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| # AMD Secure Encrypted Virtualization (AMD-SEV) | ||
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| # Concepts | ||
| - AMD-SEV is targeted at securing virtual machines by encrypting the memory of each virtual machine with a unique key. | ||
| - SEV can protect your machine from a potentially malicious hypervisor. | ||
| - SEV can calculate a signature of virtual machine's memory content which can be sent to the VM's owner as an attestation that the memory on the target host, was encrypted correctly by firmware. | ||
| - AMD-SEV SNP is an extension to SEV which adds new hardware-based security protections | ||
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| ## Keys used in SEV | ||
| The AMD SEV firmware provides a mechanism to verify that it is executing on AMD hardware that supports SEV. The following key hierarchy, rooted in an AMD-owned key, is used in this process: | ||
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| - $PDH$ (Platform Diffie Hellman) key - This key is used to negotiate a master secret which is then used with a key derivation function to establish a trusted channel | ||
| - $PEK$ (Platform Endorsement Key) - This key signs the $PDH$ to anchor the $PDH$ to the AMD root of trust and the platform owner's root of trust | ||
| - $CEK$ (Chip endorsement key) - This key signs the $PEK$ to anchor the $PEK$ to the AMD root of trust. Each chip has a unique $CEK$ which is derived from secrets stored in the chip's one-time programmable (OTP) memory | ||
| - $ASK$ (AMD Signing Key) - The $ASK$ private key signs the $CEK$ public key to demonstrate that the $CEK$ is an authentic AMD key | ||
| - $ARK$ (AMD Root Key) - The $ARK$ private key signs the $ASK$ public key to demonstrate that the $ASK$ is an authentic AMD key. This key is the root of trust of AMD and its signatures signify AMD authencity | ||
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| Therefore the following certificate chain is produced: | ||
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| $ARK \rightarrow ASK \rightarrow CEK \rightarrow PEK \rightarrow PDH$ | ||
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| Therefore if the secure channel can be established using the $PDH$ key, then it is ensured that, the attesting workload is executed on, is an authentic AMD system which has the SEV feature. | ||
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| ## AMD-SEV SNP Attestation report measurements | ||
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| ### Platform measurements | ||
| - CHIP_ID - The unique chip identifier | ||
| - PLATFORM_INFO - Indicates properties of the platform configuration, for example whether whole system memory encryption (TSME) or simultaneous multithreading (SMT) is enabled | ||
| - CURRENT_TCB - Security Version Numbers (SVNs) of the current executing platform firmware and microcode | ||
| - COMMITTED_TCB - SVNs of the anti-rollback minimum of the platform firmware and microcode | ||
| - REPORTED_TCB - SVN of the hypervisor. The hypervisor has the option to report a lower version | ||
| - LAUNCH_TCB - SVNs of the platform firmware and microcode at the time the guest was launched or imported | ||
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| ### Guest measurements | ||
| - FAMILY_ID - The family ID of the guest that is provided at launch | ||
| - IMAGE_ID - The image ID of the guest that is provided at launch | ||
| - GUEST_SVN - The guest SVN | ||
| - MEASUREMENT - Measurement of the guest address space | ||
| - ID_KEY_DIGEST - SHA-384 digest of the ID public key that signed the [ID block](https://www.amd.com/system/files/TechDocs/56860.pdf#page=91) provided in `SNP_LAUNCH_FINISH` | ||
| - AUTHOR_KEY_DIGEST - SHA-384 digest of the Author public key that certified the ID key, if provided in `SNP_LAUNCH_FINISH` | ||
| - POLICY - The [guest policy](https://www.amd.com/system/files/TechDocs/56860.pdf#page=26) | ||
| - REPORT_ID - Report ID of this guest | ||
| - REPORT_ID_MA - Report ID of this guest's migration agent, if the guest is associated with a migration agent | ||
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| More details on other elements of the produced attestation report are outlined [here](https://www.amd.com/system/files/TechDocs/56860.pdf#page=44). | ||
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| # AMD-SEV SNP from Veraison's perspective | ||
| Veraison needs two things to verify the attestation of an AMD-SEV SNP system: | ||
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| - The signed attestation report | ||
| - $VCEK$ (Versioned Chip Endorsement Key) - The [$VCEK$](https://www.amd.com/system/files/TechDocs/57230.pdf) is the key that signs the attestation report. The key is derived from chip unique secrets and a [TCB_VERSION](https://www.amd.com/system/files/TechDocs/56860.pdf#page=18) | ||
| - The following certificate chain that is tied to the signed attestation report | ||
| - $ARK \rightarrow ASK \rightarrow VCEK \rightarrow$ Attestation Report | ||
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| ## Current prototyping | ||
| A protoypte of an attestation verification flow is for AMD SEV-SNP evidence can be found [here](https://github.com/SabreenKaur/amd-snp-prototype). The library used for verification of the AMD SEV-SNP reports is [here](https://github.com/google/go-sev-guest). | ||
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| ### Structure of attestation verification flow | ||
| 1. Report is obtained from attester | ||
| 2. A full Attestation is derived from the report. In this step two things will happen to obtain the certificate chain (that forms part of this attestation): | ||
| - AMD KDS (Key Distribution Service) is queried to get the ASK and ARK certificates if they are not already in the internal library cache | ||
| - AMD KDS is queried to obtain the VCEK certificate if it is not already in the implemented time-to-live cache | ||
| 3. Non-signature fields of the report are matched against a provided JSON policy | ||
| 4. The Attestation is verified. Three things happen: | ||
| - AMD KDS is queried to get the certificate chain (ASK, ARK and VECK) if they are not already in the their respective caches | ||
| - Certificate revocation lists are checked | ||
| - The Attestation report's signature is verified based on the report's signature algorithm | ||
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| ### Structure of the TTL (Time-to-live) cache | ||
| The implemented time-to-live cache is designed to hold: | ||
| - VCEK certificates | ||
| - CRLs (Certificate Revocation Lists) | ||
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| Structure of the implemented cache: | ||
| - Key: `string` format of the url used to query the AMD KDS | ||
| - Value: `byte[]` format of the certificates or CRLs returned after querying the AMD KDS | ||
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| Expiry of cache entries: | ||
| - VCEK certificates: time to live is set to be the duration between the current time and the `vcek.NotAfter` field. VCEK certificates typically expire every 7 years after being issued | ||
| - CRLs (Certificate Revocation Lists): time to live is set to be the duration between the current time and the `crl.NextUpdate` field. CRLs typically get refreshed every year | ||
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| A [custom getter](https://github.com/SabreenKaur/amd-snp-prototype/blob/main/ttl_getter.go) is implemented to allow us to interface with the ttl cache. | ||
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| ### Veraison in a TEE with AMD SEV-SNP | ||
| - We can setup up an AWS EC2 instance with support for AMD SEV-SNP to obtain Attestation Reports. | ||
| - Instructions on running the EC2 instance with AMD SEV SNP support are outlined [here](https://confluence.arm.com/display/~sabgur01/Creating+an+amd+sev+snp+instance+using+AWS+console) | ||
| - Reports can be obtain using this golang [library](https://github.com/google/go-sev-guest) (currently a [WIP](https://github.com/google/go-sev-guest/issues/53)) or using the AMD offical tooling following these [instructions](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/snp-attestation.html). This [guide](https://confluence.arm.com/display/~sabgur01/Generating+a+VLEK+using+AMD+tools) helps with installing the modules needed to complete the steps in the instructions. | ||
| - Caveat: To verify SNP VMs within AWS, we need to obtain an VLEK instead of a VCEK certificate | ||
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| ## Open Questions | ||
| 1. What sort of background tasks need to be performed behind the kvstore exisiting functionality, when querying the AMD key service? | ||
| - Does it need to be per scheme? | ||
| - What are the common logics? | ||
| - UPDATE: We can use ttl caches to store the certificates that are queried from the AMD KDS | ||
| 2. Through what mechanism can we do the checking of CRLs (certificate revocation list) or similiar? | ||
| - UPDATE: This can be done by enabling the CheckRevocations in the Options struct | ||
| 3. How can platform (OCA) specific fields of the attestation report be verified? | ||
| - Could it be handled by policies? | ||
| 4. Which non-signature fields of the attestation report should be validated by Veraison? | ||
| 5. Where can nonce-like freshness data be represented? Possibly in the [REPORT_DATA](https://www.amd.com/system/files/TechDocs/56860.pdf#page=43) field? | ||
| 6. Logic and functions within `trustedservices_grpc.GetAttestation` need to be refactored to account for that fact that trust anchors cannot be assumed in the SEV-SNP scheme, initial thoughts on functions to be refactored: | ||
| - `initEvidenceContext(...)` - make it optional to `GetTrustAnchorID` within this function | ||
| - `getTrustAnchor(...)` - make it optional to call this function | ||
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| ## References: | ||
| 1. [AMD SEV API specification](https://www.amd.com/system/files/TechDocs/55766_SEV-KM_API_Specification.pdf) | ||
| 2. [AMD SEV-SNP ABI specification](https://www.amd.com/system/files/TechDocs/56860.pdf) | ||
| 3. [AMD VCEK certificate and KDS interface specification](https://www.amd.com/system/files/TechDocs/57230.pdf) | ||
| 4. [Presentation on attestation in AMD SEV-SNP](https://www.amd.com/content/dam/amd/en/documents/developer/lss-snp-attestation.pdf) |