CVE-2026-46154: sched_ext: Read scx_root under scx_cgroup_ops_rwsem in cgroup setters

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

schedext: Read scxroot under scxcgroupopsrwsem in cgroup setters

scxgroupset{weight,idle,bandwidth}() cache scxroot before acquiring scxcgroupopsrwsem, so the pointer can be stale by the time the op runs. If the loaded scheduler is disabled and freed (via RCU work) and another is enabled between the naked load and the rwsem acquire, the reader sees scxcgroupenabled=true (the new scheduler's) but dereferences the freed one - UAF on SCXHASOP(sch, ...) / SCXCALLOP(sch, ...).

scxcgroupenabled is toggled only under scxcgroupopsrwsem write (scxcgroup{init,exit}), so reading scxroot inside the rwsem read section correlates @sch with the enabled snapshot.