Cloudanix VulnDB

CVE-2025-21813

None

CVSS

—

EPSS

0.2%

Published

Feb 27, 2025

CISA due date

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Cloudanix analysis

Next patch cycle SSVC: Track

A Linux kernel bug can cause timers to be handled incorrectly when CPUs change power states, which may produce warnings or minor timing problems until the system is updated.

Exploitation
Other
Attacker needs
None
Complexity
Theoretical
Impact
Other
Component
OS kernel
Class
Race Condition
Remediation
Upgrade
Effort
Medium
Patch risk
Low

What it actually does

Race condition in Linux kernel timer migration allows multiple top-level roots to exist due to incorrect children counter check during root connection on CPU hotplug, breaking the single idle migrator invariant and causing mis-initialized groupmasks.

Why it matters to the business

May lead to subtle timer handling anomalies or kernel warnings on systems undergoing frequent CPU power state changes; low urgency given absence from known exploitation lists but still requires patching to maintain kernel stability across large fleets.

How an attack unfolds

An attacker with local access triggers concurrent CPU hotplug and idle entry/exit paths that exercise the flawed pre-accounting logic, resulting in disconnected or double-accounted roots that can produce the reported warning during timer interrupts.

How to fix it

Upgrade Linux kernel to version 6.12.14 or later, or apply commit c6dd70e5b465a2b77c7a7c3d868736d302e29aec on affected stable branches.

How to confirm the fix worked

Confirm kernel version is at or above the fixed release; monitor for absence of tmigr_requires_handle_remote warnings during CPU hotplug/idle cycles after update.

Typical attack path

  1. Obtain local code execution on affected kernel
  2. Force repeated CPU online/offline and idle transitions
  3. Observe timer migration state corruption via warnings or timing anomalies

How to detect it

Search kernel logs for tmigr_requires_handle_remote warnings and backtraces involving hrtimer_run_queues or tick_handle_periodic around CPU hotplug events.

  • WARNING: CPU.*PID.*at kernel/time/timer_migration.c:543 tmigr_requires_handle_remote
  • tmigr_requires_handle_remote

What your security team should do

  • Prioritize kernel updates on systems with frequent CPU hotplug or power management activity
  • Include timer_migration.c changes in automated kernel build and test pipelines
  • Add the warning condition to kernel log monitoring rules for early detection of similar issues

The record

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.

Affected packages

Ecosystem Package Introduced Fixed
Debian:13 linux 0 No fix
Debian:13 linux 0 6.12.15-1
Linux Kernel 0 6.12.14
Ubuntu:24.10 linux-realtime 0 6.11.0-1009.9

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