ALSA: seq: Fix race of get-subscription call vs port-delete ioctls

The snd_seq_ioctl_get_subscription() retrieves the port subscriber
information as a pointer, while the object isn't protected, hence it
may be deleted before the actual reference.  This race was spotted by
syzkaller and may lead to a UAF.

The fix is simply copying the data in the lookup function that
performs in the rwsem to protect against the deletion.

Reported-by: syzbot+9437020c82413d00222d@syzkaller.appspotmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/seq_clientmgr.c

@@ -1897,20 +1897,14 @@ static int snd_seq_ioctl_get_subscription(struct snd_seq_client *client,
 	int result;
 	struct snd_seq_client *sender = NULL;
 	struct snd_seq_client_port *sport = NULL;
-	struct snd_seq_subscribers *p;
 
 	result = -EINVAL;
 	if ((sender = snd_seq_client_use_ptr(subs->sender.client)) == NULL)
 		goto __end;
 	if ((sport = snd_seq_port_use_ptr(sender, subs->sender.port)) == NULL)
 		goto __end;
-	p = snd_seq_port_get_subscription(&sport->c_src, &subs->dest);
-	if (p) {
-		result = 0;
-		*subs = p->info;
-	} else
-		result = -ENOENT;
-
+	result = snd_seq_port_get_subscription(&sport->c_src, &subs->dest,
+					       subs);
       __end:
       	if (sport)
 		snd_seq_port_unlock(sport);

sound/core/seq/seq_ports.c

@@ -632,20 +632,23 @@ int snd_seq_port_disconnect(struct snd_seq_client *connector,
 
 
 /* get matched subscriber */
-struct snd_seq_subscribers *snd_seq_port_get_subscription(struct snd_seq_port_subs_info *src_grp,
-							  struct snd_seq_addr *dest_addr)
+int snd_seq_port_get_subscription(struct snd_seq_port_subs_info *src_grp,
+				  struct snd_seq_addr *dest_addr,
+				  struct snd_seq_port_subscribe *subs)
 {
-	struct snd_seq_subscribers *s, *found = NULL;
+	struct snd_seq_subscribers *s;
+	int err = -ENOENT;
 
 	down_read(&src_grp->list_mutex);
 	list_for_each_entry(s, &src_grp->list_head, src_list) {
 		if (addr_match(dest_addr, &s->info.dest)) {
-			found = s;
+			*subs = s->info;
+			err = 0;
 			break;
 		}
 	}
 	up_read(&src_grp->list_mutex);
-	return found;
+	return err;
 }
 
 /*

sound/core/seq/seq_ports.h

@@ -135,7 +135,8 @@ int snd_seq_port_subscribe(struct snd_seq_client_port *port,
 			   struct snd_seq_port_subscribe *info);
 
 /* get matched subscriber */
-struct snd_seq_subscribers *snd_seq_port_get_subscription(struct snd_seq_port_subs_info *src_grp,
-							  struct snd_seq_addr *dest_addr);
+int snd_seq_port_get_subscription(struct snd_seq_port_subs_info *src_grp,
+				  struct snd_seq_addr *dest_addr,
+				  struct snd_seq_port_subscribe *subs);
 
 #endif