CVE-2025-21813

/// Technical 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 ro

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References