I was reading through and thought "surely I missed a step here" in more than one place.

They mention in part 1 having a write primitive that can hit a 64 byte region, but never really explained how that worked. I'm guessing this was them being able to have checksum offload write TCP/UDP checksum values into the overhang region past the valid data. Maybe the memory map was set up that way, but obviously relies on getting really lucky with getting their page table in the next page after their packet.

Also it was never called out, but as far as I can tell their exploit should only be able to target the beginning of pages for reading/writing. In order to rug-pull the socket logic that reads the IP IHL field, the IP header content needs to be in a page mapped to the guest. The only useful way to use this attack is to have the region spill out of the guest page into whatever page comes next in the kernel's memory map. (Which also makes the mentioning of the skb linear header kind of confusing, since that would be in a completely unrelated kernel memory region, and just have the data pointers pointing into their data, which is what happens for zero-copy network drivers, which presumably was set up for the kvm network for them to retain guest control over the packet.)