CVE-2025-39821
📋 TL;DR
A Linux kernel vulnerability in the perf subsystem allows undefined behavior when handling disabled performance monitoring events during throttling. This can lead to kernel instability or crashes when specific event configurations trigger the bug. Systems running vulnerable Linux kernel versions with performance monitoring enabled are affected.
💻 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 →⚠️ Risk & Real-World Impact
Worst Case
Kernel panic or system crash due to undefined behavior in PMU driver operations, potentially causing denial of service.
Likely Case
Kernel warnings from UBSAN (Undefined Behavior Sanitizer) and potential system instability when specific perf event configurations are used.
If Mitigated
No impact if performance monitoring is disabled or if vulnerable configurations aren't used.
🎯 Exploit Status
Syzkaller reproducer exists, requires local access and perf subsystem privileges to trigger.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Patches available in stable kernel trees (commits b64fdd422a85025b5e91ead794db9d3ef970e369 and d689135aa9c5e4e0eab5a92bbe35dab0c8d6677f)
Vendor Advisory: https://git.kernel.org/stable/c/b64fdd422a85025b5e91ead794db9d3ef970e369
Restart Required: Yes
Instructions:
1. Update to a patched kernel version from your distribution. 2. Reboot the system. 3. Verify the fix by checking kernel version.
🔧 Temporary Workarounds
Disable perf subsystem
linuxDisable performance monitoring events subsystem at kernel build time
Rebuild kernel with CONFIG_PERF_EVENTS=n
Restrict perf usage
linuxLimit perf subsystem access via kernel.perf_event_paranoid sysctl
sysctl -w kernel.perf_event_paranoid=3
🧯 If You Can't Patch
- Restrict perf subsystem access to trusted users only
- Monitor for UBSAN shift-out-of-bounds warnings in kernel logs
🔍 How to Verify
Check if Vulnerable:
Check kernel logs for UBSAN shift-out-of-bounds warnings related to perf events, or test with syzkaller reproducer if available.Check Version:
uname -rVerify Fix Applied:
Check that kernel version includes the fix commits or that UBSAN warnings no longer appear under perf stress testing.📡 Detection & Monitoring
Log Indicators:
- UBSAN: shift-out-of-bounds warnings in kernel logs
- perf-related kernel warnings or crashes
Network Indicators:
- None - local vulnerability only
SIEM Query:
source="kernel" AND ("UBSAN" OR "shift-out-of-bounds" OR "perf")