>What’s an RDRAND?

>The microcode bug in question is a faulty response to the RDRAND instruction. Modern x86_64 CPUs—beginning with Intel's Broadwell and AMD's Zen architectures—are supposed to have high-quality onboard random number generators (RNGs), which use thermal "noise" to very rapidly offer high-entropy pseudorandom numbers to anybody with kernel-level access who wants it. RDRAND is, in turn, the instruction that provides these random numbers.

>All of this is supposed to be fairly failsafe. There's a CPUID function call that checks for the availability of RDRAND, and there's also a "carry bit" in the return value from a call to RDRAND that's supposed to let the calling application know if the CPU's RNG was unable to generate a sufficiently random number. Unfortunately, unpatched Ryzen 3000 says "yes" to the CPUID 01H call, sets the carry bit indicating it has successfully created the most artisanal, organic high-quality random number possible... and gives you a 0xFFFFFFFF for the "random" number, every single time.