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Feature status of clouds
What works on which cloud? Currently, Confidential VMs (CVMs) are available in varying quality on the different clouds and software stacks.
For Constellation, the ideal environment provides the following:
- Ability to run arbitrary software and images inside CVMs
- CVMs based on AMD SEV-SNP (available in EPYC CPUs since the Milan generation) or, in the future, Intel TDX (available in Xeon CPUs from the Sapphire Rapids generation onward)
- Ability for CVM guests to obtain raw attestation statements directly from the CPU, ideally via a TPM-like interface
- Reviewable, open-source firmware inside CVMs
(1) is a functional must-have. (2)--(4) are required for remote attestation that fully keeps the infrastructure/cloud out. Constellation can work without them or with approximations, but won't protect against certain privileged attackers anymore.
The following table summarizes the state of features for different infrastructures as of May 2023.
| Feature | Azure | GCP | AWS | OpenStack (Yoga) |
|---|---|---|---|---|
| 1. Custom images | Yes | Yes | Yes | Yes |
| 2. SEV-SNP or TDX | Yes | Yes | Yes | Depends on kernel/HV |
| 3. Raw guest attestation | Yes | Yes | Yes | Depends on kernel/HV |
| 4. Reviewable firmware | No* | No | No* | Depends on kernel/HV |
Microsoft Azure
With its CVM offering, Azure provides the best foundations for Constellation. Regarding (3), Azure provides direct access to remote-attestation statements. However, regarding (4), the standard CVMs still include closed-source firmware running in VM Privilege Level (VMPL) 0. This firmware is signed by Azure. The signature is reflected in the remote-attestation statements of CVMs. Thus, the Azure closed-source firmware becomes part of Constellation's trusted computing base (TCB).
* Recently, Azure announced the limited preview of CVMs with customizable firmware. With this CVM type, (4) switches from No to Yes. Constellation will support customizable firmware on Azure in the future.
Google Cloud Platform (GCP)
The CVMs Generally Available in GCP are based on AMD SEV but don't have SNP features enabled. CVMs with SEV-SNP enabled are currently in private preview. Regarding (3), with their SEV-SNP offering Google provides direct access to remote-attestation statements. However, regarding (4), the CVMs still include closed-source firmware.
Intel and Google have collaborated to enhance the security of TDX, and have recently revealed their plans to make TDX compatible with Google Cloud.
Amazon Web Services (AWS)
Amazon EC2 supports AMD SEV-SNP. Regarding (3), AWS provides direct access to remote-attestation statements. However, attestation is partially based on the NitroTPM for measured boot, which is a vTPM managed by the Nitro hypervisor. Hence, the hypervisor is currently part of Constellation's TCB.
* Regarding (4), the CVMs include initial firmware inside the CVM based on OVMF. Once this firmware will be reproducible and therefore verifiable, (4) switches from No to Yes.
OpenStack
OpenStack is an open-source cloud and infrastructure management software. It's used by many smaller CSPs and datacenters. In the latest Yoga version, OpenStack has basic support for CVMs. However, much depends on the employed kernel and hypervisor. Features (2)--(4) are likely to be a Yes with Linux kernel version 6.2. Thus, going forward, OpenStack on corresponding AMD or Intel hardware will be a viable underpinning for Constellation.
Conclusion
The different clouds and software like the Linux kernel and OpenStack are in the process of building out their support for state-of-the-art CVMs. Azure has already most features in place. For Constellation, the status quo means that the TCB has different shapes on different infrastructures. With broad SEV-SNP support coming to the Linux kernel, we soon expect a normalization of features across infrastructures.