Subject: CVE-2024-42245: Revert "sched/fair: Make sure to try to detach at least one movable task"

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

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

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

Revert "sched/fair: Make sure to try to detach at least one movable task"

This reverts commit b0defa7ae03ecf91b8bfd10ede430cff12fcbd06.

b0defa7ae03ec changed the load balancing logic to ignore env.max_loop if
all tasks examined to that point were pinned. The goal of the patch was
to make it more likely to be able to detach a task buried in a long list
of pinned tasks. However, this has the unfortunate side effect of
creating an O(n) iteration in detach_tasks(), as we now must fully
iterate every task on a cpu if all or most are pinned. Since this load
balance code is done with rq lock held, and often in softirq context, it
is very easy to trigger hard lockups. We observed such hard lockups with
a user who affined O(10k) threads to a single cpu.

When I discussed this with Vincent he initially suggested that we keep
the limit on the number of tasks to detach, but increase the number of
tasks we can search. However, after some back and forth on the mailing
list, he recommended we instead revert the original patch, as it seems
likely no one was actually getting hit by the original issue.

The Linux kernel CVE team has assigned CVE-2024-42245 to this issue.

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

	Issue introduced in 6.1 with commit b0defa7ae03e and fixed in 6.1.100 with commit d467194018dd
	Issue introduced in 6.1 with commit b0defa7ae03e and fixed in 6.6.41 with commit 1e116c18e32b
	Issue introduced in 6.1 with commit b0defa7ae03e and fixed in 6.9.10 with commit 0fa6dcbfa2e2
	Issue introduced in 6.1 with commit b0defa7ae03e and fixed in 6.10 with commit 2feab2492deb

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-2024-42245
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/fair.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/d467194018dd536fe6c65a2fd3aedfcdb1424903
	https://git.kernel.org/stable/c/1e116c18e32b035a2d1bd460800072c8bf96bc44
	https://git.kernel.org/stable/c/0fa6dcbfa2e2b97c1e6febbea561badf0931a38b
	https://git.kernel.org/stable/c/2feab2492deb2f14f9675dd6388e9e2bf669c27a