[amluto]

By “firmware security” I meant control of writes to the SPI flash chip that controls firmware. There are other mechanisms that try to control whether the contents of the chip are trusted for various purposes at boot, and you’re probably more familiar with those than I am.

As for my guesses about the rest:

As far as I know (and I am not privy to any non-public info here), the Intel ucode patch process sure seems like it can reprogram things other than the ucode patch SRAM. There seem to be some indications that AMD’s is different.

I wouldn’t bet real money, with fairly strong odds, that this ucode compromise gives the ability to run effectively arbitrary code in SMM CPL0, without even a whole lot of difficulty other than reverse engineering enough of the CPU to understand what the uops do and which patch slots do what. I would also bet, at somewhat less aggressive odds, that ucode patches can do things that even SMM can’t, e.g. writing to locked MSRs and even issuing special extra-privileged operations like the “Debug Read” and “Debug Write” operations that Intel CPUs support in the “Red Unlock” state.