CVE-2026-74594: sched/psi: Shut down rtpoll_timer in psi_cgroup_free()

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

sched/psi: Shut down rtpolltimer in psicgroupfree()

psischedulertpollwork() is called locklessly from the scheduler hotpath and can race psitriggerdestroy() taking down the last rtpoll trigger under rtpolltriggerlock:

psischedulertpollwork() psitriggerdestroy()

rcureadlock(); task = rcudereference(rtpolltask); rcuassignpointer(rtpolltask, NULL); timerdelete(&rtpolltimer); modtimer(&rtpolltimer, ...); rcureadunlock(); synchronizercu(); kthreadstop(tasktodestroy);

The group can then be freed with the re-armed timer still pending, and polltimerfn() runs on freed memory.

461daba06bdc ("psi: eliminate kthreadworker from psi trigger scheduling mechanism") deleted the timer synchronously after the synchronizercu(), which prevented this but raced trigger creation instead: the deletion could cancel the timer that a new trigger set armed during the grace period and, as creation also reinitialized the timer at the time, corrupt it. 8f91efd870ea ("psi: Fix race between psitriggercreate/destroy") moved the initialization into groupinit() and the deletion into the locked section, trading the creation races for the window above.

Neither placement in the destruction path works. A pending timer firing while the group is alive is harmless though. polltimerfn() just wakes the rtpoll waitqueue and doesn't re-arm itself. Bind the timer to the group's lifetime instead and shut it down in psicgroupfree(). Nothing can arm it by then. timershutdownsync() because the timer is never armed again.