Teddy Reed has a second article on UEFI, MinnowboardMax and Linux! This one is called “Minnowboard Max: Booting Linux Securely”, and talks about how the Linux shim is used, and enumerates the various ways this boot security can be bypassed.
[…] I say ‘more-or-less’ because there are tons of places where the verification can be subverted. Unfortunately, if you start examining the implementation and configuration details of the streamlined Secure Boot support, you’ll find plenty of bypasses. Let’s talk briefly about each bypass and conclude with a simple way to use Secure Boot and enforce a signed kernel execution on Ubuntu. To be clear, there are no vulnerabilities here as there is no documented intention to boot Linux securely (e.g., BUG/1401532), only to support a Secure Boot and boot Linux. […]