On Fri, Mar 22, 2024 at 1:52 AM Gerd Hoffmann <kra...@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) -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#117055): https://edk2.groups.io/g/devel/message/117055 Mute This Topic: https://groups.io/mt/105070442/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
