CVE-2024-42108

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

net: rswitch: Avoid use-after-free in rswitch_poll()

The use-after-free is actually in rswitchtxfree(), which is inlined in rswitchpoll(). Since skb and gq->skbs[gq->dirty] are in fact the same pointer, the skb is first freed using devkfreeskbany(), then the value in skb->len is used to update the interface statistics.

Let's move around the instructions to use skb->len before the skb is freed.

This bug is trivial to reproduce using KFENCE. It will trigger a splat every few packets. A simple ARP request or ICMP echo request is enough.