CVE-2026-89520: sched/core: Make core-sched flips wait for in-flight selections

CVE-2026-89520: sched/core: Make core-sched flips wait for in-flight selections

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

sched/core: Make core-sched flips wait for in-flight selections

Core scheduling's pick_next_task() operates on all sibling rqs under one
acquisition of the shared core-wide lock. A ->pick_task() that releases the
rq lock leaves every sibling __lock momentarily free, letting
__sched_core_flip(false) complete mid-selection and rebind rq_lockp() 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->core_pick_in_flight
and make __sched_core_flip() 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.

sched_core_cpu_deactivate() 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.

The Linux kernel CVE team has assigned CVE-2026-89520 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.14 with commit 539f65125d20aacab54d02d77f10a839f45b09dc and fixed in 6.18.51 with commit 9d69422ddaa13526c2223444e237d948d6e44728
	Issue introduced in 5.14 with commit 539f65125d20aacab54d02d77f10a839f45b09dc and fixed in 7.2.4 with commit 23eda24f325a6e6d686d30ec75f2e9d6745e2015
	Issue introduced in 5.14 with commit 539f65125d20aacab54d02d77f10a839f45b09dc and fixed in 7.3-rc1 with commit f3629c63a4af3e491381780bc6c123cb498c4c40

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-89520
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	kernel/sched/core.c
	kernel/sched/sched.h


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/9d69422ddaa13526c2223444e237d948d6e44728
	https://git.kernel.org/stable/c/23eda24f325a6e6d686d30ec75f2e9d6745e2015
	https://git.kernel.org/stable/c/f3629c63a4af3e491381780bc6c123cb498c4c40