CVE-2025-21657 sched_ext: Replace rq_lock() to raw_spin_rq_lock() in scx_ops_bypass()

Medium 5.5 Published 21 Jan 2025 Updated 11 May 2026

A locking error in the Linux kernel scheduler can trigger warnings and potential instability when bypassing sched_ext, requiring a patch to fix.

Exploited in the wild: Not known
Login needed: Any signed-in user
Attackable: Local or same network only
Victim action needed: Not needed
Fix: Available

What is CVE-2025-21657?

CVE-2025-21657 is a medium-severity denial of service vulnerability in Linux kernel, published on 21 Jan 2025. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

In the Linux kernel, the following vulnerability has been resolved:

sched_ext: Replace rq_lock() to raw_spin_rq_lock() in scx_ops_bypass()

scx_ops_bypass() iterates all CPUs to re-enqueue all the scx tasks. For each CPU, it acquires a lock using rq_lock() regardless of whether a CPU is offline or the CPU is currently running a task in a higher scheduler class (e.g., deadline). The rq_lock() is supposed to be used for online CPUs, and the use of rq_lock() may trigger an unnecessary warning in rq_pin_lock(). Therefore, replace rq_lock() to raw_spin_rq_lock() in scx_ops_bypass().

Without this change, we observe the following warning:

===== START ===== [ 6.615205] rq->balance_callback && rq->balance_callback != &balance_push_callback [ 6.615208] WARNING: CPU: 2 PID: 0 at kernel/sched/sched.h:1730 __schedule+0x1130/0x1c90 ===== END =====

Which versions are affected by CVE-2025-21657?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2025-21657?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2025-21657 being exploited?

There is no known exploitation of CVE-2025-21657 in the wild and no public exploit code.

All Linux kernel vulnerabilities

Sources

Also known as BELL-CVE-2025-21657, DEBIAN-CVE-2025-21657, GHSA-79cx-c495-ffvx, UBUNTU-CVE-2025-21657.