Re: [edk2-devel] [RFC PATCH] OvmfPkg/SecurityPkg: Add build option for coexistance of vTPM and RTMR.

["Dionna Glaze via groups.io" <dionnaglaze=google.com@groups.io>]

On Fri, Mar 22, 2024 at 1:52 AM Gerd Hoffmann <kraxel@redhat.com> wrote:
>
> On Fri, Mar 22, 2024 at 02:39:20AM +0000, Yao, Jiewen wrote:
> > Please aware that this option will cause potential security risk.
> >
> > In case that any the guest component only knows one of vTPM or RTMR,
> > and only extends one of vTPM or RTMR, but the other one only verifies
> > the other, then the chain of trust is broken.  This solution is secure
> > if and only if all guest components aware of coexistence, and can
> > ensure all measurements are extended to both vTPM and RTMR.  But I am
> > not sure if all guest components are ready today.
>
> As far I know (it's been a while I looked at those patches) shim.efi and
> grub.efi have support for EFI_CC_MEASUREMENT_PROTOCOL, but use the same
> logic we have in DxeTpm2MeasureBootLib, i.e. they will not measure to
> both RTMR and vTPM.

Shim will measure into CC and then continue to measure into TPM
https://github.com/rhboot/shim/blob/126a07ebc30bbd203b6966465b058da741b2654b/tpm.c#L164

GRUB2 has the same behavior. We can at least get coexistence
supporting the current boot integrity strategy that Confidential Space
is using, which is to depend on a dmverity initramfs whose root hash
is in the kernel_cmdline, and a Linux kernel built with LOADPIN. The
changes to Linux are proposed but not accepted precisely due to this
conversation we're having now.
I recall describing this to another CSP engineer at an IETF meeting
and they claimed they used the same approach, but I can't remember if
that was Oracle or another company.

>
> Looking at systemd-boot I see it will likewise not measure to both RTMR
> and vTPM, but with reversed priority (use vTPM not RTMR in case both are
> present).
>

Interesting. Thanks for this report. We'll push for the changed
semantics here if the spec is indeed changed, and request partner
distros in the CCC to include the updated systemd-boot.
I think that since the current boot integrity story stops at PCR9, we
have time to update this component before the attestation method
evolves to support a less special-purpose system composition.

> Linux kernel appears to not have EFI_CC_MEASUREMENT_PROTOCOL support.
>
> > Since this option caused a potential risk / misuse breaking the chain
> > of trust, I recommend we have at least one more company to endorse the
> > runtime co-existence of vTPM and RTMR.  Also, I would like to hear the
> > opinions from other companies.
>
> Rumors say intel is working on coconut-svsm support for tdx.  That will
> most likely allow to use a vTPM with tdx even without depending on the
> virtualization host or cloud hyperscaler providing one.  We will see VMs
> with both RTMR and vTPM and surely need a strategy how guests should
> deal with that situation, consistent across the whole boot stack and
> not every component doing something different.
>

An ephemeral TPM that Coconut-svsm offers is qualitatively different
than the persistent TPMs in CSPs today. Users of Confidential VMs all
have different threat models that allow for trusting a CSP-managed
vTPM for sealing keys but not for trusting unencrypted data in use.
The boot stack attestation story will not be fully resolved for a long
while, and a smooth transition is better than a jarring one.

> Given that the vTPM might be provided by the hypervisor and thus not be
> part of the TCB I can see that guests might want use both vTPM and RTMR.
> So, yes, for that case coexistance makes sense.  I'm not convinced it is
> a good idea to make that a compile time option though.  That will not
> help to promote a consistent story ...
>

I agree, but it does mean we have to change the event log composition
to describe the configured measurement services like described above.
I think a static Pcd is okay to begin with. The idea for tracking
these qualities is through software supply chain endorsements.
Eventually that would look like the proposed in-toto's SCAI [1] but
until then we'd have a bespoke format that we document and integrate
with in an attestation verification service.

[1] https://arxiv.org/pdf/2210.05813.pdf

-- 
-Dionna Glaze, PhD (she/her)


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