CVE-2025-37890

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

net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

As described in Gerrards report [1], we have a UAF case when an hfsc class has a netem child qdisc. The crux of the issue is that hfsc is assuming that checking for cl->qdisc->q.qlen == 0 guarantees that it hasnt inserted the class in the vttree or eltree (which is not true for the netem duplicate case).

This patch checks the n_active class variable to make sure that the code wont insert the class in the vttree or eltree twice, catering for the reentrant case.

[1] https://lore.kernel.org/netdev/CAHcdcOm+03OD2j6R0=YHKqmy=VgJ8xEOKuP6c7mSgnp-TEJJbw@mail.gmail.com/

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 Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.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/141d34391abbb315d68556b7c67ad97885407547
• https://git.kernel.org/stable/c/273bbcfa53541cde38b2003ad88a59b770306421
• https://git.kernel.org/stable/c/2e7093c7a8aba5d4f8809f271488e5babe75e202
• https://git.kernel.org/stable/c/6082a87af4c52f58150d40dec1716011d871ac21
• https://git.kernel.org/stable/c/8df7d37d626430035b413b97cee18396b3450bef
• https://git.kernel.org/stable/c/ac39fd4a757584d78ed062d4f6fd913f83bd98b5
• https://git.kernel.org/stable/c/e0cf8ee23e1915431f262a7b2dee0c7a7d699af0
• https://git.kernel.org/stable/c/e3e949a39a91d1f829a4890e7dfe9417ac72e4d0
• https://lists.debian.org/debian-lts-announce/2025/08/msg00010.html
• https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html