# CVE-2025-37890: net\_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Published: 2025-05-16 · Updated: 2026-08-05

## Affected

- Linux: version range not reported
- Linux: 5.0

## Description

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 Gerrard's 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 hasn't 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 won't 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/

## What to do

- Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below.
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## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/273bbcfa53541cde38b2003ad88a59b770306421
- https://git.kernel.org/stable/c/e0cf8ee23e1915431f262a7b2dee0c7a7d699af0
- https://git.kernel.org/stable/c/e3e949a39a91d1f829a4890e7dfe9417ac72e4d0
- https://git.kernel.org/stable/c/8df7d37d626430035b413b97cee18396b3450bef
- https://git.kernel.org/stable/c/6082a87af4c52f58150d40dec1716011d871ac21
- https://git.kernel.org/stable/c/2e7093c7a8aba5d4f8809f271488e5babe75e202
- https://git.kernel.org/stable/c/ac39fd4a757584d78ed062d4f6fd913f83bd98b5
- https://git.kernel.org/stable/c/141d34391abbb315d68556b7c67ad97885407547

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