{"id":58106,"date":"2025-02-04T23:52:11","date_gmt":"2025-02-04T23:52:11","guid":{"rendered":"https:\/\/www.threatshub.org\/blog\/google-how-to-make-any-amd-zen-cpu-always-generate-4-as-a-random-number\/"},"modified":"2025-02-04T23:52:11","modified_gmt":"2025-02-04T23:52:11","slug":"google-how-to-make-any-amd-zen-cpu-always-generate-4-as-a-random-number","status":"publish","type":"post","link":"https:\/\/www.threatshub.org\/blog\/google-how-to-make-any-amd-zen-cpu-always-generate-4-as-a-random-number\/","title":{"rendered":"Google: How to make any AMD Zen CPU always generate 4 as a random number"},"content":{"rendered":"

Googlers have not only figured out how to break AMD’s security \u2013 allowing them to load unofficial microcode into its processors to modify the silicon’s behavior as they wish \u2013 but also demonstrated this by producing a microcode patch that makes the chips always output 4 when asked for a random number.<\/p>\n

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.<\/p>\n

Background<\/h3>\n

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 \u2013 Intel has similar \u2013 and crucially only AMD is supposed to be able to produce working microcode updates for its products.<\/p>\n

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.<\/p>\n

Well, the boffins at Google have discovered a way to craft their own microcode that is accepted by AMD processors and successfully changes the silicon’s operation. They say their technique works on all Zen-based AMD chips \u2013 all Ryzen and Epyc parts, basically.<\/p>\n

How (not very) random<\/h3>\n

To back up these claims, the Googlers this week released initial details<\/a> of their findings, and a proof-of-concept microcode update for Milan-family Epyc server chips and Phoenix-family Ryzen 9 desktop processors. This demo microcode<\/a> forces a chip’s read random (RDRAND<\/a>) instruction to always output the value 4 rather than an actual random number, likely a reference to XKCD<\/a>.<\/p>\n

The Googlers carefully neutered their proof-of-concept, we note. The altered RDRAND instruction always clears the CPU core’s carry flag to zero, signaling to applications the value isn’t valid. Thus if some miscreant were to start distributing the patch, well-written apps and libraries should refuse to use the static number on hobbled processors. This is important because RDRAND is used by software to generate random values for strong secure encryption and other cryptography; a value of always 4 will silently wreck that data protection.<\/p>\n