CVE-2026-74388: ALSA: seq: oss: Fix UAF at handling events with embedded SysEx data

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

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

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

ALSA: seq: oss: Fix UAF at handling events with embedded SysEx data

The OSS sequencer processes the input MIDI bytes into a sequencer
event to be dispatched later (in snd_seq_oss_midi_putc() called from
snd_seq_oss_process_event()).  When it's a SysEx data, the event
record contains data.ext.ptr pointer to the original SysEx bytes, and
the referred data is copied into the pool afterwards at dispatching.
The problem is that, if the sequencer port gets closed concurrently
before the dispatch, the OSS sequencer core also releases the
resources (in snd_seq_oss_midi_check_exit_port()), while the pending
event may hold a stale pointer, eventually leading to a UAF at a later
dispatch.

Fortunately, there is already a refcounting mechanism (snd_use_lock_t)
for the OSS MIDI device access, and for addressing the issue above, we
just need to extend the refcount until the event gets dispatched.

This patch extends snd_seq_oss_process_event() to give back the
refcount object, which is in turn released after calling the sequencer
dispatcher with the given event in the caller side.

According to the original report, KASAN report as below:

KASAN slab-use-after-free in snd_seq_event_dup+0x40c/0x470
RIP: 0033:0x7f2cb66a6340
Read of size 6
Call trace:
  dump_stack_lvl+0x73/0xb0 (?:?)
  print_report+0xd1/0x650 (?:?)
  srso_alias_return_thunk+0x5/0xfbef5 (?:?)
  __virt_addr_valid+0x1a7/0x340 (?:?)
  kasan_complete_mode_report_info+0x64/0x200 (?:?)
  kasan_report+0xf7/0x130 (?:?)
  snd_seq_event_dup+0x40c/0x470 (?:?)
  kasan_check_range+0x10c/0x1c0 (?:?)
  __asan_memcpy+0x27/0x70 (?:?)
  snd_seq_event_dup+0x9/0x470 (?:?)
  snd_seq_client_enqueue_event+0x139/0x240 (?:?)
  _raw_spin_unlock_irqrestore+0x4b/0x60 (?:?)
  snd_seq_kernel_client_enqueue+0x102/0x120 (?:?)
  snd_seq_oss_write+0x416/0x4e0 (?:?)
  apparmor_file_permission+0x20/0x30 (?:?)
  odev_write+0x3b/0x60 (?:?)
  vfs_write+0x1ce/0x850 (?:?)
  lock_release+0xc8/0x2a0 (?:?)
  __kasan_check_write+0x18/0x20 (?:?)
  __mutex_unlock_slowpath+0x129/0x510 (?:?)
  ksys_write+0xe1/0x180 (?:?)
  mutex_unlock+0x16/0x20 (?:?)
  odev_ioctl+0x65/0xc0 (?:?)
  __x64_sys_write+0x46/0x60 (?:?)
  x64_sys_call+0x7d/0x20d0 (?:?)
  do_syscall_64+0xc1/0x360 (arch/x86/entry/syscall_64.c:87)
  entry_SYSCALL_64_after_hwframe+0x77/0x7f (?:?)

The Linux kernel CVE team has assigned CVE-2026-74388 to this issue.


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

	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 6.18.40 with commit 6dc781778b595be94c31395b2cb167f65145d91f
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.1.5 with commit 7aad70cabd8f34cf11a9593fcd3f2ac3f5496943
	Issue introduced in 2.6.12 with commit 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 and fixed in 7.2-rc1 with commit 7c349b4f2a603202fb8c363bd2774a22ac2fddf3

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-2026-74388
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/core/seq/oss/seq_oss_event.c
	sound/core/seq/oss/seq_oss_event.h
	sound/core/seq/oss/seq_oss_ioctl.c
	sound/core/seq/oss/seq_oss_midi.c
	sound/core/seq/oss/seq_oss_midi.h
	sound/core/seq/oss/seq_oss_rw.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/6dc781778b595be94c31395b2cb167f65145d91f
	https://git.kernel.org/stable/c/7aad70cabd8f34cf11a9593fcd3f2ac3f5496943
	https://git.kernel.org/stable/c/7c349b4f2a603202fb8c363bd2774a22ac2fddf3