sd-boot setup and PCRs

[Felix Rubio <felix@xxxxxxxxx>]

Hi everybody,

After some days offline, today I have gone through the emails exchanged 
a couple of weeks ago and agreed: UKI is the way to go. Last time I 
checked about it I read about possible problems related to when some 
modules would be loaded and so, but I see that my knowledge was 
outdated.

This said, right now my setup looks like: SecureBoot is enabled, I am 
using Shim, Systemd-Boot as shim's second stage, and a UKI. As the disk 
is encrypted, for now I am making the decryption predicated to PCRs 7 
and 14, so that the decryption will only fail when either SB state 
changes, or when shim certificates/hashes change. So far so good.

Out of curiosity now, I am wondering: what would happen in case somebody 
boots the system from, e.g., a USB drive that contains a signed image? 
Even if the shim is the same version, I assume it will fail to unlock 
because the MOK will not contain my certificate? Should that certificate 
had been stolen and present, be enough to then unlock the disk?

I am trying to assess if I should put in the mix PCR 4, so that I can 
keep track of the UKI image that gets loaded. Do you guys think this 
would be needed, or is overkill?

Regards,

Felix