sched_ext: Build the cid tables privately and publish them with RCU

Overview

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

sched_ext: Build the cid tables privately and publish them with RCU

The cid tables are visible to the cid kfuncs while being modified: the first enable publishes the global pointers before filling them, ops.init_cids() overrides rewrite them in place, and re-enables rebuild them in place. A racing TRACING or SYSCALL program can read unfilled entries, including uninitialized memory in the kmalloc'd tables, or torn topo updates.

Tie the tables' lifetimes to the root sched instead: each root enable builds a fresh set privately and publishes the per-table __rcu globals once the layout is final, and root disable unpublishes and RCU-frees the set. A non-NULL global is now always a fully built table which stays valid for the reader's RCU read section, and lookups stay two loads. Kfuncs treat NULL as no-mapping, also after the scheduler exits instead of reporting the stale last mapping.

The cid kfuncs are available whether the root scheduler is cid-form or cpu-form, the latter to allow gradual migration to cids. Every root therefore builds and publishes a default mapping.

Every reader must either be gated on scheduler liveness or NULL-check inside an RCU read section. Fix the two kfuncs that were neither: scx_bpf_this_cid() read the table with no RCU or preemption protection and scx_bpf_task_cid() relied on KF_RCU, which doesn't put a sleepable program in an RCU read section. The hotplug callbacks are instead serialized by retiring the tables inside the cpus_read_lock() section that clears scx_root.

v2: Document why every root builds the tables (desc + cid.c comment).

Affected

• Linux >= e9b55af47edf6e60c9a47b5604c3e15c5162ec86 < 0e1a74369bdbc357d3d9713734418d914a981553
• Linux >= e9b55af47edf6e60c9a47b5604c3e15c5162ec86 < 3a773220d39ba993dfe5d135f8b610be10794d5d
• Linux 7.2

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.