CVE-2026-68200: ALSA: timer: don't re-enter an instance callback that is still running

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

ALSA: timer: don't re-enter an instance callback that is still running

The userspace-driven timer (utimer) TRIGGER ioctl calls sndtimerinterrupt() directly with no serialization, so two threads triggering the same utimer can run sndtimerinterrupt() on one sndtimer concurrently.

sndtimerprocesscallbacks() drops timer->lock around each instance callback and marks the in-flight callback with the single SNDRVTIMERIFLGCALLBACK bit; sndtimercloselocked() waits on that bit to drain an in-flight callback before freeing the instance. The bit cannot represent two concurrent callbacks: when a second interrupt re-queues an instance whose callback is still running, both run at once, the first to finish clears the bit, and the close-path drain then frees the instance (and its callbackdata) while the other callback is still live - a use-after-free reachable by any user able to open /dev/snd/timer, both via a user timer instance and via a sequencer queue timer bound to the utimer.

sndtimerinterrupt() sets IFLGCALLBACK before dropping timer->lock, so a concurrent interrupt already observes it under the lock. Skip re-queuing an instance (and its slaves) to the ack/sack list while its callback is in flight; the accumulated pticks are delivered on the next tick, so no event is lost.