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

[William Roberts]

On Thu, Jan 26, 2023 at 11:12 AM Jarkko Sakkinen <jarkko@xxxxxxxxxx> wrote:
>
> On Thu, Jan 26, 2023 at 05:07:43PM +0000, Jarkko Sakkinen wrote:
> > On Mon, Jan 23, 2023 at 11:48:25AM -0600, William Roberts wrote:
> > > On Fri, Jan 20, 2023 at 9:29 PM Jarkko Sakkinen <jarkko@xxxxxxxxxx> 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@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.
> > >
> > > 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".
> >
> > What about having opt-in flag that distributions can then enable?
>
> This is more intrusive but still worth of consideration: add opt-in
> kernel command-line flag for no locality. I.e. require locality support
> unless explicitly stated otherwise.
>
> I'd presume that legacy production cases are a rarity but really is
> something that is beyond me, and could potentially draw wrong conclusions.
>

One thing that was never answered for me, is that there was nowhere safe
to store some information about the expected key or a secret. That would
be the most obvious solution, so I am assuming that's a no.