UBUNTU-CVE-2026-89520: High severity ubuntu/linux-hwe-edge vulnerability

In the Linux kernel, the following vulnerability has been resolved: sched/core: Make core-sched flips wait for in-flight selections Core scheduling's picknexttask() operates on all sibling rqs under one acquisition of the shared core-wide lock. A ->picktask() that releases the rq lock leaves every sibling lock momentarily free, letting schedcoreflip(false) complete mid-selection and rebind rqlockp() under it. The selection resumes on the split locks, touching sibling state it no longer protects, and schedule() finally releases a lock that was never taken while leaking the one that was. Count in-flight core-wide selections in the leader's rq->corepickinflight and make schedcoreflip() wait for the count to drain. The count only changes under the shared lock, which the flip holds while sampling, so no other ordering is needed. The wait can repeat while selections overlap, but the flip backs off between samples and flips are rare cookie-lifetime events. schedcorecpudeactivate() moves the count to the new leader - a stale copy left behind would bias it forever if that CPU later returns as its own leader.