ALSA: timer: Limit max instances per timer

commit 9b7d869ee5a77ed4a462372bb89af622e705bfb8 upstream.

Currently we allow unlimited number of timer instances, and it may
bring the system hogging way too much CPU when too many timer
instances are opened and processed concurrently.  This may end up with
a soft-lockup report as triggered by syzkaller, especially when
hrtimer backend is deployed.

Since such insane number of instances aren't demanded by the normal
use case of ALSA sequencer and it merely  opens a risk only for abuse,
this patch introduces the upper limit for the number of instances per
timer backend.  As default, it's set to 1000, but for the fine-grained
timer like hrtimer, it's set to 100.

Reported-by: syzbot
Tested-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

2 files changed, 55 insertions, 13 deletions

diff --git a/sound/core/hrtimer.c b/sound/core/hrtimer.c
index 38514ed6e55c..9b3445922645 100644
--- a/sound/core/hrtimer.c
+++ b/sound/core/hrtimer.c
@@ -144,6 +144,7 @@ static int __init snd_hrtimer_init(void)
 	timer->hw = hrtimer_hw;
 	timer->hw.resolution = resolution;
 	timer->hw.ticks = NANO_SEC / resolution;
+	timer->max_instances = 100; /* lower the limit */
 
 	err = snd_timer_global_register(timer);
 	if (err < 0) {
diff --git a/sound/core/timer.c b/sound/core/timer.c
index 51df6918bc60..762da0a59ba0 100644
--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -181,7 +181,7 @@ static void snd_timer_request(struct snd_timer_id *tid)
  *
  * call this with register_mutex down.
  */
-static void snd_timer_check_slave(struct snd_timer_instance *slave)
+static int snd_timer_check_slave(struct snd_timer_instance *slave)
 {
 	struct snd_timer *timer;
 	struct snd_timer_instance *master;
@@ -191,16 +191,21 @@ static void snd_timer_check_slave(struct snd_timer_instance *slave)
 		list_for_each_entry(master, &timer->open_list_head, open_list) {
 			if (slave->slave_class == master->slave_class &&
 			    slave->slave_id == master->slave_id) {
+				if (master->timer->num_instances >=
+				    master->timer->max_instances)
+					return -EBUSY;
 				list_move_tail(&slave->open_list,
 					       &master->slave_list_head);
+				master->timer->num_instances++;
 				spin_lock_irq(&slave_active_lock);
 				slave->master = master;
 				slave->timer = master->timer;
 				spin_unlock_irq(&slave_active_lock);
-				return;
+				return 0;
 			}
 		}
 	}
+	return 0;
 }
 
 /*
@@ -209,7 +214,7 @@ static void snd_timer_check_slave(struct snd_timer_instance *slave)
  *
  * call this with register_mutex down.
  */
-static void snd_timer_check_master(struct snd_timer_instance *master)
+static int snd_timer_check_master(struct snd_timer_instance *master)
 {
 	struct snd_timer_instance *slave, *tmp;
 
@@ -217,7 +222,11 @@ static void snd_timer_check_master(struct snd_timer_instance *master)
 	list_for_each_entry_safe(slave, tmp, &snd_timer_slave_list, open_list) {
 		if (slave->slave_class == master->slave_class &&
 		    slave->slave_id == master->slave_id) {
+			if (master->timer->num_instances >=
+			    master->timer->max_instances)
+				return -EBUSY;
 			list_move_tail(&slave->open_list, &master->slave_list_head);
+			master->timer->num_instances++;
 			spin_lock_irq(&slave_active_lock);
 			spin_lock(&master->timer->lock);
 			slave->master = master;
@@ -229,8 +238,11 @@ static void snd_timer_check_master(struct snd_timer_instance *master)
 			spin_unlock_irq(&slave_active_lock);
 		}
 	}
+	return 0;
 }
 
+static int snd_timer_close_locked(struct snd_timer_instance *timeri);
+
 /*
  * open a timer instance
  * when opening a master, the slave id must be here given.
@@ -241,6 +253,7 @@ int snd_timer_open(struct snd_timer_instance **ti,
 {
 	struct snd_timer *timer;
 	struct snd_timer_instance *timeri = NULL;
+	int err;
 
 	if (tid->dev_class == SNDRV_TIMER_CLASS_SLAVE) {
 		/* open a slave instance */
@@ -260,10 +273,14 @@ int snd_timer_open(struct snd_timer_instance **ti,
 		timeri->slave_id = tid->device;
 		timeri->flags |= SNDRV_TIMER_IFLG_SLAVE;
 		list_add_tail(&timeri->open_list, &snd_timer_slave_list);
-		snd_timer_check_slave(timeri);
+		err = snd_timer_check_slave(timeri);
+		if (err < 0) {
+			snd_timer_close_locked(timeri);
+			timeri = NULL;
+		}
 		mutex_unlock(&register_mutex);
 		*ti = timeri;
-		return 0;
+		return err;
 	}
 
 	/* open a master instance */
@@ -289,6 +306,10 @@ int snd_timer_open(struct snd_timer_instance **ti,
 			return -EBUSY;
 		}
 	}
+	if (timer->num_instances >= timer->max_instances) {
+		mutex_unlock(&register_mutex);
+		return -EBUSY;
+	}
 	timeri = snd_timer_instance_new(owner, timer);
 	if (!timeri) {
 		mutex_unlock(&register_mutex);
@@ -315,26 +336,28 @@ int snd_timer_open(struct snd_timer_instance **ti,
 	}
 
 	list_add_tail(&timeri->open_list, &timer->open_list_head);
-	snd_timer_check_master(timeri);
+	timer->num_instances++;
+	err = snd_timer_check_master(timeri);
+	if (err < 0) {
+		snd_timer_close_locked(timeri);
+		timeri = NULL;
+	}
 	mutex_unlock(&register_mutex);
 	*ti = timeri;
-	return 0;
+	return err;
 }
 
 static int _snd_timer_stop(struct snd_timer_instance *timeri, int event);
 
 /*
  * close a timer instance
+ * call this with register_mutex down.
  */
-int snd_timer_close(struct snd_timer_instance *timeri)
+static int snd_timer_close_locked(struct snd_timer_instance *timeri)
 {
 	struct snd_timer *timer = NULL;
 	struct snd_timer_instance *slave, *tmp;
 
-	if (snd_BUG_ON(!timeri))
-		return -ENXIO;
-
-	mutex_lock(&register_mutex);
 	list_del(&timeri->open_list);
 
 	/* force to stop the timer */
@@ -342,6 +365,7 @@ int snd_timer_close(struct snd_timer_instance *timeri)
 
 	timer = timeri->timer;
 	if (timer) {
+		timer->num_instances--;
 		/* wait, until the active callback is finished */
 		spin_lock_irq(&timer->lock);
 		while (timeri->flags & SNDRV_TIMER_IFLG_CALLBACK) {
@@ -357,6 +381,7 @@ int snd_timer_close(struct snd_timer_instance *timeri)
 		list_for_each_entry_safe(slave, tmp, &timeri->slave_list_head,
 					 open_list) {
 			list_move_tail(&slave->open_list, &snd_timer_slave_list);
+			timer->num_instances--;
 			slave->master = NULL;
 			slave->timer = NULL;
 			list_del_init(&slave->ack_list);
@@ -384,10 +409,25 @@ int snd_timer_close(struct snd_timer_instance *timeri)
 		module_put(timer->module);
 	}
 
-	mutex_unlock(&register_mutex);
 	return 0;
 }
 
+/*
+ * close a timer instance
+ */
+int snd_timer_close(struct snd_timer_instance *timeri)
+{
+	int err;
+
+	if (snd_BUG_ON(!timeri))
+		return -ENXIO;
+
+	mutex_lock(&register_mutex);
+	err = snd_timer_close_locked(timeri);
+	mutex_unlock(&register_mutex);
+	return err;
+}
+
 unsigned long snd_timer_resolution(struct snd_timer_instance *timeri)
 {
 	struct snd_timer * timer;
@@ -866,6 +906,7 @@ int snd_timer_new(struct snd_card *card, char *id, struct snd_timer_id *tid,
 	spin_lock_init(&timer->lock);
 	tasklet_init(&timer->task_queue, snd_timer_tasklet,
 		     (unsigned long)timer);
+	timer->max_instances = 1000; /* default limit per timer */
 	if (card != NULL) {
 		timer->module = card->module;
 		err = snd_device_new(card, SNDRV_DEV_TIMER, timer, &ops);