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 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.