AZL-98145

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