CVE-2025-37797

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 wasnt emptied.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/20d584a33e480ae80d105f43e0e7b56784da41b9
• https://git.kernel.org/stable/c/28b09a067831f7317c3841812276022d6c940677
• https://git.kernel.org/stable/c/39b9095dd3b55d9b2743df038c32138efa34a9de
• https://git.kernel.org/stable/c/3aa852e3605000d5c47035c3fc3a986d14ccfa9f
• https://git.kernel.org/stable/c/3df275ef0a6ae181e8428a6589ef5d5231e58b5c
• https://git.kernel.org/stable/c/86cd4641c713455a4f1c8e54c370c598c2b1cee0
• https://git.kernel.org/stable/c/bb583c88d23b72d8d16453d24856c99bd93dadf5
• https://git.kernel.org/stable/c/fcc8ede663569c704fb00a702973bd6c00373283
• https://lists.debian.org/debian-lts-announce/2025/05/msg00030.html
• https://lists.debian.org/debian-lts-announce/2025/05/msg00045.html