ALSA: pcm: Return -EBUSY for OSS ioctls changing busy streams

OSS PCM stream management isn't modal but it allows ioctls issued at
any time for changing the parameters.  In the previous hardening
patch ("ALSA: pcm: Avoid potential races between OSS ioctls and
read/write"), we covered these races and prevent the corruption by
protecting the concurrent accesses via params_lock mutex.  However,
this means that some ioctls that try to change the stream parameter
(e.g. channels or format) would be blocked until the read/write
finishes, and it may take really long.

Basically changing the parameter while reading/writing is an invalid
operation, hence it's even more user-friendly from the API POV if it
returns -EBUSY in such a situation.

This patch adds such checks in the relevant ioctls with the addition
of read/write access refcount.

Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/pcm_oss.h

@@ -57,6 +57,7 @@ struct snd_pcm_oss_runtime {
 	char *buffer;				/* vmallocated period */
 	size_t buffer_used;			/* used length from period buffer */
 	struct mutex params_lock;
+	atomic_t rw_ref;		/* concurrent read/write accesses */
 #ifdef CONFIG_SND_PCM_OSS_PLUGINS
 	struct snd_pcm_plugin *plugin_first;
 	struct snd_pcm_plugin *plugin_last;

sound/core/oss/pcm_oss.c

@@ -1370,6 +1370,7 @@ static ssize_t snd_pcm_oss_write1(struct snd_pcm_substream *substream, const cha
 	if (atomic_read(&substream->mmap_count))
 		return -ENXIO;
 
+	atomic_inc(&runtime->oss.rw_ref);
 	while (bytes > 0) {
 		if (mutex_lock_interruptible(&runtime->oss.params_lock)) {
 			tmp = -ERESTARTSYS;
@@ -1433,6 +1434,7 @@ static ssize_t snd_pcm_oss_write1(struct snd_pcm_substream *substream, const cha
 		}
 		tmp = 0;
 	}
+	atomic_dec(&runtime->oss.rw_ref);
 	return xfer > 0 ? (snd_pcm_sframes_t)xfer : tmp;
 }
 
@@ -1478,6 +1480,7 @@ static ssize_t snd_pcm_oss_read1(struct snd_pcm_substream *substream, char __use
 	if (atomic_read(&substream->mmap_count))
 		return -ENXIO;
 
+	atomic_inc(&runtime->oss.rw_ref);
 	while (bytes > 0) {
 		if (mutex_lock_interruptible(&runtime->oss.params_lock)) {
 			tmp = -ERESTARTSYS;
@@ -1526,6 +1529,7 @@ static ssize_t snd_pcm_oss_read1(struct snd_pcm_substream *substream, char __use
 		}
 		tmp = 0;
 	}
+	atomic_dec(&runtime->oss.rw_ref);
 	return xfer > 0 ? (snd_pcm_sframes_t)xfer : tmp;
 }
 
@@ -1632,8 +1636,11 @@ static int snd_pcm_oss_sync(struct snd_pcm_oss_file *pcm_oss_file)
 			goto __direct;
 		if ((err = snd_pcm_oss_make_ready(substream)) < 0)
 			return err;
-		if (mutex_lock_interruptible(&runtime->oss.params_lock))
+		atomic_inc(&runtime->oss.rw_ref);
+		if (mutex_lock_interruptible(&runtime->oss.params_lock)) {
+			atomic_dec(&runtime->oss.rw_ref);
 			return -ERESTARTSYS;
+		}
 		format = snd_pcm_oss_format_from(runtime->oss.format);
 		width = snd_pcm_format_physical_width(format);
 		if (runtime->oss.buffer_used > 0) {
@@ -1645,10 +1652,8 @@ static int snd_pcm_oss_sync(struct snd_pcm_oss_file *pcm_oss_file)
 						   runtime->oss.buffer + runtime->oss.buffer_used,
 						   size);
 			err = snd_pcm_oss_sync1(substream, runtime->oss.period_bytes);
-			if (err < 0) {
-				mutex_unlock(&runtime->oss.params_lock);
-				return err;
-			}
+			if (err < 0)
+				goto unlock;
 		} else if (runtime->oss.period_ptr > 0) {
 #ifdef OSS_DEBUG
 			pcm_dbg(substream->pcm, "sync: period_ptr\n");
@@ -1658,10 +1663,8 @@ static int snd_pcm_oss_sync(struct snd_pcm_oss_file *pcm_oss_file)
 						   runtime->oss.buffer,
 						   size * 8 / width);
 			err = snd_pcm_oss_sync1(substream, size);
-			if (err < 0) {
-				mutex_unlock(&runtime->oss.params_lock);
-				return err;
-			}
+			if (err < 0)
+				goto unlock;
 		}
 		/*
 		 * The ALSA's period might be a bit large than OSS one.
@@ -1675,7 +1678,11 @@ static int snd_pcm_oss_sync(struct snd_pcm_oss_file *pcm_oss_file)
 			else if (runtime->access == SNDRV_PCM_ACCESS_RW_NONINTERLEAVED)
 				snd_pcm_lib_writev(substream, NULL, size);
 		}
+unlock:
 		mutex_unlock(&runtime->oss.params_lock);
+		atomic_dec(&runtime->oss.rw_ref);
+		if (err < 0)
+			return err;
 		/*
 		 * finish sync: drain the buffer
 		 */
@@ -1723,6 +1730,8 @@ static int snd_pcm_oss_set_rate(struct snd_pcm_oss_file *pcm_oss_file, int rate)
 			rate = 192000;
 		if (mutex_lock_interruptible(&runtime->oss.params_lock))
 			return -ERESTARTSYS;
+		if (atomic_read(&runtime->oss.rw_ref))
+			return -EBUSY;
 		if (runtime->oss.rate != rate) {
 			runtime->oss.params = 1;
 			runtime->oss.rate = rate;
@@ -1757,6 +1766,8 @@ static int snd_pcm_oss_set_channels(struct snd_pcm_oss_file *pcm_oss_file, unsig
 		runtime = substream->runtime;
 		if (mutex_lock_interruptible(&runtime->oss.params_lock))
 			return -ERESTARTSYS;
+		if (atomic_read(&runtime->oss.rw_ref))
+			return -EBUSY;
 		if (runtime->oss.channels != channels) {
 			runtime->oss.params = 1;
 			runtime->oss.channels = channels;
@@ -1847,6 +1858,8 @@ static int snd_pcm_oss_set_format(struct snd_pcm_oss_file *pcm_oss_file, int for
 			if (substream == NULL)
 				continue;
 			runtime = substream->runtime;
+			if (atomic_read(&runtime->oss.rw_ref))
+				return -EBUSY;
 			if (mutex_lock_interruptible(&runtime->oss.params_lock))
 				return -ERESTARTSYS;
 			if (runtime->oss.format != format) {
@@ -1901,6 +1914,8 @@ static int snd_pcm_oss_set_subdivide(struct snd_pcm_oss_file *pcm_oss_file, int
 		if (substream == NULL)
 			continue;
 		runtime = substream->runtime;
+		if (atomic_read(&runtime->oss.rw_ref))
+			return -EBUSY;
 		if (mutex_lock_interruptible(&runtime->oss.params_lock))
 			return -ERESTARTSYS;
 		err = snd_pcm_oss_set_subdivide1(substream, subdivide);
@@ -1939,6 +1954,8 @@ static int snd_pcm_oss_set_fragment(struct snd_pcm_oss_file *pcm_oss_file, unsig
 		if (substream == NULL)
 			continue;
 		runtime = substream->runtime;
+		if (atomic_read(&runtime->oss.rw_ref))
+			return -EBUSY;
 		if (mutex_lock_interruptible(&runtime->oss.params_lock))
 			return -ERESTARTSYS;
 		err = snd_pcm_oss_set_fragment1(substream, val);
@@ -2333,6 +2350,7 @@ static void snd_pcm_oss_init_substream(struct snd_pcm_substream *substream,
 	runtime->oss.maxfrags = 0;
 	runtime->oss.subdivision = 0;
 	substream->pcm_release = snd_pcm_oss_release_substream;
+	atomic_set(&runtime->oss.rw_ref, 0);
 }
 
 static int snd_pcm_oss_release_file(struct snd_pcm_oss_file *pcm_oss_file)