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

net_sched: drr: 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 drr, there won't be a UAF, but the code will add the same
classifier to the list twice, which will cause memory corruption.

In addition to checking for qlen being zero, this patch checks whether the
class was already added to the active_list (cl_is_active) before adding
to the list to cover for the reentrant case.

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

https://git.kernel.org/stable/c/26e75716b94d6ff9be5ea07d63675c4d189f30b4

https://git.kernel.org/stable/c/2968632880f1792007eedd12eeedf7f6e2b7e9f3

https://git.kernel.org/stable/c/4f0ecf50cdf76da95828578a92f130b653ac2fcf

https://git.kernel.org/stable/c/ab2248110738d4429668140ad22f530a9ee730e1

https://git.kernel.org/stable/c/db205b92dfe0501e5b92fb7cf00971d0e44ba3eb

https://git.kernel.org/stable/c/f99a3fbf023e20b626be4b0f042463d598050c9a