CVE-2025-37890: debian linux Use-After-Free — Fix

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/

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CVSS v3.1 Score

7.8

HIGH

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Weakness Type (CWE)

CWE-416 Use After Free

Affected Products

Vendor	Product	Versions
linux	linux_kernel	5.0.1 — 5.4.294
linux	linux_kernel	5.5 — 5.10.238
linux	linux_kernel	5.11 — 5.15.182
linux	linux_kernel	5.16 — 6.1.138
linux	linux_kernel	6.2 — 6.6.90
linux	linux_kernel	6.7 — 6.12.28
linux	linux_kernel	6.13 — 6.14.6
linux	linux_kernel	All
linux	linux_kernel	All
linux	linux_kernel	All
linux	linux_kernel	All
linux	linux_kernel	All
linux	linux_kernel	All
linux	linux_kernel	All
linux	linux_kernel	All
linux	linux_kernel	All
linux	linux_kernel	All
linux	linux_kernel	All
debian	debian_linux	All

References

Advisories & Patches

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

Other References

https://lists.debian.org/debian-lts-announce/2025/08/msg00010.html https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html

Frequently Asked Questions

What is 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 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/ It has a CVSS v3.1 base score of 7.8 (HIGH).

How severe is CVE-2025-37890? +

CVE-2025-37890 has a CVSS v3.1 score of 7.8 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching.

What products are affected by CVE-2025-37890? +

CVE-2025-37890 affects products from debian, linux, specifically: debian_linux, linux_kernel. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2025-37890? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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