CVE-2026-80973: ALSA: 6fire: bound the MIDI event length from the device

CVE-2026-80973: ALSA: 6fire: bound the MIDI event length from the device

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

ALSA: 6fire: bound the MIDI event length from the device

usb6fire_comm_receiver_handler() forwards a MIDI event using a length
byte the device supplies, with no bound and no check that the transfer
delivered that many bytes:

	if (!urb->status) {
		if (rt->receiver_buffer[0] == 0x10) /* midi in event */
			if (midi_rt)
				midi_rt->in_received(midi_rt,
						rt->receiver_buffer + 2,
						rt->receiver_buffer[1]);
	}

receiver_buffer is a 64-byte kzalloc() buffer (COMM_RECEIVER_BUFSIZE), so
only 62 bytes follow the two-byte header.  receiver_buffer[1] is a u8 the
device chooses, so a device that answers with 0x10 and a length of 0xFF
makes snd_rawmidi_receive() read 255 bytes starting two bytes into a
64-byte object.  The bytes past the buffer are handed to userspace
through the rawmidi read path.

urb->actual_length is not consulted either, so a short transfer leaves
both the type byte and the length byte at their previous values and the
handler acts on stale data.

The receiver URB is submitted from usb6fire_comm_init() at probe, so the
read happens on plug with no user action; forwarding to userspace also
needs a MIDI input substream open, since usb6fire_midi_in_received()
only calls snd_rawmidi_receive() when rt->in is set.

KASAN on 7.2.0-rc5 (arm64), single packet from an emulated device:

  BUG: KASAN: slab-out-of-bounds in snd_rawmidi_receive
  Read of size 255 at addr ffff000009f64682 by task bash/183
   __asan_memcpy
   snd_rawmidi_receive
   usb6fire_midi_in_received [snd_usb_6fire]
   usb6fire_comm_receiver_handler [snd_usb_6fire]
  Allocated by task 11:
   usb6fire_comm_init [snd_usb_6fire]
   usb6fire_chip_probe [snd_usb_6fire]
  The buggy address is located 2 bytes inside of
   allocated 64-byte region [ffff000009f64680, ffff000009f646c0)

Reject the event when the length exceeds the bytes that follow the
header, and require the transfer to have delivered the header plus that
many bytes.  The receiver URB is submitted with a 64-byte
transfer_buffer_length, so a genuine device cannot deliver an event
longer than those 62 bytes and nothing valid is dropped.

Discovered by XBOW, triaged by Baul Lee <baul.lee@xbow.com>

The Linux kernel CVE team has assigned CVE-2026-80973 to this issue.


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

	Issue introduced in 2.6.39 with commit c6d43ba816d1cf1d125bfbfc938f2a28a87facf9 and fixed in 6.12.109 with commit 466e911bbbbb779bb06337e35a286e5ca7af16b3
	Issue introduced in 2.6.39 with commit c6d43ba816d1cf1d125bfbfc938f2a28a87facf9 and fixed in 6.18.50 with commit 2a6f6fba3bd31d2e8c957fefa29156e35e5e75d5
	Issue introduced in 2.6.39 with commit c6d43ba816d1cf1d125bfbfc938f2a28a87facf9 and fixed in 7.2.4 with commit 34816e2cfeabafb8eccf54687186ce25699e8363
	Issue introduced in 2.6.39 with commit c6d43ba816d1cf1d125bfbfc938f2a28a87facf9 and fixed in 7.3-rc1 with commit a478893b59e36cfe7d77a76b352f2db55502e879

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-2026-80973
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/6fire/comm.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/466e911bbbbb779bb06337e35a286e5ca7af16b3
	https://git.kernel.org/stable/c/2a6f6fba3bd31d2e8c957fefa29156e35e5e75d5
	https://git.kernel.org/stable/c/34816e2cfeabafb8eccf54687186ce25699e8363
	https://git.kernel.org/stable/c/a478893b59e36cfe7d77a76b352f2db55502e879