CVE-2025-37890: net_sched: hfsc: Fix a UAF vulnerability in class with netem as child qdisc

Severity: high (CVSS 7.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2025-05-16 · updated 2026-07-30
Weakness: CWE-416

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/

Affected products

Debian Linux: 11.0
Linux kernel: from 5.0.1 before 5.4.294; from 5.5 before 5.10.238; from 5.11 before 5.15.182; from 5.16 before 6.1.138; from 6.2 before 6.6.90; from 6.7 before 6.12.28 — fixed in 5.10.238, 5.15.182, 5.4.294, 6.1.138, 6.12.28, 6.14.6

How to fix

Upgrade to a fixed version: Linux kernel 5.10.238, 5.15.182, 5.4.294, 6.1.138. Check the vendor advisory for mitigations.

References

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