CVE-2025-38000

A flaw was found in the HFSC queueing discipline implementation in the Linux kernel. When a packet is enqueued and the child qdisc's peek() function is called before properly updating the HFSC queue's length and backlog counters, a race condition can occur. In some cases, the peek operation may trigger an immediate dequeue and drop, leading to inconsistent queue accounting. This may leave an empty HFSC class in the active list, eventually causing use-after-free (UAF) conditions. Due to the nature of this memory corruption (use-after-free or list corruption) in kernel scheduler code, a successful exploit could lead to privilege escalation, data leakage, or denial of service. Therefore, the CIA impact is assessed as HHH to reflect a worst-case.

On Red Hat Enterprise Linux 8 and later releases, regular (non-root) users can exploit this issue by abusing unprivileged user namespaces. On Red Hat Enterprise Linux 7, unprivileged user namespaces are disabled by default. Red Hat Enterprise Linux 6 did not include support for them at all, meaning that root privileges are necessary to trigger this flaw.

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.

To mitigate this issue, prevent module sch_hfsc from being loaded. Please see https://access.redhat.com/solutions/41278 for how to blacklist a kernel module to prevent it from loading automatically.