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

[Baul Lee <baul.lee@xbow.com>]

snd_usbmidi_akai_output() computes its fill-loop bound

	buf_end = ep->max_transfer - MAX_AKAI_SYSEX_LEN - 1;

as a signed int, so a small device-advertised bulk-OUT max_transfer
makes buf_end negative.  The loop guard then compares the u32
urb->transfer_buffer_length against that negative int: the usual
arithmetic conversion turns buf_end into a large unsigned value, so the
guard stays true and each iteration keeps appending SysEx framing and
payload bytes past the end of the URB transfer buffer, which is only
max_transfer bytes long.

A USB device that advertises a tiny bulk-OUT endpoint can therefore
trigger an attacker-length- and content-controlled heap out-of-bounds
write when a process writes to the created /dev/snd/midiC*D* node.

Perform the comparison in signed arithmetic so that a negative buf_end
stops the loop instead of wrapping to a huge unsigned bound.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>

Fixes: 4434ade8c933 ("ALSA: usb-audio: add support for Akai MPD16")
Reported-by: Federico Kirschbaum <federico.kirschbaum@xbow.com>
Reported-by: Baul Lee <baul.lee@xbow.com>
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
---
 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 d87e3f357cf7..cc7df77632a1 100644
--- a/sound/usb/midi.c
+++ b/sound/usb/midi.c
@@ -799,7 +799,7 @@ static void snd_usbmidi_akai_output(struct snd_usb_midi_out_endpoint *ep,
 	buf_end = ep->max_transfer - MAX_AKAI_SYSEX_LEN - 1;
 
 	/* only try adding more data when there's space for at least 1 SysEx */
-	while (urb->transfer_buffer_length < buf_end) {
+	while ((int)urb->transfer_buffer_length < buf_end) {
 		count = snd_rawmidi_transmit_peek(substream,
 						  tmp, MAX_AKAI_SYSEX_LEN);
 		if (!count) {
-- 
2.50.1 (Apple Git-155)