Comment by tapoxi
21 hours ago
The call asks the TPM to display the signed boot chain, you can't fake that because it wouldnt be cryptographically valid. The TPM is that independent hardware.
21 hours ago
The call asks the TPM to display the signed boot chain, you can't fake that because it wouldnt be cryptographically valid. The TPM is that independent hardware.
How would that be implemented? I'd be curious to know.
I'm not aware that a TPM is capable of hiding a key without the OS being able to access/unseal it at some point. It can display a signed boot chain but what would it be signed with?
If it's not signed with a key out of the reach of the system, you can always implement a fake driver pretty easily to spoof it.
I guess something like that: https://tpm2-software.github.io/tpm2-tss/getting-started/201...
Basically TPM includes key that's also signed with manufacturer key. You can't just extract it and signature ensures that this key is "trusted". When asked, TPM will return boot chain (including bootloader or UKI hash), signed by its own key which you can present to remote party. The whole protocol is more complicated and includes challenge.
Tpm isn't designed for this use case. You can use it for disk encryption or for identity attestation but step 1 for id attestation is asking the tpm to generate a key and then trusting that fingerprint from then on after doing a test sign with a binary blob. The running kernel is just a binary that can be hashed and whitelisted by a user space application. Don't need tpm for that.
This is called the Endorsement Key, and you're correct, it never leaves the TPM. The TPM is a "black box" to the OS.
Ah, got it. With enough motivation this is still pretty easily defeated though. The key is in some kind of NVRAM, which can be read with specialized equipment, and once it's out, you can use it to spoof signatures on a different machine and cheat as usual. The TPM implementations of a lot of consumer hardware is also rather questionable.
These attestation methods would probably work well enough if you pin a specific key like for a hardened anti-evil-maid setup in a colo, but I doubt it'd work if it trusts a large number of vendor keys by default.
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