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Fencer Vulns
medium 5.5

CVE-2025-21813 json

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

published 2025-02-27 20 mo ago modified 2026-06-17 4 mo ago also EUVD-2025-5971

cvss
5.5
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 8%
public exploits
0
none known

Summary

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.

Patch / fix

  • Linux Linux 6.12.14, 6.13.3, 6.14
  • linux Debian 14, Debian 13 6.12.15-1

debian advisories DEBIAN-CVE-2025-21813

  • linux-gcp-6.11 Ubuntu 24.04 LTS 6.11.0-1014.14~24.04.1
  • linux-hwe-6.11 Ubuntu 24.04 LTS 6.11.0-25.25~24.04.1
  • linux-oem-6.11 Ubuntu 24.04 LTS 6.11.0-1021.21
  • linux-azure-6.11 Ubuntu 24.04 LTS 6.11.0-1014.14~24.04.1
  • linux-lowlatency-hwe-6.11 Ubuntu 24.04 LTS 6.11.0-1013.14~24.04.1

ubuntu advisories UBUNTU-CVE-2025-21813

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2025-21813 updated cvss 4.7

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

  • Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more Red Hat Enterprise Linux 8 perf, kernel, bpftool, kernel-rt, kernel-core, kernel-tools, +29 more Red Hat Enterprise Linux 9 rv, perf, rtla, kernel, bpftool, libperf, +83 more Red Hat Enterprise Linux 10 kernel
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

0.19% chance of exploitation in the next 30 days, higher than 8% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 5.5 CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Affected products

  • Linux Linux

Classification

Weakness
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.