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

snd_usbmidi_novation_output() lays out a two-byte header at
transfer_buffer[0..1] and passes &transfer_buffer[2] together with a
length of ep->max_transfer - 2 to snd_rawmidi_transmit():

	count = snd_rawmidi_transmit(ep->ports[0].substream,
				     &transfer_buffer[2],
				     ep->max_transfer - 2);

ep->max_transfer comes from the output endpoint's wMaxPacketSize via
usb_maxpacket(). A malformed or malicious device can advertise a bulk
OUT endpoint with a wMaxPacketSize of 1 - the USB core only clamps this
value downwards - so ep->max_transfer becomes 1 and the count argument
becomes -1.

snd_rawmidi_transmit() passes the negative count on to
__snd_rawmidi_transmit_peek(), where "if (count1 > count) count1 = count"
leaves count1 negative; get_aligned_size() keeps it negative for a
byte-stream substream, so the following memcpy(buffer, ..., count1) runs
with a (size_t)-1 length and writes far past the transfer buffer, which
was allocated with usb_alloc_coherent(ep->max_transfer).

This is the same class of bug that was fixed for snd_usbmidi_akai_output()
in commit 0970274613fb ("ALSA: usb-audio: fix OOB write in
snd_usbmidi_akai_output()"); the novation output routine was left
unguarded. Bail out when the endpoint cannot hold the two-byte header
plus at least one payload byte.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: [email protected]
Signed-off-by: Marouane El Moufid <[email protected]>
---
 sound/usb/midi.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/sound/usb/midi.c b/sound/usb/midi.c
index f8996416c3be..8a9bc37f0b6e 100644
--- a/sound/usb/midi.c
+++ b/sound/usb/midi.c
@@ -875,6 +875,8 @@ static void snd_usbmidi_novation_output(struct snd_usb_midi_out_endpoint *ep,
 
 	if (!ep->ports[0].active)
 		return;
+	if (ep->max_transfer < 3)
+		return;
 	transfer_buffer = urb->transfer_buffer;
 	count = snd_rawmidi_transmit(ep->ports[0].substream,
 				     &transfer_buffer[2],
 				     ep->max_transfer - 2);
-- 
2.47.3