CVE-2025-21897

CVE Details

Release Date:	2025-04-01
Impact:	Moderate	What is this?

Description

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.

See more information about CVE-2025-21897 from MITRE CVE dictionary and NIST NVD

NOTE: The following CVSS metrics and score provided are preliminary and subject to review.

CVSS v3 metrics

Base Score:	5.5
Vector String:	CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Version:	3.1
Attack Vector:	Local
Attack Complexity:	Low
Privileges Required:	Low
User Interaction:	None
Scope:	Unchanged
Confidentiality Impact:	None
Integrity Impact:	None
Availability Impact:	High

Errata information

Platform	Errata	Release Date
Oracle Linux version 10 (kernel-uek)	ELSA-2025-20480	2025-07-17
Oracle Linux version 9 (kernel-uek)	ELSA-2025-20480	2025-07-17