[PATCH] ALSA: usb-audio: Avoid dropping MIDI events at closing multiple ports

[Takashi Iwai <tiwai@xxxxxxx>]

We fixed the UAF issue in USB MIDI code by canceling the pending work
at closing each MIDI output device in the commit below.  However, this
assumed that it's the only one that is tied with the endpoint, and it
resulted in unexpected data truncations when multiple devices are
assigned to a single endpoint and opened simultaneously.

For addressing the unexpected MIDI message drops, simply replace
cancel_work_sync() with flush_work().  The drain callback should have
been already invoked before the close callback, hence the port->active
flag must be already cleared.  So this just assures that the pending
work is finished before freeing the resources.

Fixes: 0125de38122f ("ALSA: usb-audio: Cancel pending work at closing a MIDI substream")
Reported-and-tested-by: John Keeping <jkeeping@xxxxxxxxxxxxxxxxx>
Closes: https://lore.kernel.org/20250217111647.3368132-1-jkeeping@xxxxxxxxxxxxxxxxx
Signed-off-by: Takashi Iwai <tiwai@xxxxxxx>
---
 sound/usb/midi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sound/usb/midi.c b/sound/usb/midi.c
index 737dd00e97b1..779d97d31f17 100644
--- a/sound/usb/midi.c
+++ b/sound/usb/midi.c
@@ -1145,7 +1145,7 @@ static int snd_usbmidi_output_close(struct snd_rawmidi_substream *substream)
 {
 	struct usbmidi_out_port *port = substream->runtime->private_data;
 
-	cancel_work_sync(&port->ep->work);
+	flush_work(&port->ep->work);
 	return substream_open(substream, 0, 0);
 }
 
-- 
2.43.0