CVE-2025-37913

A flaw was found in the Linux kernel's Quick Fair Queueing (QFQ) network traffic scheduler. When nested queuing disciplines trigger reentrant packet enqueue operations, the scheduler fails to verify whether a class is already active before adding it, resulting in duplicate list insertion. A local user could exploit this flaw to cause memory corruption, leading to a system crash and Denial of Service (DoS).

A moderate severity flaw was found in the Linux kernel's Quick Fair Queueing (QFQ) network scheduler, affecting the kernel and kernel-rt packages in Red Hat Enterprise Linux 8 (including its update streams) and the kernel used by Red Hat's Hosted OpenShift Clusters (ARO and OSD). When a qfq qdisc is configured with a netem child qdisc, the netem processing can make the parent qdisc's enqueue callback reentrant. The QFQ scheduler fails to check whether a class is already on the active aggregate list before adding it, so the same class can be inserted into the list twice, resulting in memory corruption. A local user with the ability to configure such nested queuing disciplines could exploit this flaw to crash the system and cause a denial of service. Red Hat rates this flaw moderate (CVSS 6.3) rather than the upstream important severity because exploitation requires local access with privileges to set up the specific nested qdisc configuration and has high attack complexity, with no direct confidentiality impact. Fixes for the affected Red Hat Enterprise Linux 8 and Hosted OpenShift streams have been deferred, so the residual risk remains for systems where qfq qdiscs with netem child qdiscs are configured. Red Hat Enterprise Linux 6 and 7 are out of support scope, and Red Hat Enterprise Linux 9, Red Hat Enterprise Linux 10, and Red Hat In-Vehicle OS are not affected because the vulnerable code is not present in those kernels.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.