[alsa-devel] [PATCH 1/3] ALSA: seq: Avoid concurrent access to queue flags

[Takashi Iwai]

The queue flags are represented in bit fields and the concurrent
access may result in unexpected results.  Although the current code
should be mostly OK as it's only reading a field while writing other
fields as KCSAN reported, it's safer to cover both with a proper
spinlock protection.

This patch fixes the possible concurrent read by protecting with
q->owner_lock.  Also the queue owner field is protected as well since
it's the field to be protected by the lock itself.

Reported-by: syzbot+65c6c92d04304d0a8efc at syzkaller.appspotmail.com
Reported-by: syzbot+e60ddfa48717579799dd at syzkaller.appspotmail.com
Signed-off-by: Takashi Iwai <tiwai at suse.de>
---
 sound/core/seq/seq_queue.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/sound/core/seq/seq_queue.c b/sound/core/seq/seq_queue.c
index caf68bf42f13..20c552cf8398 100644
--- a/sound/core/seq/seq_queue.c
+++ b/sound/core/seq/seq_queue.c
@@ -392,6 +392,7 @@ int snd_seq_queue_check_access(int queueid, int client)
 int snd_seq_queue_set_owner(int queueid, int client, int locked)
 {
 	struct snd_seq_queue *q = queueptr(queueid);
+	unsigned long flags;
 
 	if (q == NULL)
 		return -EINVAL;
@@ -401,8 +402,10 @@ int snd_seq_queue_set_owner(int queueid, int client, int locked)
 		return -EPERM;
 	}
 
+	spin_lock_irqsave(&q->owner_lock, flags);
 	q->locked = locked ? 1 : 0;
 	q->owner = client;
+	spin_unlock_irqrestore(&q->owner_lock, flags);
 	queue_access_unlock(q);
 	queuefree(q);
 
@@ -539,15 +542,17 @@ void snd_seq_queue_client_termination(int client)
 	unsigned long flags;
 	int i;
 	struct snd_seq_queue *q;
+	bool matched;
 
 	for (i = 0; i < SNDRV_SEQ_MAX_QUEUES; i++) {
 		if ((q = queueptr(i)) == NULL)
 			continue;
 		spin_lock_irqsave(&q->owner_lock, flags);
-		if (q->owner == client)
+		matched = (q->owner == client);
+		if (matched)
 			q->klocked = 1;
 		spin_unlock_irqrestore(&q->owner_lock, flags);
-		if (q->owner == client) {
+		if (matched) {
 			if (q->timer->running)
 				snd_seq_timer_stop(q->timer);
 			snd_seq_timer_reset(q->timer);
@@ -739,6 +744,8 @@ void snd_seq_info_queues_read(struct snd_info_entry *entry,
 	int i, bpm;
 	struct snd_seq_queue *q;
 	struct snd_seq_timer *tmr;
+	bool locked;
+	int owner;
 
 	for (i = 0; i < SNDRV_SEQ_MAX_QUEUES; i++) {
 		if ((q = queueptr(i)) == NULL)
@@ -750,9 +757,14 @@ void snd_seq_info_queues_read(struct snd_info_entry *entry,
 		else
 			bpm = 0;
 
+		spin_lock_irq(&q->owner_lock);
+		locked = q->locked;
+		owner = q->owner;
+		spin_unlock_irq(&q->owner_lock);
+
 		snd_iprintf(buffer, "queue %d: [%s]\n", q->queue, q->name);
-		snd_iprintf(buffer, "owned by client    : %d\n", q->owner);
-		snd_iprintf(buffer, "lock status        : %s\n", q->locked ? "Locked" : "Free");
+		snd_iprintf(buffer, "owned by client    : %d\n", owner);
+		snd_iprintf(buffer, "lock status        : %s\n", locked ? "Locked" : "Free");
 		snd_iprintf(buffer, "queued time events : %d\n", snd_seq_prioq_avail(q->timeq));
 		snd_iprintf(buffer, "queued tick events : %d\n", snd_seq_prioq_avail(q->tickq));
 		snd_iprintf(buffer, "timer state        : %s\n", tmr->running ? "Running" : "Stopped");
-- 
2.16.4