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

Overview

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 snd_timer_interrupt() directly with no serialization, so two threads triggering the same utimer can run snd_timer_interrupt() on one snd_timer concurrently.

snd_timer_process_callbacks() drops timer->lock around each instance callback and marks the in-flight callback with the single SNDRV_TIMER_IFLG_CALLBACK bit; snd_timer_close_locked() 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 callback_data) 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.

snd_timer_interrupt() sets IFLG_CALLBACK 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.

Affected

• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < 996c24377eea4d4506b7c3ccbbf1e490440b5e0b
• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < 1395327a96614885552bae5fbb650e6dd182d49b
• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < c1078130a4cd7e738f4b73afe99b3e68cbfbf884
• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < 70d28bfcd6224eed75986b3b987b997e59643fa4
• Linux 6.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.