CVE-2025-37913

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

net_sched: qfq: Fix double list add in class with netem as child qdisc

As described in Gerrards report [1], there are use cases where a netem child qdisc will make the parent qdiscs enqueue callback reentrant. In the case of qfq, there wont be a UAF, but the code will add the same classifier to the list twice, which will cause memory corruption.

This patch checks whether the class was already added to the agg->active list (cl_is_active) before doing the addition to cater for the reentrant case.

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

Weakness

The product calls free() twice on the same memory address.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/005a479540478a820c52de098e5e767e63e36f0a
• https://git.kernel.org/stable/c/041f410aec2c1751ee22b8b73ba05d38c3a6a602
• https://git.kernel.org/stable/c/0aa23e0856b7cedb3c88d8e3d281c212c7e4fbeb
• https://git.kernel.org/stable/c/0bf32d6fb1fcbf841bb9945570e0e2a70072c00f
• https://git.kernel.org/stable/c/370218e8ce711684acc4cdd3cc3c6dd7956bc165
• https://git.kernel.org/stable/c/53bc0b55178bd59bdd4bcd16349505cabf54b1a2
• https://git.kernel.org/stable/c/a43783119e01849fbf2fe8855634e8989b240cb4
• https://git.kernel.org/stable/c/f139f37dcdf34b67f5bf92bc8e0f7f6b3ac63aa4
• https://lists.debian.org/debian-lts-announce/2025/08/msg00010.html
• https://lists.debian.org/debian-lts-announce/2025/10/msg00007.html