ALSA: usb-audio: Fix race against the error recovery URB submission

USB MIDI driver has an error recovery mechanism to resubmit the URB in
the delayed timer handler, and this may race with the standard start /
stop operations.  Although both start and stop operations themselves
don't race with each other due to the umidi->mutex protection, but
this isn't applied to the timer handler.

For fixing this potential race, the following changes are applied:

- Since the timer handler can't use the mutex, we apply the
  umidi->disc_lock protection at each input stream URB submission;
  this also needs to change the GFP flag to GFP_ATOMIC
- Add a check of the URB refcount and skip if already submitted
- Move the timer cancel call at disconnection to the beginning of the
  procedure; this assures the in-flight timer handler is gone properly
  before killing all pending URBs

Reported-by: syzbot+0f4ecfe6a2c322c81728@syzkaller.appspotmail.com
Reported-by: syzbot+5f1d24c49c1d2c427497@syzkaller.appspotmail.com
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200710160656.16819-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/midi.c

@@ -1499,6 +1499,8 @@ void snd_usbmidi_disconnect(struct list_head *p)
 	spin_unlock_irq(&umidi->disc_lock);
 	up_write(&umidi->disc_rwsem);
 
+	del_timer_sync(&umidi->error_timer);
+
 	for (i = 0; i < MIDI_MAX_ENDPOINTS; ++i) {
 		struct snd_usb_midi_endpoint *ep = &umidi->endpoints[i];
 		if (ep->out)
@@ -1525,7 +1527,6 @@ void snd_usbmidi_disconnect(struct list_head *p)
 			ep->in = NULL;
 		}
 	}
-	del_timer_sync(&umidi->error_timer);
 }
 EXPORT_SYMBOL(snd_usbmidi_disconnect);
 
@@ -2301,16 +2302,22 @@ void snd_usbmidi_input_stop(struct list_head *p)
 }
 EXPORT_SYMBOL(snd_usbmidi_input_stop);
 
-static void snd_usbmidi_input_start_ep(struct snd_usb_midi_in_endpoint *ep)
+static void snd_usbmidi_input_start_ep(struct snd_usb_midi *umidi,
+				       struct snd_usb_midi_in_endpoint *ep)
 {
 	unsigned int i;
+	unsigned long flags;
 
 	if (!ep)
 		return;
 	for (i = 0; i < INPUT_URBS; ++i) {
 		struct urb *urb = ep->urbs[i];
+		spin_lock_irqsave(&umidi->disc_lock, flags);
+		if (!atomic_read(&urb->use_count)) {
 			urb->dev = ep->umidi->dev;
-		snd_usbmidi_submit_urb(urb, GFP_KERNEL);
+			snd_usbmidi_submit_urb(urb, GFP_ATOMIC);
+		}
+		spin_unlock_irqrestore(&umidi->disc_lock, flags);
 	}
 }
 
@@ -2326,7 +2333,7 @@ void snd_usbmidi_input_start(struct list_head *p)
 	if (umidi->input_running || !umidi->opened[1])
 		return;
 	for (i = 0; i < MIDI_MAX_ENDPOINTS; ++i)
-		snd_usbmidi_input_start_ep(umidi->endpoints[i].in);
+		snd_usbmidi_input_start_ep(umidi, umidi->endpoints[i].in);
 	umidi->input_running = 1;
 }
 EXPORT_SYMBOL(snd_usbmidi_input_start);