How to make any AMD Zen CPU always generate 4 from RDRAND

Googlers have not only figured out how to break AMD's security – allowing them to load unofficial microcode into its processors to modify the silicon's behavior as they wish – but also demonstrated this by producing a microcode patch that makes the chips always output 4 when asked for a random number. And this ability to change the microcode not only allows Google and others to customize the operation of their AMD chips, for good and non-good reasons, but it also smashes the Epyc maker's secure encrypted virtualization and root-of-trust security features. Microcode is a special block of programs typically loaded into a processor during system startup that defines the way the chip works. By providing microcode to users, AMD can add some features, fix certain issues, and extend some functionality without having to redesign and reissue the physical silicon. It is a patch that updates your chip – Intel has similar – and crucially only AMD is supposed to be able to produce working microcode updates for its products. AMD bakes a cryptographic security mechanism into its processors that checks a microcode update truly came from AMD before accepting it. The format of the microcode is also not documented publicly and is highly proprietary and protected. All of this is to stop someone from coming up with their own viable microcode and making an AMD processor do things it shouldn't or in a non-standard way. Well, the boffins at Google have discovered a way to craft their own microcode t...