Google hacks Zen 2 to 4 microcode

The microcode of the Zen 2 to Zen 4 processors has been hacked by
security people from Google:

https://bughunters.google.com/blog/5424842357473280/zen-and-the-art-of-microcode-hacking

Apart from breaking getting updates past  AMD's signature
verificiation process, they also created a toolbox for
playing around with the microcode.  There is also quite
some information on the internal microcode format at
https://github.com/google/security-research/blob/master/pocs/cpus/entrysign/zentool/