsem);
>   	return res;
>   }

Re: [PATCH 3/4] ALSA: pcm: Fix races among concurrent prepare and hw_params/hw_free calls

[Takashi Iwai]

On Wed, 23 Mar 2022 09:08:25 +0100,
Amadeusz SX2awiX4ski wrote:
> 
> On 3/22/2022 6:07 PM, Takashi Iwai wrote:
> > Like the previous fixes to hw_params and hw_free ioctl races, we need
> > to paper over the concurrent prepare ioctl calls against hw_params and
> > hw_free, too.
> >
> > This patch implements the locking with the existing
> > runtime->buffer_mutex for prepare ioctls.  Unlike the previous case
> > for snd_pcm_hw_hw_params() and snd_pcm_hw_free(), snd_pcm_prepare() is
> > performed to the linked streams, hence the lock can't be applied
> > simply on the top.  For tracking the lock in each linked substream, we
> > modify snd_pcm_action_group() slightly and apply the buffer_mutex for
> > the case stream_lock=false (formerly there was no lock applied)
> > there.
> >
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > ---
> >   sound/core/pcm_native.c | 32 ++++++++++++++++++--------------
> >   1 file changed, 18 insertions(+), 14 deletions(-)
> >
> > diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> > index 266895374b83..0e4fbf5fd87b 100644
> > --- a/sound/core/pcm_native.c
> > +++ b/sound/core/pcm_native.c
> > @@ -1190,15 +1190,17 @@ struct action_ops {
> >   static int snd_pcm_action_group(const struct action_ops *ops,
> >   				struct snd_pcm_substream *substream,
> >   				snd_pcm_state_t state,
> > -				bool do_lock)
> > +				bool stream_lock)
> >   {
> >   	struct snd_pcm_substream *s = NULL;
> >   	struct snd_pcm_substream *s1;
> >   	int res = 0, depth = 1;
> >     	snd_pcm_group_for_each_entry(s, substream) {
> > -		if (do_lock && s != substream) {
> > -			if (s->pcm->nonatomic)
> > +		if (s != substream) {
> > +			if (!stream_lock)
> > +				mutex_lock_nested(&s->runtime->buffer_mutex, depth);
> > +			else if (s->pcm->nonatomic)
> >   				mutex_lock_nested(&s->self_group.mutex, depth);
> >   			else
> >   				spin_lock_nested(&s->self_group.lock, depth);
> 
> Maybe
> 	if (!stream_lock)
> 		mutex_lock_nested(&s->runtime->buffer_mutex, depth);
> 	else
> 		snd_pcm_group_lock(&s->self_group, s->pcm->nonatomic);
> ?

No, it must be nested locks with the given subclass.  That's why it
has been the open code beforehand, too.

> > @@ -1226,18 +1228,18 @@ static int snd_pcm_action_group(const struct action_ops *ops,
> >   		ops->post_action(s, state);
> >   	}
> >    _unlock:
> > -	if (do_lock) {
> > -		/* unlock streams */
> > -		snd_pcm_group_for_each_entry(s1, substream) {
> > -			if (s1 != substream) {
> > -				if (s1->pcm->nonatomic)
> > -					mutex_unlock(&s1->self_group.mutex);
> > -				else
> > -					spin_unlock(&s1->self_group.lock);
> > -			}
> > -			if (s1 == s)	/* end */
> > -				break;
> > +	/* unlock streams */
> > +	snd_pcm_group_for_each_entry(s1, substream) {
> > +		if (s1 != substream) {
> > +			if (!stream_lock)
> > +				mutex_unlock(&s1->runtime->buffer_mutex);
> > +			else if (s1->pcm->nonatomic)
> > +				mutex_unlock(&s1->self_group.mutex);
> > +			else
> > +				spin_unlock(&s1->self_group.lock);
> 
> And similarly to above, use snd_pcm_group_unlock() here?

This side would be possible to use that macro but it's still better to
have the consistent call pattern.


thanks,

Takashi

Re: [PATCH 3/4] ALSA: pcm: Fix races among concurrent prepare and hw_params/hw_free calls

[Takashi Iwai]

On Wed, 23 Mar 2022 09:15:19 +0100,
Takashi Iwai wrote:
> 
> On Wed, 23 Mar 2022 09:08:25 +0100,
> Amadeusz SX2awiX4ski wrote:
> > 
> > On 3/22/2022 6:07 PM, Takashi Iwai wrote:
> > > Like the previous fixes to hw_params and hw_free ioctl races, we need
> > > to paper over the concurrent prepare ioctl calls against hw_params and
> > > hw_free, too.
> > >
> > > This patch implements the locking with the existing
> > > runtime->buffer_mutex for prepare ioctls.  Unlike the previous case
> > > for snd_pcm_hw_hw_params() and snd_pcm_hw_free(), snd_pcm_prepare() is
> > > performed to the linked streams, hence the lock can't be applied
> > > simply on the top.  For tracking the lock in each linked substream, we
> > > modify snd_pcm_action_group() slightly and apply the buffer_mutex for
> > > the case stream_lock=false (formerly there was no lock applied)
> > > there.
> > >
> > > Cc: <stable@vger.kernel.org>
> > > Signed-off-by: Takashi Iwai <tiwai@suse.de>
> > > ---
> > >   sound/core/pcm_native.c | 32 ++++++++++++++++++--------------
> > >   1 file changed, 18 insertions(+), 14 deletions(-)
> > >
> > > diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c
> > > index 266895374b83..0e4fbf5fd87b 100644
> > > --- a/sound/core/pcm_native.c
> > > +++ b/sound/core/pcm_native.c
> > > @@ -1190,15 +1190,17 @@ struct action_ops {
> > >   static int snd_pcm_action_group(const struct action_ops *ops,
> > >   				struct snd_pcm_substream *substream,
> > >   				snd_pcm_state_t state,
> > > -				bool do_lock)
> > > +				bool stream_lock)
> > >   {
> > >   	struct snd_pcm_substream *s = NULL;
> > >   	struct snd_pcm_substream *s1;
> > >   	int res = 0, depth = 1;
> > >     	snd_pcm_group_for_each_entry(s, substream) {
> > > -		if (do_lock && s != substream) {
> > > -			if (s->pcm->nonatomic)
> > > +		if (s != substream) {
> > > +			if (!stream_lock)
> > > +				mutex_lock_nested(&s->runtime->buffer_mutex, depth);
> > > +			else if (s->pcm->nonatomic)
> > >   				mutex_lock_nested(&s->self_group.mutex, depth);
> > >   			else
> > >   				spin_lock_nested(&s->self_group.lock, depth);
> > 
> > Maybe
> > 	if (!stream_lock)
> > 		mutex_lock_nested(&s->runtime->buffer_mutex, depth);
> > 	else
> > 		snd_pcm_group_lock(&s->self_group, s->pcm->nonatomic);
> > ?
> 
> No, it must be nested locks with the given subclass.

FWIW, the reason is that lockdep would complain otherwise as if it
were a deadlock.  That is, this is a workaround for avoiding false
lockdep warnings.


Takashi

[PATCH 4/4] ALSA: pcm: Fix races among concurrent prealloc proc writes

[Takashi Iwai]

We have no protection against concurrent PCM buffer preallocation
changes via proc files, and it may potentially lead to UAF or some
weird problem.  This patch applies the PCM open_mutex to the proc
write operation for avoiding the racy proc writes and the PCM stream
open (and further operations).

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
---
 sound/core/pcm_memory.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/sound/core/pcm_memory.c b/sound/core/pcm_memory.c
index b70ce3b69ab4..8848d2f3160d 100644
--- a/sound/core/pcm_memory.c
+++ b/sound/core/pcm_memory.c
@@ -163,19 +163,20 @@ static void snd_pcm_lib_preallocate_proc_write(struct snd_info_entry *entry,
 	size_t size;
 	struct snd_dma_buffer new_dmab;
 
+	mutex_lock(&substream->pcm->open_mutex);
 	if (substream->runtime) {
 		buffer->error = -EBUSY;
-		return;
+		goto unlock;
 	}
 	if (!snd_info_get_line(buffer, line, sizeof(line))) {
 		snd_info_get_str(str, line, sizeof(str));
 		size = simple_strtoul(str, NULL, 10) * 1024;
 		if ((size != 0 && size < 8192) || size > substream->dma_max) {
 			buffer->error = -EINVAL;
-			return;
+			goto unlock;
 		}
 		if (substream->dma_buffer.bytes == size)
-			return;
+			goto unlock;
 		memset(&new_dmab, 0, sizeof(new_dmab));
 		new_dmab.dev = substream->dma_buffer.dev;
 		if (size > 0) {
@@ -189,7 +190,7 @@ static void snd_pcm_lib_preallocate_proc_write(struct snd_info_entry *entry,
 					 substream->pcm->card->number, substream->pcm->device,
 					 substream->stream ? 'c' : 'p', substream->number,
 					 substream->pcm->name, size);
-				return;
+				goto unlock;
 			}
 			substream->buffer_bytes_max = size;
 		} else {
@@ -201,6 +202,8 @@ static void snd_pcm_lib_preallocate_proc_write(struct snd_info_entry *entry,
 	} else {
 		buffer->error = -EINVAL;
 	}
+ unlock:
+	mutex_unlock(&substream->pcm->open_mutex);
 }
 
 static inline void preallocate_info_init(struct snd_pcm_substream *substream)
-- 
2.31.1

Re: [PATCH 0/4] ALSA: pcm: Fix ioctl races

[Jaroslav Kysela]

On 22. 03. 22 18:07, Takashi Iwai wrote:
> Hi,
> 
> this is a patch set to address the recently reported bug for the racy
> PCM ioctls.  In short, the current ALSA PCM core doesn't take enough
> care of concurrent ioctl calls, and the concurrent calls may result in
> a use-after-free.  The reported problem was the concurrent hw_free
> calls, but there can be similar cases with other code paths like
> hw_params, prepare, etc, too.
> 
> The patch set introduces the new runtime->buffer_mutex for protecting
> those.  The first patch is the fix for the reported issue (the races
> with hw_free), while the rest three are more hardening for the other
> similar executions.

Thank you Takashi.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>

-- 
Jaroslav Kysela <perex@perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.