Hello Trammell,
Thursday, May 11, 2017, 5:42:38 PM, you wrote:
TH> On Thu, May 11, 2017 at 10:30:48AM -0500, Allen Krell wrote:
[...] There are multiple keys
ME - public/private key pair - Fused in by Intel and checked by Intel silicon - Probably different across models
It's a little simpler than that: the ME ROM has a hardcoded list of pubkey hashes and accepts ME manifests signed by any of them. I think (but haven't checked) that the keys change with each major ME version.
TH> If an attacker can sign an ME binary, they can provide invalid fuses to TH> the CPU microcode so that it won't check the ACM key (or provide their TH> own bootguard key so that the TPM locality will be set for the IBB TH> measurement).
I'm don't think this is possible. the OEM keys (or rather, their hashes) are set in the data area of ME and are copied to the PCH/MCH fuses on first boot. These fuses are one-time programmable so can't be overwritten (supposedly) even if you manage to get ME codeexec.
TH> If the attacker can sign the ACM, they can ignore the bootguard key on TH> the IBB and provide invalid measurements to the CRTM. TH> And if they can sign an IBB they can implement their own policy (but TH> not avoid TPM measurement of the IBB by the ACM).
This sounds correct (I did not look into BootGuard in much detail).
So, back to AMT bug. I believe Boot Guard (by itself) doesn't help. An exploiter "may" be able to reflash only the ME region and enable AMT even if the OEM has disabled AMT and implemented Boot Guard. Not confirmed, just a educated hunch.
TH> That might be possible, although ideally the startup ACM or IBB can TH> ensure that the ME region is included in its measurements and this would TH> cause key unsealing or remote attestation to fail. That's one of TH> the reasons that I recommend changing the flash descriptor to allow TH> the host CPU to read the ME region.
In fact I think this is exactly the reason why flashing cleaned ME fails on BootGuard-protected systems - they check ME's hash (which ME provides in the PCI register space) and fail when it changes. Though that makes me wonder how they handle ME firmware updates...