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From:   Matthew Garrett <mgarrett@aurora.tech>
Date:   Sat, 14 Jan 2023 19:05:03 -0800
Message-ID: <CAHSSk058UoBY2nDx8U7-siG_dbjNSKZaPukZVjSnq=f=CBSKsw@mail.gmail.com>
Subject: Re: [PATCH v5 03/11] tpm: Allow PCR 23 to be restricted to
 kernel-only use
To:     jejb@linux.ibm.com
Cc:     William Roberts <bill.c.roberts@gmail.com>,
        Evan Green <evgreen@chromium.org>,
        linux-kernel@vger.kernel.org, corbet@lwn.net,
        linux-integrity@vger.kernel.org,
        Eric Biggers <ebiggers@kernel.org>, gwendal@chromium.org,
        dianders@chromium.org, apronin@chromium.org,
        Pavel Machek <pavel@ucw.cz>, Ben Boeckel <me@benboeckel.net>,
        rjw@rjwysocki.net, Kees Cook <keescook@chromium.org>,
        dlunev@google.com, zohar@linux.ibm.com, jarkko@kernel.org,
        linux-pm@vger.kernel.org, Matthew Garrett <mjg59@google.com>,
        Jason Gunthorpe <jgg@ziepe.ca>, Peter Huewe <peterhuewe@gmx.de>
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On Sat, Jan 14, 2023 at 6:55 AM James Bottomley <jejb@linux.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.