ALSA: caiaq: fix out-of-bounds read in the Traktor Kontrol S4 input parser

snd_usb_caiaq_tks4_dispatch() decodes the Traktor Kontrol S4 input
stream in fixed 16-byte (TKS4_MSGBLOCK_SIZE) message blocks. On every
iteration it advances buf and subtracts the block size while looping on
"while (len)".

len is urb->actual_length. That value is supplied by the device and is
not guaranteed to be a multiple of 16. When a final short block leaves
len between 1 and 15, the loop runs once more, reads up to buf[15], and
then does "len -= TKS4_MSGBLOCK_SIZE". As len is unsigned this underflows
to a huge value. The loop then keeps iterating and walking buf far past
the end of the 512-byte ep4_in_buf, reading out of bounds until a bogus
block id happens to be hit.

Iterate only while a full message block is available. This stops the
unsigned underflow and silently drops any trailing partial block, which
carries no complete control value anyway.

The sibling endpoint-4 parsers are not affected. The Traktor Kontrol X1
and Maschine arms in snd_usb_caiaq_ep4_reply_dispatch() floor
urb->actual_length before dispatching.

Fixes: 15c5ab6070 ("ALSA: snd-usb-caiaq: Add support for Traktor Kontrol S4")
Cc: stable@vger.kernel.org
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
Link: https://patch.msgid.link/178176259547.3343534.2724779296835237429@maoyixie.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/caiaq/input.c

@@ -330,7 +330,7 @@ static void snd_usb_caiaq_tks4_dispatch(struct snd_usb_caiaqdev *cdev,
 {
 	struct device *dev = caiaqdev_to_dev(cdev);
 
-	while (len) {
+	while (len >= TKS4_MSGBLOCK_SIZE) {
 		unsigned int i, block_id = (buf[0] << 8) | buf[1];
 
 		switch (block_id) {