CVE-2026-68421: sched_ext: Don't warn on core-sched forced idle in put_prev_task_scx()

From: Greg Kroah-Hartman <[email protected]>

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

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

sched_ext: Don't warn on core-sched forced idle in put_prev_task_scx()

put_prev_task_scx() warns when a runnable task drops to a lower sched_class
without SCX_OPS_ENQ_LAST, on the assumption that balance_one() would have
kept it running. Core scheduling breaks that: a forced-idle SMT sibling
reschedules through the core_pick fast path in pick_next_task(), which skips
pick_task_scx() and thus balance_one(), so a runnable task can drop to idle
with ENQ_LAST unset.

Gate the warning on sched_cpu_cookie_match(): a cookie mismatch means core
scheduling forced the idle, while a match (or core scheduling off) still
catches a genuine missing-ENQ_LAST drop.

The Linux kernel CVE team has assigned CVE-2026-68421 to this issue.


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

	Issue introduced in 6.12 with commit 7c65ae81ea86a6ed8086e1a5651acd766187f19b and fixed in 6.18.42 with commit 2907e9d0f05b506dfd58aec589a23042a56ef36b
	Issue introduced in 6.12 with commit 7c65ae81ea86a6ed8086e1a5651acd766187f19b and fixed in 7.1.6 with commit e2f188cdbf8312289532c36eb4e9eb1c9544d43a
	Issue introduced in 6.12 with commit 7c65ae81ea86a6ed8086e1a5651acd766187f19b and fixed in 7.2-rc4 with commit b7d9c359e5cf867f7eb23df3bb1c6b9e58af24da

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-2026-68421
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/ext/ext.c


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/2907e9d0f05b506dfd58aec589a23042a56ef36b
	https://git.kernel.org/stable/c/e2f188cdbf8312289532c36eb4e9eb1c9544d43a
	https://git.kernel.org/stable/c/b7d9c359e5cf867f7eb23df3bb1c6b9e58af24da