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

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Message-ID: <CAFftDdooms8AfKRhp3N8G7roCQUaV3CHayeF14gXVqoK3PeTwA@mail.gmail.com>
Date:   Sun, 15 Jan 2023 08:41:41 -0600
From:   William Roberts <bill.c.roberts@...il.com>
To:     Matthew Garrett <mgarrett@...ora.tech>
Cc:     jejb@...ux.ibm.com, Evan Green <evgreen@...omium.org>,
        linux-kernel@...r.kernel.org, corbet@....net,
        linux-integrity@...r.kernel.org,
        Eric Biggers <ebiggers@...nel.org>, gwendal@...omium.org,
        dianders@...omium.org, apronin@...omium.org,
        Pavel Machek <pavel@....cz>, Ben Boeckel <me@...boeckel.net>,
        rjw@...ysocki.net, Kees Cook <keescook@...omium.org>,
        dlunev@...gle.com, zohar@...ux.ibm.com, jarkko@...nel.org,
        linux-pm@...r.kernel.org, Matthew Garrett <mjg59@...gle.com>,
        Jason Gunthorpe <jgg@...pe.ca>, Peter Huewe <peterhuewe@....de>
Subject: Re: [PATCH v5 03/11] tpm: Allow PCR 23 to be restricted to
 kernel-only use

On Sat, Jan 14, 2023 at 9:05 PM Matthew Garrett <mgarrett@...ora.tech> wrote:
>
> On Sat, Jan 14, 2023 at 6:55 AM James Bottomley <jejb@...ux.ibm.com> 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.

Instead of walling off a PCR, why not wall off an NV Index PCR and
use a policy?