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

[Jarkko Sakkinen]

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@xxxxxxxxx> 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.

BR, Jarkko