# CVE-2025-21813: timers/migration: Fix off-by-one root mis-connection

> An off-by-one error in Linux kernel timer migration logic can cause incorrect root connections, potentially leading to system instability or warnings.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Feb 2025
- **Updated:** 23 May 2026
- **Also known as:** BELL-CVE-2025-21813, DEBIAN-CVE-2025-21813, GHSA-xjrx-58mf-555f, UBUNTU-CVE-2025-21813, USN-7489-1, USN-7489-2

## What is CVE-2025-21813?

CVE-2025-21813 is a medium-severity vulnerability in Linux kernel, published on 27 Feb 2025. It can only be exploited from the same machine or local network, with no action from the victim.

### Official 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.

Weakness: CWE-193 (Off-by-one Error)

## Which versions are affected by CVE-2025-21813?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2025-21813?

Apply the vendor's patch.

## Is CVE-2025-21813 being exploited?

There is no known exploitation of CVE-2025-21813 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-21813 and fix it if it is affected.

What it is: CVE-2025-21813 is a medium-severity vulnerability in Linux kernel, published on 27 Feb 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-21813 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-21813/ (Markdown: https://insights.sbomify.com/CVE-2025-21813/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-21813
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-21813
- OSV: https://osv.dev/vulnerability/CVE-2025-21813

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-21813/
Summaries are generated and may contain mistakes; the official description above is the record.