[Skiboot] [PATCH v2 6/6] secvar/edk2: enforce a PK update enrolled in setup mode to be signed by itself

[Daniel Axtens]

Hi Eric,

> When the system is in setup mode, that is, no PK has been enrolled in the PK
> variable, variable signature enforcement is disabled. Therefore, variable
> update signatures do not need to be authorized by a variable already stored in
> a variable.
>
> However, variable updates must still be in the AUTH format, which still
> contains a pkcs7 signature. Bare ESL files are *not* accepted as an update to
> a variable. So, a variable update must still be signed, albeit with any key.
>
> Not enforcing signature checks is intended to be a convenience feature, however
> enrolling a variable into the PK carries extra weight. Once data is enrolled
> into the PK variable, signatures are now required for all further variable
> updates.
>
> Due to this extra importance when enrolling a PK, the PK should most definitely
> contain valid data. Therefore, as a sanity check, when enrolling a PK for the
> first time (i.e. in setup mode), the PK variable update must be signed by its
> own private key, which ensures ownership of the private key and validity of the
> public key to be stored in the PK variable.

Hmm. In my work for [REDACTED], I had explictly _not_ required this. I
could do this, I'll just need to get things renamed from "Allow
unauthenticated PK updates" to "Allow self-signed PK updates".

Although, as with your case, I had also done all my tests (such as they
are at the moment) with a self-signed key.

Does it take us closer to EDK2 compat or further away? I am guessing
EDK2 also requires that there is an AUTH structure but just throws it
away after maybe doing some basic syntactical sanity checks?

I'm not _really_ sold on the value of this but I'm open to being
convinced.

Beyond that it looks OK, I'll have a closer look once I get through the
refactorings.

Kind regards,
Daniel