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

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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

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]>

The Linux kernel CVE team has assigned CVE-2026-74499 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 2.6.35 with commit 4434ade8c9334a3ab975d8993de456f06841899e and fixed in 6.6.151 with commit 29a4c29943631301e85f5e9d10f25741bd78e7ba
	Issue introduced in 2.6.35 with commit 4434ade8c9334a3ab975d8993de456f06841899e and fixed in 6.12.103 with commit 9b22a5c8310b0d55d04f5f0159b913a2fb8b444f
	Issue introduced in 2.6.35 with commit 4434ade8c9334a3ab975d8993de456f06841899e and fixed in 6.18.44 with commit b5305a0d0bb8e90a6fc9f88270d5f6c9b8c40081
	Issue introduced in 2.6.35 with commit 4434ade8c9334a3ab975d8993de456f06841899e and fixed in 7.1.8 with commit 2b7a0f330dd90dd1a7657cec0db019ee1efa4372
	Issue introduced in 2.6.35 with commit 4434ade8c9334a3ab975d8993de456f06841899e and fixed in 7.2-rc6 with commit 0970274613fb463d376211450cab066d34ebfe6a

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74499
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	sound/usb/midi.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/29a4c29943631301e85f5e9d10f25741bd78e7ba
	https://git.kernel.org/stable/c/9b22a5c8310b0d55d04f5f0159b913a2fb8b444f
	https://git.kernel.org/stable/c/b5305a0d0bb8e90a6fc9f88270d5f6c9b8c40081
	https://git.kernel.org/stable/c/2b7a0f330dd90dd1a7657cec0db019ee1efa4372
	https://git.kernel.org/stable/c/0970274613fb463d376211450cab066d34ebfe6a