CVE-2026-43418: sched/mmcid: Prevent CID stalls due to concurrent forks

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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

sched/mmcid: Prevent CID stalls due to concurrent forks

A newly forked task is accounted as MMCID user before the task is visible
in the process' thread list and the global task list. This creates the
following problem:

 CPU1			CPU2
 fork()
   sched_mm_cid_fork(tnew1)
     tnew1->mm.mm_cid_users++;
     tnew1->mm_cid.cid = getcid()
-> preemption
			fork()
			  sched_mm_cid_fork(tnew2)
			    tnew2->mm.mm_cid_users++;
                            // Reaches the per CPU threshold
			    mm_cid_fixup_tasks_to_cpus()
			    for_each_other(current, p)
			         ....

As tnew1 is not visible yet, this fails to fix up the already allocated CID
of tnew1. As a consequence a subsequent schedule in might fail to acquire a
(transitional) CID and the machine stalls.

Move the invocation of sched_mm_cid_fork() after the new task becomes
visible in the thread and the task list to prevent this.

This also makes it symmetrical vs. exit() where the task is removed as CID
user before the task is removed from the thread and task lists.

The Linux kernel CVE team has assigned CVE-2026-43418 to this issue.


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

	Issue introduced in 6.19 with commit fbd0e71dc370af73f6b316e4de9eed273dd90340 and fixed in 6.19.9 with commit f0189d49282e0458f3a737bd486c1ec048148f66
	Issue introduced in 6.19 with commit fbd0e71dc370af73f6b316e4de9eed273dd90340 and fixed in 7.0 with commit b2e48c429ec54715d16fefa719dd2fbded2e65be

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-2026-43418
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:
	include/linux/sched.h
	kernel/fork.c
	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/f0189d49282e0458f3a737bd486c1ec048148f66
	https://git.kernel.org/stable/c/b2e48c429ec54715d16fefa719dd2fbded2e65be