Re: Firmware signing -- Re: [PATCH 00/27] security, efi: Add kernel lockdown

[Matthew Garrett]

On Tue, Nov 14, 2017 at 2:14 PM, James Bottomley
<James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 2017-11-14 at 15:55 -0500, Matthew Garrett wrote:
>> TPM-backed Trusted Boot means you don't /need/ to sign anything,
>> since the measurements of what you loaded will end up in the TPM. But
>> signatures make it a lot easier, since you can just assert that only
>> signed material will be loaded and so you only need to measure the
>> kernel and the trusted keys.
>
> Actually, I'd disagree with that quite a lot: measured boot only works
> if you're attesting to something outside of your system that has the
> capability for doing something about a wrong measurement.  Absent that,
> measured boot has no safety whatsoever.  Secure boot, on the other
> hand, can enforce not booting with elements that fail the signature
> check.

Measured boot has a great deal of value in the sealing of private
material, even in the absence of attestation. The way Microsoft make
use of PCR7 is a good example of how signatures make this easier -
achieving the same goal with a full measurement of the boot chain
instead of relying on signature validation results in significantly
more fragility.