Re: [PATCH v7 02/13] Documentation/x86: Secure Launch kernel documentation

Alyssa Ross
2023-11-12T18:08:08Z

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> +Load-time Integrity
> +-------------------
> +
> +It is critical to understand what load-time integrity establishes about a
> +system and what is assumed, i.e. what is being trusted. Load-time integrity is
> +when a trusted entity, i.e. an entity with an assumed integrity, takes an
> +action to assess an entity being loaded into memory before it is used. A
> +variety of mechanisms may be used to conduct the assessment, each with
> +different properties. A particular property is whether the mechanism creates an
> +evidence of the assessment. Often either cryptographic signature checking or
> +hashing are the common assessment operations used.
> +
> +A signature checking assessment functions by requiring a representation of the
> +accepted authorities and uses those representations to assess if the entity has
> +been signed by an accepted authority. The benefit to this process is that
> +assessment process includes an adjudication of the assessment. The drawbacks
> +are that 1) the adjudication is susceptible to tampering by the Trusted
> +Computing Base (TCB), 2) there is no evidence to assert that an untampered
> +adjudication was completed, and 3) the system must be an active participant in
> +the key management infrastructure.
> +
> +A cryptographic hashing assessment does not adjudicate the assessment but
> +instead, generates evidence of the assessment to be adjudicated independently.
> +The benefits to this approach is that the assessment may be simple such that it
> +may be implemented in an immutable mechanism, e.g. in hardware. Additionally,
> +it is possible for the adjudication to be conducted where it cannot be tampered
> +with by the TCB. The drawback is that a compromised environment will be allowed
> +to execute until an adjudication can be completed.
> +
> +Ultimately, load-time integrity provides confidence that the correct entity was
> +loaded and in the absence of a run-time integrity mechanism assumes, i.e.
> +trusts, that the entity will never become corrupted.

I'm somewhat familiar with this area, but not massively (so probably the
sort of person this documentation is aimed at!), and this was the only
section of the documentation I had trouble understanding.

The thing that confused me was that the first time I read this, I was
thinking that a hashing assessment would be comparing the generated hash
to a baked-in known good hash, simliar to how e.g. a verity root hash
might be specified on the kernel command line, baked in to the OS image.
This made me wonder why it wasn't considered to be adjudicated during
assessment. Upon reading it a second time, I now understand that what
it's actually talking about is generating a hash, but not comparing it
automatically against anything, and making it available for external
adjudication somehow.

I don't know if the approach I first thought of is used in early boot
at all, but it might be worth contrasting the cryptographic hashing
assessment described here with it, because I imagine that I'm not going
to be the only reader who's more used to thinking about integrity
slightly later in the boot process where adjudicating based on a static
hash is common, and who's mind is going to go to that when they read
about a "cryptographic hashing assessment".

The rest of the documentation was easy to understand and very helpful to
understanding system launch integrity. Thanks!

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