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 Gerrard's report [1], there are use cases where a netem
child qdisc will make the parent qdisc's enqueue callback reentrant.
In the case of qfq, there won't 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/

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