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SubjectRe: [PATCH v5 03/11] tpm: Allow PCR 23 to be restricted to kernel-only use
On Fri, Jan 20, 2023 at 9:29 PM Jarkko Sakkinen <jarkko@kernel.org> wrote:
>
> On Sat, Jan 14, 2023 at 09:55:37AM -0500, James Bottomley wrote:
> > On Tue, 2023-01-03 at 13:10 -0800, Matthew Garrett wrote:
> > > On Tue, Jan 3, 2023 at 1:05 PM William Roberts
> > > <bill.c.roberts@gmail.com> wrote:
> > >
> > > > What's the use case of using the creation data and ticket in this
> > > > context? Who gets the creationData and the ticket?
> > > > Could a user supplied outsideInfo work? IIRC I saw some patches
> > > > flying around where the sessions will get encrypted and presumably
> > > > correctly as well. This would allow the transfer of that
> > > > outsideInfo, like the NV Index PCR value to be included and
> > > > integrity protected by the session HMAC.
> > >
> > > The goal is to ensure that the key was generated by the kernel. In
> > > the absence of the creation data, an attacker could generate a
> > > hibernation image using their own key and trick the kernel into
> > > resuming arbitrary code. We don't have any way to pass secret data
> > > from the hibernate kernel to the resume kernel, so I don't think
> > > there's any easy way to do it with outsideinfo.
> >
> > 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.
>
> Kernel could try to use locality 2 and use locality 0 as fallback.

I don't think that would work for Matthew, they need something
reliable to indicate key provenance.

I was informed that all 5 localities should be supported starting
with Gen 7 Kaby Lake launched in 2016. Don't know if this is
still "too new".

>
> BR, Jarkko

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