CVE-2026-53180: Linux Kernel Timer Migration Livelock Vulnerability

Technical summary

The vulnerability resides in tmigr_handle_remote_up() within the Linux kernel's timer migration infrastructure. The function assumes that when processing remote CPU timers, the local softirq path has already handled the current CPU's timers via run_timer_base(BASE_GLOBAL). However, because jiffies can advance between the local timer wheel update and remote timer evaluation, a timer can expire in the window between these two operations. When this occurs, the timer is neither processed locally nor remotely, but fetch_next_timer_interrupt_remote() continues to report it as expired. This causes the event to be re-queued with expires == now on each iteration, triggering an infinite loop. The fix unconditionally calls timer_expire_remote() to catch all expired timers, with minimal overhead for the common case since __run_timer_base() returns immediately when there are no timers to expire.

Business impact

A livelock in the timer migration subsystem can severely degrade system performance, particularly on multi-CPU systems handling time-sensitive workloads. Affected systems experience high CPU utilization from the spinning loop, which can starve legitimate processes and make the system unresponsive. For organizations running databases, message queues, real-time applications, or other latency-sensitive services, this vulnerability can cause service degradation or outages. The issue is especially problematic in cloud and containerized environments where resource contention is already high.

Affected systems

Linux systems running the affected kernel versions are vulnerable. The vulnerability impacts all architectures and configurations where the timer migration code path is active, which is typical for multi-CPU systems (nearly all modern servers and many embedded systems). Single-CPU systems or those with timer migration disabled may be less affected. Specific affected kernel versions should be verified against vendor advisories and your kernel version via uname -r.

Exploitability

This is not an exploitable vulnerability in the traditional sense; it is a logic flaw that triggers under specific timing conditions rather than via intentional exploit code. The livelock can manifest during normal system operation when timer expiration timing aligns with CPU scheduling decisions. No authentication or special privileges are required—the condition can occur spontaneously on vulnerable systems. The unpredictable nature of the timing window means the issue may be intermittent and difficult to reproduce in lab environments, but production systems with sustained timer activity are at higher risk.

Remediation

Remediation requires applying a kernel patch that implements the fix described in the upstream kernel commit. The fix modifies tmigr_handle_remote_up() to unconditionally invoke timer_expire_remote() rather than skipping it based on CPU identity. This ensures no expired timers are overlooked due to jiffies advancement. Kernel recompilation and system reboot are necessary to apply the patch. Verify the patch version against your Linux distributor's advisory (Red Hat, Canonical, SUSE, etc.), as backport versions and timing may vary by distribution.

Patch guidance

Apply the kernel update provided by your Linux distributor. Check your current kernel version with uname -r and consult your vendor's security advisory for the specific patched version. Most distributions will provide the fix through standard package management (apt, yum, zypper). After patching, schedule a maintenance window for system reboot to activate the new kernel. Test in a non-production environment first if possible. If a reboot is not immediately feasible, monitor system load and timer-related metrics for signs of livelock (sustained high CPU with low actual work throughput).

Detection guidance

Monitor for signs of the livelock condition: sustained high CPU utilization on one or more cores with minimal legitimate process activity, high softirq time in top or iostat output, and processes becoming unresponsive despite available CPU. Use perf or ftrace to capture timer-related function calls and identify if timer_expire_remote() is being repeatedly invoked without clearing timers. Kernel logs may contain warnings about hung tasks or watchdog triggers if the livelock persists long enough. Baseline system behavior before and after patching to confirm the issue is resolved.

Why prioritize this

Despite a CVSS score of 7.5 (HIGH), this vulnerability's impact is primarily denial of service through system livelock rather than data breach. However, the unpredictable triggering and severe performance degradation make it a significant operational risk. Prioritize patching for production systems running multi-CPU workloads with active timer usage (databases, web servers, middleware). Systems with SLAs requiring high availability or low latency should be prioritized. Less critical development or testing systems can be deferred slightly, but should not be left unpatched indefinitely.

Risk score, explained

The CVSS 3.1 score of 7.5 (HIGH) reflects a network-accessible, unauthenticated availability impact with no confidentiality or integrity compromise. The score appropriately captures the denial-of-service nature of the livelock. However, the unpredictable and intermittent triggering, combined with the difficulty in reproducing the issue in controlled environments, suggests real-world exploitability is lower than a typical network-accessible DoS. The impact severity is mitigated by the fact that remediation requires only a standard kernel patch and reboot, not architectural changes.

Frequently asked questions