ALSA: usb-audio: Clear MIDI port active flag after draining

When a rawmidi output stream is closed, it calls the drain at first,
then does trigger-off only when the drain returns -ERESTARTSYS as a
fallback. It implies that each driver should turn off the stream
properly after the drain. Meanwhile, USB-audio MIDI interface didn't
change the port->active flag after the drain. This may leave the
output work picking up the port that is closed right now, which
eventually leads to a use-after-free for the already released rawmidi
object.

This patch fixes the bug by properly clearing the port->active flag
after the output drain.

Reported-by: [e-mail]
Cc: <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/usb/midi.c

···
 		} while (drain_urbs && timeout);
 		finish_wait(&ep->drain_wait, &wait);
 	}
+	port->active = 0;
 	spin_unlock_irq(&ep->buffer_lock);
 }