Linux Kernel up to 6.12.17/6.13.5 sched_ext pick_task_scx deadlock

Summaryinfo

A vulnerability was found in Linux Kernel up to 6.12.17/6.13.5. It has been classified as critical. This affects the function pick_task_scx of the component sched_ext. This manipulation causes deadlock. The identification of this vulnerability is CVE-2025-21897. There is no exploit available. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability has been found in Linux Kernel up to 6.12.17/6.13.5 and classified as critical. Affected by this vulnerability is the function pick_task_scx of the component sched_ext. The manipulation with an unknown input leads to a deadlock vulnerability. The CWE definition for the vulnerability is CWE-833. The product contains multiple threads or executable segments that are waiting for each other to release a necessary lock, resulting in deadlock. As an impact it is known to affect availability. The summary by CVE is:

In the Linux kernel, the following vulnerability has been resolved: sched_ext: Fix pick_task_scx() picking non-queued tasks when it's called without balance() a6250aa251ea ("sched_ext: Handle cases where pick_task_scx() is called without preceding balance_scx()") added a workaround to handle the cases where pick_task_scx() is called without prececing balance_scx() which is due to a fair class bug where pick_taks_fair() may return NULL after a true return from balance_fair(). The workaround detects when pick_task_scx() is called without preceding balance_scx() and emulates SCX_RQ_BAL_KEEP and triggers kicking to avoid stalling. Unfortunately, the workaround code was testing whether @prev was on SCX to decide whether to keep the task running. This is incorrect as the task may be on SCX but no longer runnable. This could lead to a non-runnable task to be returned from pick_task_scx() which cause interesting confusions and failures. e.g. A common failure mode is the task ending up with (!on_rq && on_cpu) state which can cause potential wakers to busy loop, which can easily lead to deadlocks. Fix it by testing whether @prev has SCX_TASK_QUEUED set. This makes @prev_on_scx only used in one place. Open code the usage and improve the comment while at it.

The advisory is shared at git.kernel.org. This vulnerability is known as CVE-2025-21897 since 12/29/2024. The exploitation appears to be difficult. Technical details are known, but no exploit is available.

The vulnerability scanner Nessus provides a plugin with the ID 250071 (Linux Distros Unpatched Vulnerability : CVE-2025-21897), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 6.12.18 or 6.13.6 eliminates this vulnerability. Applying the patch 5324c459f90d16b0c43a78b494c598915d782b7a/de60a31cb0bcacfaf9487546eac5e70e0a9c66d7/8fef0a3b17bb258130a4fcbcb5addf94b25e9ec5 is able to eliminate this problem. The bugfix is ready for download at git.kernel.org. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the databases at Tenable (250071) and CERT Bund (WID-SEC-2025-0683). If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Affected

• Google Container-Optimized OS
• Debian Linux
• Amazon Linux 2
• Red Hat Enterprise Linux
• Ubuntu Linux
• SUSE Linux
• Oracle Linux
• RESF Rocky Linux
• Dell Avamar
• Open Source Linux Kernel
• SolarWinds Security Event Manager
• Dell NetWorker
• Dell Secure Connect Gateway
• IBM Security Verify Access

Productinfo

Type

• Operating System

Vendor

• Linux

Name

• Kernel

Version

• 6.12.0
• 6.12.1
• 6.12.2
• 6.12.3
• 6.12.4
• 6.12.5
• 6.12.6
• 6.12.7
• 6.12.8
• 6.12.9
• 6.12.10
• 6.12.11
• 6.12.12
• 6.12.13
• 6.12.14
• 6.12.15
• 6.12.16
• 6.12.17
• 6.13.0
• 6.13.1
• 6.13.2
• 6.13.3
• 6.13.4
• 6.13.5

License

• open-source

Website

• Vendor: https://www.kernel.org/

CPE 2.3info

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CPE 2.2info

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Entryinfo

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