Subject: CVE-2024-35985: sched/eevdf: Prevent vlag from going out of bounds in reweight_eevdf()

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

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

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

sched/eevdf: Prevent vlag from going out of bounds in reweight_eevdf()

It was possible to have pick_eevdf() return NULL, which then causes a
NULL-deref. This turned out to be due to entity_eligible() returning
falsely negative because of a s64 multiplcation overflow.

Specifically, reweight_eevdf() computes the vlag without considering
the limit placed upon vlag as update_entity_lag() does, and then the
scaling multiplication (remember that weight is 20bit fixed point) can
overflow. This then leads to the new vruntime being weird which then
causes the above entity_eligible() to go side-ways and claim nothing
is eligible.

Thus limit the range of vlag accordingly.

All this was quite rare, but fatal when it does happen.

The Linux kernel CVE team has assigned CVE-2024-35985 to this issue.

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

	Issue introduced in 6.6.4 with commit 14204acc09f6 and fixed in 6.6.30 with commit 470d347b14b0
	Issue introduced in 6.7 with commit eab03c23c2a1 and fixed in 6.8.9 with commit 06f27e6d7bf0
	Issue introduced in 6.7 with commit eab03c23c2a1 and fixed in 6.9 with commit 1560d1f6eb6b

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-2024-35985
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/470d347b14b0ecffa9b39cf8f644fa2351db3efb
	https://git.kernel.org/stable/c/06f27e6d7bf0abf54488259ef36bbf0e1fccb35c
	https://git.kernel.org/stable/c/1560d1f6eb6b398bddd80c16676776c0325fe5fe