CVE-2026-74499: ALSA: usb-audio: fix OOB write in snd_usbmidi_akai_output()

In the Linux kernel, the following vulnerability has been resolved:

ALSA: usb-audio: fix OOB write in sndusbmidiakaioutput()

sndusbmidiakaioutput() computes its fill-loop bound

bufend = ep->maxtransfer - MAXAKAISYSEXLEN - 1;

as a signed int, so a small device-advertised bulk-OUT maxtransfer makes bufend negative. The loop guard then compares the u32 urb->transferbufferlength against that negative int: the usual arithmetic conversion turns bufend 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 maxtransfer 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/midiCD 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 <baul.lee@xbow.com>