ALSA: timer: drain a slave's callback before its master detaches it

Overview

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

ALSA: timer: drain a slave's callback before its master detaches it

snd_timer_close_locked() drains the closing instance's own in-flight callback (IFLG_CALLBACK) before freeing it, but not its slaves'. When a master instance is closed, remove_slave_links() clears each slave's ->timer; the slave's own close then reads timer == NULL and takes the branch that skips the drain entirely (snd_timer_stop_slave() also no-ops on a NULL timer). So a slave whose callback is still running when the master is closed is freed underneath the live callback, leading to use-after-free.

Drain the slaves too before remove_slave_links() severs them. snd_timer_stop() has already taken this instance off the active list, so no new slave callback can be queued. Take the slaves off the ack list so a pending one can't fire either, then wait for any that is already in flight.

Affected

• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < cd461bcfcdf8d6b6b5365941c1d3859f8bc77aa0
• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < 426c0ff1c433d6030610ad4f9375746dfe931caa
• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < 2b298997786876b225cff2446e11a0fa6f602f6d
• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < bdefe1346a8e6b8dc8593406dc2617e985fcbcab
• Linux 6.12

Remediation

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