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

Affected: Linux

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/

USN-7608-1 — linux, linux-aws, linux-gcp, linux-gcp-5.15, linux-gke, linux-gkeop, linux-hwe-5.15, linux-ibm, linux-kvm, linux-lowlat…

Affected: Ubuntu:Pro:20.04:LTS: linux-gcp-5.15, Ubuntu:Pro:20.04:LTS: linux-hwe-5.15, Ubuntu:Pro:20.04:LTS: linux-lowlatency-hwe-5.15, Ubuntu:Pro:20.04:LTS: linux-oracle-5.15, Ubuntu:22.04:LTS: linux, Ubuntu:22.04:LTS: linux-aws, Ubuntu:22.04:LTS: linux-gcp, Ubuntu:22.04:LTS: linux-gke, Ubuntu:22.04:LTS: linux-gkeop, Ubuntu:22.04:LTS: linux-ibm and 4 more

Several security issues were discovered in the Linux kernel. An attacker could possibly use these to compromise the system. This update corrects flaws in the following subsystems: - GPU drivers; - SMB network file system; - Memory management; - Netfilter; - Network traffic control; (CVE-2025-37890, CVE-2024-46787, CVE-2025-37798, CVE-2025-38000, CVE-2025-37932, CVE-2025-38001, CVE-2025-37997, CVE-2024-50047, CVE-2024-53051)

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

USN-7608-1 — linux, linux-aws, linux-gcp, linux-gcp-5.15, linux-gke, linux-gkeop, linux-hwe-5.15, linux-ibm, linux-kvm, linux-lowlat…