I have never seen someone make a distinction between Secure Boot and boot verification on UEFI/x86, although I suppose it’s a valid one? I suspect the parent post is referring to Secure Boot in the colloquial sense it’s usually used for on x86: validation of the UEFI boot binary using Secure Boot keys followed by OS level trust chaining, usually accomplished by entangling PCR state with disk encryption in some way.
And I think this would deliver a slight level of protection from the BMC: tampering with the firmware image or key enrollment / secure boot state _should_ both break the UEFI root of trust and alter the PCR state and break everything downstream. Of course, all UEFI implementations are holier than Swiss cheese and there are probably a lot of ways to use the BMC to dump or modify memory post-boot anyway, but it does do something.
You mean the same Secure Boot that I mean. This is a software mechanism (almost universally implemented as an unholy mixture of ordinary code in firmware and a pile of immensely complex SMM code that, IMO, is entirely unnecessary if the hardware or the BMC gives even the slightest bit of help).
And Secure Boot is implemented in, and configured by, the firmware that the BMC can overwrite at its whim while entirely bypassing all the fancy CPU-hardware and SMM protections that are supposed to prevent writing arbitrary data to it.
To the extent that a mechanism not controlled by firmware will detect such an attack and extend PCRs accordingly before executing a single instruction from the compromised flash chip, it might partially mitigate attacks. But that part isn’t Secure Boot.
Oh, yes, we 100% agree on this, the true root of trust for firmware execution exists before and independently of “secure boot,” and therefore, often not at all (and “secure boot” is a terrible name).
And I think this would deliver a slight level of protection from the BMC: tampering with the firmware image or key enrollment / secure boot state _should_ both break the UEFI root of trust and alter the PCR state and break everything downstream. Of course, all UEFI implementations are holier than Swiss cheese and there are probably a lot of ways to use the BMC to dump or modify memory post-boot anyway, but it does do something.