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

net_sched: ets: 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 ets, 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
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/1a6d0c00fa07972384b0c308c72db091d49988b6

https://git.kernel.org/stable/c/1f01e9f961605eb397c6ecd1d7b0233dfbf9077c

https://git.kernel.org/stable/c/554acc5a2ea9703e08023eb9a003f9e5a830a502

https://git.kernel.org/stable/c/72c3da7e6ceb74e74ddbb5a305a35c9fdfcac6e3

https://git.kernel.org/stable/c/9efb6a0fa88e0910d079fdfeb4f7ce4d4ac6c990

https://git.kernel.org/stable/c/bc321f714de693aae06e3786f88df2975376d996