Re: [PATCH v5 03/11] tpm: Allow PCR 23 to be restricted to kernel-only use

[Matthew Garrett]

On Sat, Jan 14, 2023 at 6:55 AM James Bottomley <jejb@xxxxxxxxxxxxx> wrote:
> Can we go back again to why you can't use locality?  It's exactly
> designed for this since locality is part of creation data.  Currently
> everything only uses locality 0, so it's impossible for anyone on Linux
> to produce a key with anything other than 0 in the creation data for
> locality.  However, the dynamic launch people are proposing that the
> Kernel should use Locality 2 for all its operations, which would allow
> you to distinguish a key created by the kernel from one created by a
> user by locality.
>
> I think the previous objection was that not all TPMs implement
> locality, but then not all laptops have TPMs either, so if you ever
> come across one which has a TPM but no locality, it's in a very similar
> security boat to one which has no TPM.

It's not a question of TPM support, it's a question of platform
support. Intel chipsets that don't support TXT simply don't forward
requests with non-0 locality. Every Windows-sticker laptop since 2014
has shipped with a TPM, but the number that ship with TXT support is a
very small percentage of that. I agree that locality is the obvious
solution for a whole bunch of problems, but it's just not usable in
the generic case.