CVE-2026-64477: x86,fs/resctrl: Prevent out-of-bounds access while offlining CPU when SNC enabled

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

x86,fs/resctrl: Prevent out-of-bounds access while offlining CPU when SNC enabled

The architecture updates the cpumask in a domain's header to track which online CPUs are associated with the domain. When this mask becomes empty the architecture initiates offline of the domain that includes calling on resctrl fs to offline the domain. If it is a monitoring domain in which LLC occupancy is tracked resctrl fs forces the limbo handler to clear all busy RMID state associated with the domain.

The limbo handler always reads the current event value associated with a busy RMID irrespective of it being checked as part of regular "is it still busy" check or whether it will be forced released anyway. When reading an RMID on a system with SNC enabled the "logical RMID" is converted to the "physical RMID" and this conversion requires the NUMA node ID of the resctrl monitoring domain that is in turn determined by querying the NUMA node ID of any CPU belonging to the monitoring domain.

When the monitoring domain is going offline its cpumask is empty causing the NUMA node ID query via cputonode() to be done with "nrcpuids" as argument resulting in an out-of-bounds access.

Refactor the limbo handler to skip reading the RMID when the RMID will just be forced to no longer be dirty in the domain anyway. Add a safety check to the architecture's RMID reader to protect against this scenario.