TCB Versioning

Within the SEV-SNP architecture, several firmware components are considered upgradeable and trusted in the threat model. These components, such as the AMD-SP API and CPU microcode patch, collectively form the Trusted Computing Base (TCB) of the architecture. To maintain security and integrity, new versions of these firmware components may be released periodically to address bugs, introduce improvements, or implement new features. In the event of discovering a security bug in one of these components, the guest owner needs assurance that their virtual machine (VM) is running on a patched firmware version and not a vulnerable one.

Previous SEV and SEV-ES features relied on a self-reported AMD-SP version number to manage TCB versioning. In SEV-SNP, this check has been significantly enhanced to provide cryptographic strength. The version numbers of all TCB components are now combined with a fused secret called the Chip Endorsement Key (CEK) to generate a Versioned Chip Endorsement Key (VCEK). The VCEK is a private ECDSA key unique to each AMD chip, running a specific TCB version. The construction of the VCEK leverages cryptographic hash functions to prevent any attempt to impersonate a newer TCB version. This ensures that a given TCB version cannot be misrepresented as a more recent one. The VCEK plays a crucial role in various aspects, including signing attestation reports, contributing to the overall security and integrity of the SEV-SNP ecosystem.