ALSA: seq: oss: Fix racy open/close of MIDI devices

Although snd_seq_oss_midi_open() and snd_seq_oss_midi_close() can be
called concurrently from different code paths, we have no proper data
protection against races.  Introduce open_mutex to each seq_oss_midi
object for avoiding the races.
Reported-by: "Gong, Sishuai" <sishuai@purdue.edu>
Closes: https://lore.kernel.org/r/7DC9AF71-F481-4ABA-955F-76C535661E33@purdue.edu
Link: https://lore.kernel.org/r/20230612125533.27461-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/oss/seq_oss_midi.c

@@ -37,6 +37,7 @@ struct seq_oss_midi {
 	struct snd_midi_event *coder;	/* MIDI event coder */
 	struct seq_oss_devinfo *devinfo;	/* assigned OSSseq device */
 	snd_use_lock_t use_lock;
+	struct mutex open_mutex;
 };
 
 
@@ -172,6 +173,7 @@ snd_seq_oss_midi_check_new_port(struct snd_seq_port_info *pinfo)
 	mdev->flags = pinfo->capability;
 	mdev->opened = 0;
 	snd_use_lock_init(&mdev->use_lock);
+	mutex_init(&mdev->open_mutex);
 
 	/* copy and truncate the name of synth device */
 	strscpy(mdev->name, pinfo->name, sizeof(mdev->name));
@@ -322,15 +324,17 @@ snd_seq_oss_midi_open(struct seq_oss_devinfo *dp, int dev, int fmode)
 	int perm;
 	struct seq_oss_midi *mdev;
 	struct snd_seq_port_subscribe subs;
+	int err;
 
 	mdev = get_mididev(dp, dev);
 	if (!mdev)
 		return -ENODEV;
 
+	mutex_lock(&mdev->open_mutex);
 	/* already used? */
 	if (mdev->opened && mdev->devinfo != dp) {
-		snd_use_lock_free(&mdev->use_lock);
-		return -EBUSY;
+		err = -EBUSY;
+		goto unlock;
 	}
 
 	perm = 0;
@@ -340,14 +344,14 @@ snd_seq_oss_midi_open(struct seq_oss_devinfo *dp, int dev, int fmode)
 		perm |= PERM_READ;
 	perm &= mdev->flags;
 	if (perm == 0) {
-		snd_use_lock_free(&mdev->use_lock);
-		return -ENXIO;
+		err = -ENXIO;
+		goto unlock;
 	}
 
 	/* already opened? */
 	if ((mdev->opened & perm) == perm) {
-		snd_use_lock_free(&mdev->use_lock);
-		return 0;
+		err = 0;
+		goto unlock;
 	}
 
 	perm &= ~mdev->opened;
@@ -372,13 +376,17 @@ snd_seq_oss_midi_open(struct seq_oss_devinfo *dp, int dev, int fmode)
 	}
 
 	if (! mdev->opened) {
-		snd_use_lock_free(&mdev->use_lock);
-		return -ENXIO;
+		err = -ENXIO;
+		goto unlock;
 	}
 
 	mdev->devinfo = dp;
+	err = 0;
+
+ unlock:
+	mutex_unlock(&mdev->open_mutex);
 	snd_use_lock_free(&mdev->use_lock);
-	return 0;
+	return err;
 }
 
 /*
@@ -393,10 +401,9 @@ snd_seq_oss_midi_close(struct seq_oss_devinfo *dp, int dev)
 	mdev = get_mididev(dp, dev);
 	if (!mdev)
 		return -ENODEV;
-	if (! mdev->opened || mdev->devinfo != dp) {
-		snd_use_lock_free(&mdev->use_lock);
-		return 0;
-	}
+	mutex_lock(&mdev->open_mutex);
+	if (!mdev->opened || mdev->devinfo != dp)
+		goto unlock;
 
 	memset(&subs, 0, sizeof(subs));
 	if (mdev->opened & PERM_WRITE) {
@@ -415,6 +422,8 @@ snd_seq_oss_midi_close(struct seq_oss_devinfo *dp, int dev)
 	mdev->opened = 0;
 	mdev->devinfo = NULL;
 
+ unlock:
+	mutex_unlock(&mdev->open_mutex);
 	snd_use_lock_free(&mdev->use_lock);
 	return 0;
 }