lockdown bypass on ubuntu 18.04's 4.15 kernel for loading unsigned modules

["Jason A. Donenfeld" <Jason () zx2c4 com>]

Hey folks,

I noticed that Ubuntu 18.04's 4.15 kernels forgot to protect
efivar_ssdt with lockdown, making that a vector for disabling lockdown
on an efi secure boot machine. I wrote a little PoC exploit to
demonstrate these types of ACPI shenanigans:

https://git.zx2c4.com/american-unsigned-language/tree/american-unsigned-language.sh

The comment on the top has description of exploit strategy and such. I
haven't yet looked into other kernels and distros that might be
affected, though afaict, Canonical's kernel seems to deviate a lot
from upstream.

Jason