CVE-2025-21813
📋 TL;DR
A race condition in the Linux kernel timer migration subsystem can cause multiple top-level timer groups to exist, defeating the single idle migrator design. This affects Linux systems with specific kernel versions and can cause kernel warnings or potential timer handling issues. All Linux users with affected kernel versions are vulnerable.
💻 Affected Systems
- Linux kernel
📦 What is this software?
Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →Linux Kernel by Linux
The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...
Learn more about Linux Kernel →⚠️ Risk & Real-World Impact
Worst Case
System instability, kernel panics, or timer malfunctions leading to service disruption or denial of service conditions.
Likely Case
Kernel warning messages in system logs and potential minor timer inaccuracies without immediate system failure.
If Mitigated
No impact if patched; unpatched systems may experience occasional warnings but typically continue functioning.
🎯 Exploit Status
Exploitation requires local access and precise timing to trigger the race condition. Primarily a reliability issue rather than a security bypass.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Fixed in kernel commits: 6f449d8fa1808a7f9ee644866bbc079285dbefdd, 868c9037df626b3c245ee26a290a03ae1f9f58d3, c6dd70e5b465a2b77c7a7c3d868736d302e29aec
Vendor Advisory: https://git.kernel.org/stable/c/6f449d8fa1808a7f9ee644866bbc079285dbefdd
Restart Required: Yes
Instructions:
1. Update to a kernel version containing the fix commits. 2. Check with your Linux distribution for specific patched kernel packages. 3. Reboot the system after kernel update.
🔧 Temporary Workarounds
Disable CPU hotplug
linuxPrevents the race condition by disabling CPU hotplug functionality
echo 0 > /sys/devices/system/cpu/cpuX/online (for each CPU)
🧯 If You Can't Patch
- Monitor system logs for WARNING messages from tmigr_requires_handle_remote
- Implement strict access controls to prevent local user exploitation
🔍 How to Verify
Check if Vulnerable:
Check kernel version and if it's between vulnerable commits: uname -r and examine kernel changelogCheck Version:
uname -rVerify Fix Applied:
Verify kernel version includes fix commits or check for absence of WARNING messages in dmesg📡 Detection & Monitoring
Log Indicators:
- WARNING: CPU: X PID: 0 at kernel/time/timer_migration.c:543 tmigr_requires_handle_remote
Network Indicators:
- None - local kernel issue
SIEM Query:
source="kernel" AND "tmigr_requires_handle_remote" AND "WARNING"