> For example, it would be difficult to make an instruction like fyl2x or fadd cause a privilege level change. The reason is that floating point instructions ar

> For example, it would be difficult to make an instruction like fyl2x or fadd cause a privilege level change. The reason is that floating point instructions are executed on a separate unit (the FPU), with a separate decoder. This is not true. On modern Intel CPUs, FP instructions are decoded by the same decoders as all other instructions. As for the rest of the pipeline, the whole point of picking a microcoded instruction is that microcode can emit any ops. > It would make more sense to encode the backdoor on an illegal opcode, i.e. an opcode that under normal conditions would generate a UD# exception, but with the correct values in the registers it would trigger some undocumented behavior. This would only be exploitable through native code, and if suspicions arose, it would be detectable and it would be possible to extract the magic instruction from it. A much better vector is a microcoded slow operation with lots of input bits, and that gets emitted by javascript jits. > Another question is how to hide this backdoor in the microcode. Presumably, at some point someone might stumble upon the backdoor and ask around about it. If the backdoor depends on some "magic values", it would be relatively easy to spot just by looking at the microcode. Microcode for modern CPUs is signed and encoded in some manner (encrypted?). So long as the bad guys get between the team that designs the microcode and whatever process that encodes it, no-one will ever be even able to see the evil ucode. > There's also the point that the author mentioned of "fixing" the processor at some point during the production process. I don't think that the author understands the way mass production of microchips works. It's very much not possible to do something like this while keeping the production price on the same level (or someone noticing this extra step in the production process). It doesn't take extra steps, it takes small modifications in the masks of a few metal layers. If someone could make their few modifications into the files before the masks are written, this would be damn near impossible to spot.