CVE-2025-40083: 
Linux Kernel vulnerability analysis and mitigation

CVE-2025-40083 is a vulnerability discovered in the Linux kernel's networking scheduler component (net/sched). The vulnerability was disclosed on October 29, 2025, affecting the Quick Fair Queueing (QFQ) scheduler implementation. The issue specifically involves a null pointer dereference in the aggdequeue function within the net/sched/schqfq.c file (NVD, Red Hat).

The vulnerability stems from a null pointer dereference in the aggdequeue function when cl->qdisc->ops->peek(cl->qdisc) returns NULL. The issue occurs in the net/sched/schqfq.c file of the Linux kernel. The vulnerability has been assigned a CVSS v3 base score of 5.5, indicating moderate severity. Red Hat has classified this as a moderate security issue (Red Hat).

The vulnerability could potentially lead to a system crash when the affected component is triggered, resulting in a denial of service condition. The issue affects various versions of Red Hat Enterprise Linux, including versions 6 through 10 (Red Hat).

A fix has been implemented that involves checking the return value before using it, similar to the existing approach in schhfsc.c. The solution includes changing qdiscwarnnonwc into a static inline function, moving qdiscpeeklen from net/sched/schhfsc.c to include/net/pktsched.h, and applying qdiscpeeklen in aggdequeue to prevent crashes (NVD).