cve/published/2025/CVE-2025-21813.mbox

From bippy-5f407fcff5a0 Mon Sep 17 00:00:00 2001
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: <linux-cve-announce@vger.kernel.org>
Reply-to: <cve@kernel.org>, <linux-kernel@vger.kernel.org>
Subject: CVE-2025-21813: timers/migration: Fix off-by-one root mis-connection

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

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

timers/migration: Fix off-by-one root mis-connection

Before attaching a new root to the old root, the children counter of the
new root is checked to verify that only the upcoming CPU's top group have
been connected to it. However since the recently added commit b729cc1ec21a
("timers/migration: Fix another race between hotplug and idle entry/exit")
this check is not valid anymore because the old root is pre-accounted
as a child to the new root. Therefore after connecting the upcoming
CPU's top group to the new root, the children count to be expected must
be 2 and not 1 anymore.

This omission results in the old root to not be connected to the new
root. Then eventually the system may run with more than one top level,
which defeats the purpose of a single idle migrator.

Also the old root is pre-accounted but not connected upon the new root
creation. But it can be connected to the new root later on. Therefore
the old root may be accounted twice to the new root. The propagation of
such overcommit can end up creating a double final top-level root with a
groupmask incorrectly initialized. Although harmless given that the final
top level roots will never have a parent to walk up to, this oddity
opportunistically reported the core issue:

  WARNING: CPU: 8 PID: 0 at kernel/time/timer_migration.c:543 tmigr_requires_handle_remote
  CPU: 8 UID: 0 PID: 0 Comm: swapper/8
  RIP: 0010:tmigr_requires_handle_remote
  Call Trace:
   <IRQ>
   ? tmigr_requires_handle_remote
   ? hrtimer_run_queues
   update_process_times
   tick_periodic
   tick_handle_periodic
   __sysvec_apic_timer_interrupt
   sysvec_apic_timer_interrupt
  </IRQ>

Fix the problem by taking the old root into account in the children count
of the new root so the connection is not omitted.

Also warn when more than one top level group exists to better detect
similar issues in the future.

The Linux kernel CVE team has assigned CVE-2025-21813 to this issue.


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

	Issue introduced in 6.12.11 with commit 12ead225b7996252a8bc1a49b03aad57c0794880 and fixed in 6.12.14 with commit c6dd70e5b465a2b77c7a7c3d868736d302e29aec
	Issue introduced in 6.13 with commit b729cc1ec21a5899b7879ccfbe1786664928d597 and fixed in 6.13.3 with commit 6f449d8fa1808a7f9ee644866bbc079285dbefdd
	Issue introduced in 6.13 with commit b729cc1ec21a5899b7879ccfbe1786664928d597 and fixed in 6.14 with commit 868c9037df626b3c245ee26a290a03ae1f9f58d3

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-2025-21813
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/time/timer_migration.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/c6dd70e5b465a2b77c7a7c3d868736d302e29aec
	https://git.kernel.org/stable/c/6f449d8fa1808a7f9ee644866bbc079285dbefdd
	https://git.kernel.org/stable/c/868c9037df626b3c245ee26a290a03ae1f9f58d3