In the Linux kernel, the following vulnerability has been resolved: ALSA: seq: oss: Serialize readq reset state with q->lock snd_seq_oss_r

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

ALSA: seq: oss: Serialize readq reset state with q->lock

snd_seq_oss_readq_clear() resets qlen, head, and tail without
q->lock even though the normal reader and producer paths serialize the
same ring state under that spinlock. A reset can therefore race
snd_seq_oss_readq_free() or snd_seq_oss_readq_put_event() and leave
stale records in the queue, drop freshly queued ones, or report the
wrong readiness after wakeup. KCSAN reports a data race between
snd_seq_oss_readq_clear() and snd_seq_oss_readq_free().

Take q->lock while clearing the ring and resetting input_time. Factor
the enqueue logic into a caller-locked helper so
snd_seq_oss_readq_put_timestamp() updates its suppression state under
the same lock instead of racing the reset path.

The buggy scenario involves two paths, with each column showing the
order within that path:

reset path: locked readq updater:

1. snd_seq_oss_reset() or 1. A reader or callback producer
   release reaches takes q->lock on the same queue.
   snd_seq_oss_readq_clear().
2. snd_seq_oss_readq_clear() 2. The updater tests or modifies
   resets qlen, head, tail, qlen, head, and tail.
   and input_time.
3. snd_seq_oss_readq_clear() 3. The updater completes its
   wakes sleepers on read-modify-write sequence.
   q->midi_sleep.
4. Without q->lock, the reset 4. The resulting ring state drives
   can overlap the locked later reads and readiness.
   update.

KCSAN reports:

BUG: KCSAN: data-race in snd_seq_oss_readq_clear /
snd_seq_oss_readq_free

write to 0xffff8881069fe608 of 4 bytes by task 120516 on cpu 0:
snd_seq_oss_readq_free+0x6c/0x80
snd_seq_oss_read+0xcb/0x250
odev_read+0x38/0x60
vfs_read+0xff/0x600
ksys_read+0xb4/0x140
__x64_sys_read+0x46/0x60
do_syscall_64+0xbb/0x2f0
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffff8881069fe608 of 4 bytes by task 120517 on cpu 1:
snd_seq_oss_readq_clear+0x1f/0x90
snd_seq_oss_reset+0xa7/0xf0
snd_seq_oss_ioctl+0x6f6/0x7e0
odev_ioctl+0x56/0xc0
__x64_sys_ioctl+0xd1/0x120
do_syscall_64+0xbb/0x2f0
entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x00000001 -> 0x00000000