CVE-2025-37797

/// Technical description

In the Linux kernel, the following vulnerability has been resolved:

net_sched: hfsc: Fix a UAF vulnerability in class handling

This patch fixes a Use-After-Free vulnerability in the HFSC qdisc class

handling. The issue occurs due to a time-of-check/time-of-use condition

in hfsc_change_class() when working with certain child qdiscs like netem

or codel.

The vulnerability works as follows:

1. hfsc_change_class() checks if a class has packets (q.qlen != 0)

2. It then calls qdisc_peek_len(), which for certain qdiscs (e.g.,

codel, netem) might drop packets and empty the queue

3. The code continues assuming the queue is still non-empty, adding

the class to vttree

4. This breaks HFSC scheduler assumptions that only non-empty classes

are in vttree

5. Later, when the class is destroyed, this can lead to a Use-After-Free

The fix adds a second queue length check after qdisc_peek_len() to verify

the queue wasn't emptied.

/// Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

/// References