[PATCH] ALSA: usb-audio: fix OOB write in snd_usbmidi_us122l_output()

[HyeongJun An <sammiee5311@gmail.com>]

The snd_usbmidi_us122l_output() picks a count of 2 on anything slower
than high speed and never relates it to ep->max_transfer.  The URB
buffer holds exactly max_transfer bytes, so a device declaring a one
byte bulk endpoint takes two bytes from snd_rawmidi_transmit(), and the
memset that pads the rest computes 1 - 2 in int and wraps to SIZE_MAX.

Only 0x800e and 0x800f are pinned to nine bytes.  The US-122MKII at
0x0644:0x8021 falls to the default and takes usb_maxpacket(), which the
USB core only clamps downward.

The akai and novation output ops in this file were given the same guard
recently.  Do the same here.

Fixes: 030a07e44129 ("ALSA: Add USB US122L driver")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-5
Signed-off-by: HyeongJun An <sammiee5311@gmail.com>
---
 sound/usb/midi.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sound/usb/midi.c b/sound/usb/midi.c
index f8996416c3be..1a5210556981 100644
--- a/sound/usb/midi.c
+++ b/sound/usb/midi.c
@@ -969,6 +969,8 @@ static void snd_usbmidi_us122l_output(struct snd_usb_midi_out_endpoint *ep,
 	default:
 		count = 2;
 	}
+	if (ep->max_transfer < count)
+		return;
 	count = snd_rawmidi_transmit(ep->ports[0].substream,
 				     urb->transfer_buffer,
 				     count);
-- 
2.43.0