CVE-2025-21657
📋 TL;DR
This CVE addresses a kernel warning triggered by improper lock usage in the Linux kernel's scheduler extension (sched_ext) subsystem. The vulnerability occurs when scx_ops_bypass() uses rq_lock() on offline CPUs or CPUs running higher-priority tasks, causing unnecessary warnings but not enabling privilege escalation or code execution. Only systems using the sched_ext scheduler extension 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 →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 →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 warning/panic leading to system instability or denial of service, potentially causing system crashes or reboots.
Likely Case
Kernel warning messages in system logs during scheduler operations, with possible minor performance impact but no security compromise.
If Mitigated
No security impact when patched; warnings eliminated without affecting system functionality.
🎯 Exploit Status
This is a stability issue causing warnings, not a security vulnerability that can be exploited for privilege escalation or code execution.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Kernel commits 6268d5bc10354fc2ab8d44a0cd3b042d49a0417e and d9e446dd63cee7161717a6a8414ba9c6435af764
Vendor Advisory: https://git.kernel.org/stable/c/6268d5bc10354fc2ab8d44a0cd3b042d49a0417e
Restart Required: Yes
Instructions:
1. Update Linux kernel to version containing the fix commits. 2. Reboot system to load new kernel. 3. Verify kernel version matches patched version.
🔧 Temporary Workarounds
Disable sched_ext scheduler
linuxDisable the sched_ext scheduler extension if not required
echo 'blacklist sched_ext' >> /etc/modprobe.d/blacklist.conf
reboot
🧯 If You Can't Patch
- Monitor system logs for kernel warnings related to scheduler operations
- Ensure sched_ext scheduler extension is disabled if not required
🔍 How to Verify
Check if Vulnerable:
Check kernel version and if sched_ext is enabled: 'uname -r' and check /proc/sys/kernel/sched_extCheck Version:
uname -rVerify Fix Applied:
Verify kernel version contains fix commits and check system logs for absence of scheduler warnings📡 Detection & Monitoring
Log Indicators:
- Kernel warnings containing 'rq->balance_callback && rq->balance_callback != &balance_push_callback'
- Warnings at kernel/sched/sched.h:1730
Network Indicators:
- None - this is a local kernel issue
SIEM Query:
source="kernel" AND "WARNING: CPU:" AND "balance_callback"