CVE-2024-57951

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

hrtimers: Handle CPU state correctly on hotplug

Consider a scenario where a CPU transitions from CPUHP_ONLINE to halfway

through a CPU hotunplug down to CPUHP_HRTIMERS_PREPARE, and then back to

CPUHP_ONLINE:

Since hrtimers_prepare_cpu() does not run, cpu_base.hres_active remains set

to 1 throughout. However, during a CPU unplug operation, the tick and the

clockevents are shut down at CPUHP_AP_TICK_DYING. On return to the online

state, for instance CFS incorrectly assumes that the hrtick is already

active, and the chance of the clockevent device to transition to oneshot

mode is also lost forever for the CPU, unless it goes back to a lower state

than CPUHP_HRTIMERS_PREPARE once.

This round-trip reveals another issue; cpu_base.online is not set to 1

after the transition, which appears as a WARN_ON_ONCE in enqueue_hrtimer().

Aside of that, the bulk of the per CPU state is not reset either, which

means there are dangling pointers in the worst case.

Address this by adding a corresponding startup() callback, which resets the

stale per CPU state and sets the online flag.

[ tglx: Make the new callback unconditionally available, remove the online

modification in the prepare() callback and clear the remaining

state in the starting callback instead of the prepare callback ]