[PATCH v2] ALSA: usb-audio: fix OOB write in snd_usbmidi_akai_output()

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.

Return early when there is no room for even one SysEx, so the loop is
never entered with a bound that would wrap.  The loop is the last
statement of the function, so bailing out is equivalent to it not
running.

Discovered by XBOW, triaged by Baul Lee <[email protected]>

Fixes: 4434ade8c933 ("ALSA: usb-audio: add support for Akai MPD16")
Suggested-by: Takashi Iwai <[email protected]>
Reported-by: Federico Kirschbaum <[email protected]>
Reported-by: Baul Lee <[email protected]>
Cc: [email protected]
Signed-off-by: Baul Lee <[email protected]>
---
v2: use an explicit "buf_end <= 0" early return instead of casting the
    loop guard to int, as suggested by Takashi Iwai. Verified with the
    reproducer under KASAN on v7.2-rc4: without the patch the kernel
    reports a slab-out-of-bounds write in snd_usbmidi_akai_output(),
    with it the same run is clean.

 sound/usb/midi.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sound/usb/midi.c b/sound/usb/midi.c
index d87e3f357cf7..f8996416c3be 100644
--- a/sound/usb/midi.c
+++ b/sound/usb/midi.c
@@ -797,6 +797,8 @@ static void snd_usbmidi_akai_output(struct snd_usb_midi_out_endpoint *ep,
 
 	msg = urb->transfer_buffer + urb->transfer_buffer_length;
 	buf_end = ep->max_transfer - MAX_AKAI_SYSEX_LEN - 1;
+	if (buf_end <= 0)
+		return;
 
 	/* only try adding more data when there's space for at least 1 SysEx */
 	while (urb->transfer_buffer_length < buf_end) {
-- 
2.50.1 (Apple Git-155)