CVE-2026-43417: sched/mmcid: Handle vfork()/CLONE_VM correctly

CVE-2026-43417: sched/mmcid: Handle vfork()/CLONE_VM correctly

[Greg Kroah-Hartman]

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

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

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

sched/mmcid: Handle vfork()/CLONE_VM correctly

Matthieu and Jiri reported stalls where a task endlessly loops in
mm_get_cid() when scheduling in.

It turned out that the logic which handles vfork()'ed tasks is broken. It
is invoked when the number of tasks associated to a process is smaller than
the number of MMCID users. It then walks the task list to find the
vfork()'ed task, but accounts all the already processed tasks as well.

If that double processing brings the number of to be handled tasks to 0,
the walk stops and the vfork()'ed task's CID is not fixed up. As a
consequence a subsequent schedule in fails to acquire a (transitional) CID
and the machine stalls.

Cure this by removing the accounting condition and make the fixup always
walk the full task list if it could not find the exact number of users in
the process' thread list.

The Linux kernel CVE team has assigned CVE-2026-43417 to this issue.


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

	Issue introduced in 6.19 with commit fbd0e71dc370af73f6b316e4de9eed273dd90340 and fixed in 6.19.9 with commit e6761cdce78a8919a537989afb6aaf6881469f83
	Issue introduced in 6.19 with commit fbd0e71dc370af73f6b316e4de9eed273dd90340 and fixed in 7.0 with commit 28b5a1395036d6c7a6c8034d85ad3d7d365f192c

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-2026-43417
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


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/e6761cdce78a8919a537989afb6aaf6881469f83
	https://git.kernel.org/stable/c/28b5a1395036d6c7a6c8034d85ad3d7d365f192c