CVE-2025-21816 is a vulnerability in the Linux kernel's high-resolution timers (hrtimers) subsystem related to CPU hotplug operations. The issue arises during the CPU hotplug process, specifically at the CPUHP_AP_HRTIMERS_DYING stage, where hrtimers queued on a CPU that is going offline are supposed to be migrated to an online CPU to ensure timely handling of bandwidth timers. However, the vulnerability stems from the fact that wakeups can still be performed by the outgoing CPU after this stage, potentially re-arming bandwidth timers on an offline CPU. This can lead to timers being ignored or mishandled, causing timing inconsistencies and potential delays in critical kernel operations. The problem notably affected the Read-Copy-Update (RCU) subsystem, which had to implement multiple complex workarounds to defer wakeups and handle timers correctly when CPUs go offline. The vulnerability also impacts other kernel components such as the stop machine kthread, which manages CPU hotplug synchronization and timer wakeups. The fix involves changing the hrtimers infrastructure to always migrate timers to an online CPU whenever they are enqueued from an offline CPU, eliminating the need for prior workarounds and improving kernel stability and correctness during CPU hotplug events. This vulnerability affects multiple Linux kernel versions identified by specific commit hashes, indicating a broad impact across kernel releases prior to the fix. No known exploits are reported in the wild as of the publication date.

For European organizations, this vulnerability could impact systems running affected Linux kernel versions, particularly those utilizing CPU hotplug features common in virtualized environments, cloud infrastructure, and high-availability servers. The mishandling of hrtimers during CPU offline events could lead to kernel timing anomalies, potentially causing performance degradation, delayed task scheduling, or instability in critical kernel subsystems like RCU. This may affect the reliability and availability of services dependent on precise timing and CPU resource management, such as telecommunications, financial trading platforms, and industrial control systems prevalent in Europe. While the vulnerability does not directly enable code execution or privilege escalation, the resulting kernel instability could be leveraged by attackers to cause denial of service or disrupt critical operations. Given the widespread use of Linux in European data centers, cloud providers, and embedded systems, the impact could be significant if unpatched systems experience CPU hotplug events under load.

European organizations should prioritize updating their Linux kernel to versions that include the fix for CVE-2025-21816. Specifically, kernel updates that implement the migration of timers away from offline CPUs during enqueue operations should be applied promptly. For environments where immediate kernel upgrades are challenging, organizations should monitor CPU hotplug events and avoid frequent CPU offline/online cycles that could trigger the vulnerability. Additionally, system administrators should audit and test critical workloads for stability during CPU hotplug operations and consider implementing kernel live patching solutions where available to minimize downtime. Organizations running custom or older kernel versions should backport the relevant patches from the mainline kernel to ensure protection. Finally, monitoring kernel logs for warnings related to hrtimer_start_range_ns or CPU hotplug anomalies can help detect attempts to exploit or trigger the vulnerability.