ALSA: seq: Fix copy_from_user() call inside lock

The event handler in the virmidi sequencer code takes a read-lock for
the linked list traverse, while it's calling snd_seq_dump_var_event()
in the loop. The latter function may expand the user-space data
depending on the event type. It eventually invokes copy_from_user(),
which might be a potential dead-lock.

The sequencer core guarantees that the user-space data is passed only
with atomic=0 argument, but snd_virmidi_dev_receive_event() ignores it
and always takes read-lock(). For avoiding the problem above, this
patch introduces rwsem for non-atomic case, while keeping rwlock for
atomic case.

Also while we're at it: the superfluous irq flags is dropped in
snd_virmidi_input_open().

Reported-by: Jia-Ju Bai <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Takashi Iwai <[e-mail]>

include/sound/seq_virmidi.h

···
 	int port;			/* created/attached port */
 	unsigned int flags;		/* SNDRV_VIRMIDI_* */
 	rwlock_t filelist_lock;
+	struct rw_semaphore filelist_sem;
 	struct list_head filelist;
 };
 

sound/core/seq/seq_virmidi.c

···
  * decode input event and put to read buffer of each opened file
  */
 static int snd_virmidi_dev_receive_event(struct snd_virmidi_dev *rdev,
-					 struct snd_seq_event *ev)
+					 struct snd_seq_event *ev,
+					 bool atomic)
 {
 	struct snd_virmidi *vmidi;
 	unsigned char msg[4];
 	int len;
 
-	read_lock(&rdev->filelist_lock);
+	if (atomic)
+		read_lock(&rdev->filelist_lock);
+	else
+		down_read(&rdev->filelist_sem);
 	list_for_each_entry(vmidi, &rdev->filelist, list) {
 		if (!vmidi->trigger)
 			continue;
···
 				snd_rawmidi_receive(vmidi->substream, msg, len);
 		}
 	}
-	read_unlock(&rdev->filelist_lock);
+	if (atomic)
+		read_unlock(&rdev->filelist_lock);
+	else
+		up_read(&rdev->filelist_sem);
 
 	return 0;
 }
···
 	struct snd_virmidi_dev *rdev;
 
 	rdev = rmidi->private_data;
-	return snd_virmidi_dev_receive_event(rdev, ev);
+	return snd_virmidi_dev_receive_event(rdev, ev, true);
 }
 #endif  /*  0  */
 
···
 	rdev = private_data;
 	if (!(rdev->flags & SNDRV_VIRMIDI_USE))
 		return 0; /* ignored */
-	return snd_virmidi_dev_receive_event(rdev, ev);
+	return snd_virmidi_dev_receive_event(rdev, ev, atomic);
 }
 
 /*
···
 	struct snd_virmidi_dev *rdev = substream->rmidi->private_data;
 	struct snd_rawmidi_runtime *runtime = substream->runtime;
 	struct snd_virmidi *vmidi;
-	unsigned long flags;
 
 	vmidi = kzalloc(sizeof(*vmidi), GFP_KERNEL);
 	if (vmidi == NULL)
···
 	vmidi->client = rdev->client;
 	vmidi->port = rdev->port;	
 	runtime->private_data = vmidi;
-	write_lock_irqsave(&rdev->filelist_lock, flags);
+	down_write(&rdev->filelist_sem);
+	write_lock_irq(&rdev->filelist_lock);
 	list_add_tail(&vmidi->list, &rdev->filelist);
-	write_unlock_irqrestore(&rdev->filelist_lock, flags);
+	write_unlock_irq(&rdev->filelist_lock);
+	up_write(&rdev->filelist_sem);
 	vmidi->rdev = rdev;
 	return 0;
 }
···
 	struct snd_virmidi_dev *rdev = substream->rmidi->private_data;
 	struct snd_virmidi *vmidi = substream->runtime->private_data;
 
+	down_write(&rdev->filelist_sem);
 	write_lock_irq(&rdev->filelist_lock);
 	list_del(&vmidi->list);
 	write_unlock_irq(&rdev->filelist_lock);
+	up_write(&rdev->filelist_sem);
 	snd_midi_event_free(vmidi->parser);
 	substream->runtime->private_data = NULL;
 	kfree(vmidi);
···
 	rdev->rmidi = rmidi;
 	rdev->device = device;
 	rdev->client = -1;
+	init_rwsem(&rdev->filelist_sem);
 	rwlock_init(&rdev->filelist_lock);
 	INIT_LIST_HEAD(&rdev->filelist);
 	rdev->seq_mode = SNDRV_VIRMIDI_SEQ_DISPATCH;