CVE-2026-98972: cgroup/cpuset: Don't access cpuset_cgrp_subsys.root in is_in_v2_mode()

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

cgroup/cpuset: Don't access cpusetcgrpsubsys.root in isinv2mode()

After seeing the patch [1] to guard isinv2mode() with RCU, it makes me realize that isinv2mode() may be called in a context where a new cgroup filesystem is being rebound with stale cpusetcgrpsubsys.root pointer. Avoid this potential UaF situation by adding a new cpusetv2mode flag which is set when the cpusetv2mode mount option is used. This flag is written into only when cpusetbind() is being called with a stable cpusetcgrpsubsys.root value. The isinv2mode() helper is modified to read the new cpusetv2mode flag instead of accessing cpusetcgrpsubsys.root directly.

[1] https://lore.kernel.org/lkml/20260929084124.626693-2-arighi@nvidia.com