“Wow, we (+@h0t_max and @_Dmit) have found two undocumented x86 instructions in Intel CPUs which completely control microarchitectural state (yes, they can modify microcode)”
Intel never documented two instructions that rewrite the processor’s own microcode. Outside researchers found them. Every security model on that chip assumes the microcode is what Intel says it is. Hidden doors in hardware are the hardest kind to close.