CVE-2026-74731: sched_ext: Skip sub-disable teardown for never-linked sub-schedulers

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

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

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

sched_ext: Skip sub-disable teardown for never-linked sub-schedulers

A sub-scheduler enable can fail before scx_link_sched() links the sched into
the hierarchy, e.g. when the parent is already being disabled, and cleanup
still runs the full scx_sub_disable().

That is racy against root disable: drain_descendants() is the only ordering
between a sub's disable-time task walk and root disable's all-task teardown,
and an unlinked sub is invisible to it. Root's teardown can thus run between
the never-linked sub's drain and its walk, exiting every task to no
scheduler.

The walk then trips the membership WARN and re-homes the exited tasks onto
the dying hierarchy, a use-after-free.

Skip the cgroup ownership reset and the task walk if @sch was never linked,
indicated by the empty ->sibling as unlinking only happens later in the same
function. The membership WARN remains valid: a linked sub is always waited
on by an ancestor's drain.

The Linux kernel CVE team has assigned CVE-2026-74731 to this issue.


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

	Issue introduced in 7.1 with commit 337ec00b1d9c676f637651c2cefddb8612b867ee and fixed in 7.1.9 with commit 6428093a4a986c38c9089b5eb32b56d914ef437a
	Issue introduced in 7.1 with commit 337ec00b1d9c676f637651c2cefddb8612b867ee and fixed in 7.2 with commit 8c13364db9c9a43ed286f3a8d0fb9477b1adc43c

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-2026-74731
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/6428093a4a986c38c9089b5eb32b56d914ef437a
	https://git.kernel.org/stable/c/8c13364db9c9a43ed286f3a8d0fb9477b1adc43c