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

[William Roberts <bill.c.roberts@gmail.com>]

On Tue, Jan 24, 2023 at 6:38 AM James Bottomley <jejb@linux.ibm.com> wrote:
>
> On Mon, 2023-01-23 at 11:48 -0600, William Roberts wrote:
> > 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.
>
> No, I think it would be good enough: locality 0 means anyone (including
> the kernel on a machine which doesn't function correctly) could have
> created this key.  Locality 2 would mean only the kernel could have
> created this key.

That's exactly what I was saying, for this feature to be functional
2 localities need to be supported.

>
> By the time the kernel boots and before it loads the hibernation image
> it will know the answer to the question "does my TPM support locality
> 2", so it can use that in its security assessment: if the kernel
> supports locality 2 and the key wasn't created in locality 2 then
> assume an attack.  Obviously, if the kernel doesn't support locality 2
> then the hibernation resume has to accept any old key, but that's the
> same as the situation today.
>

Yep, we had this conversation offline on a thread, i'm in agreement here
as well.

> > 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".
>
> It's probably good enough.  Current laptops which can't use locality 2
> are in the same position as now, but newer ones can provide more
> security guarantees.
>
> There is, however, another wrinkle: can Kaby Lake be persuaded, though
> bios settings perhaps, to shut off the non zero localities?

I have no idea, and I don't have one handy, but I can ask around.

> This would
> allow for a downgrade attack where you shut off locality 2 then present
> a forged locality 0 key and hibernation image; the kernel will think,
> because it can't access locality 2, that it's in a reduced security
> environment so the key might be OK.  We could fix this by requiring
> Kaby Lake and beyond to have locality 2 and refusing to hibernate if it
> can't be accessed and building "is this Kaby lake or beyond" into the
> check for should I have locality 2, but this is getting complex and
> error prone.
>
> James
>