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.

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digiown 2 hours ago | parent [-]

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.

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vbezhenar an hour ago | parent | next [-]

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.

	▲	hparadiz 4 minutes ago \| parent [-]
		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.

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tapoxi 2 hours ago | parent | prev [-]

This is called the Endorsement Key, and you're correct, it never leaves the TPM. The TPM is a "black box" to the OS.