Something worth noting:
CPUs have no non-volatile memory -- microcode fully resets when the power is cycled. So, in a sensible world, the impact of this bug would be limited to people temporarily compromising systems on which they already had CPL0 (kernel) access. This would break (possibly very severely and maybe even unpatchably) SEV, and maybe it would break TPM-based security if it persisted across a soft reboot, but would not do much else of consequence.
But we do not live in a sensible world. The entire UEFI and Secure Boot ecosystem is a complete dumpster fire in which the CPU, via mechanisms that are so baroque that they should have been disposed of in, well, the baroque era, enforces its own firmware security instead of delegating to an independent coprocessor. So the actual impact is that getting CPL0 access to an unpatched system [0] will allow a complete compromise of the system flash, which will almost certainly allow a permanent, irreversible compromise of that system, including persistent installation of malicious microcode that will pretend to be patched. Maybe a really nice Verified Boot (or whatever AMD calls its version) implementation would make this harder. Maybe not.
(Okay, it's not irreversible if someone physically rewrites the flash using external hardware. Good luck.)
[0] For this purpose, "unpatched" means running un-fixed microcode at the time at which CPL0 access is gained.
CPUs have no non-volatile memory -- microcode fully resets when the power is cycled. So, in a sensible world, the impact of this bug would be limited to people temporarily compromising systems on which they already had CPL0 (kernel) access. This would break (possibly very severely and maybe even unpatchably) SEV, and maybe it would break TPM-based security if it persisted across a soft reboot, but would not do much else of consequence.
But we do not live in a sensible world. The entire UEFI and Secure Boot ecosystem is a complete dumpster fire in which the CPU, via mechanisms that are so baroque that they should have been disposed of in, well, the baroque era, enforces its own firmware security instead of delegating to an independent coprocessor. So the actual impact is that getting CPL0 access to an unpatched system [0] will allow a complete compromise of the system flash, which will almost certainly allow a permanent, irreversible compromise of that system, including persistent installation of malicious microcode that will pretend to be patched. Maybe a really nice Verified Boot (or whatever AMD calls its version) implementation would make this harder. Maybe not.
(Okay, it's not irreversible if someone physically rewrites the flash using external hardware. Good luck.)
[0] For this purpose, "unpatched" means running un-fixed microcode at the time at which CPL0 access is gained.