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Attestation

The second step in ensuring boot integrity of a platform is to guarantee that the executed and launched components are trusted components. This process is called attestation, and without this step there is no assurance that the platform is in a trusted state. Why is attestation important from a cloud perspective? There are two main considerations for use cases to be instantiated and delivered in a cloud:

• How would the entity needing this information know if a specific platform is Intel TXT enabled, or if a specific server has a defined or compliant BIOS or VMM running on it (i.e., can it be trusted)?

• Why should the entity requesting this information (which, in a cloud environment, could be a resource scheduler or orchestrator trying to schedule a service on a set of available nodes or servers) trust the response from the platform?

An attestation service provides definitive answers to these questions. Chapter 4 covers attestation in detail, including description of a reference attestation platform for Intel-based platforms, code-named Mt. Wilson. But here is a quick summary of the capability.

Attestation ratchets up the notion of roots of trust by making the information from various roots of trust visible and usable by other entities. In a TPM-based implementation of RTS and RTR, it provides a digital signature of platform configuration registers (PCR), with a set of registers in a TPM extended with specific measurements for various launch modules of the software and the requestor validating the signature and the PCR contents. To validate, first the requestor invokes, via an agent on the host or device, the TPM_Quote command, specifying an attestation identity key to perform the digital signature on the set of PCRs to quote, and a cryptographic nonce to ensure freshness of the digital signature. Next, the attestation service validates the signature and determines the trust of the launched server by comparing the measurements from the TPM quote with known-good measurements. It is a critical IT operations challenge to manage the known-good measurement for hypervisors, operating systems, and BIOS software to ensure they are all protected from tampering and spoofing. This capability can be internal to a company, offered by a service provider, or delivered remotely as a service by a trusted third party (TTP). The process is described in detail in Chapter 4.

The measured boot and the attestation thus enable a server/host to demonstrate its boot integrity. Failure of a measured boot process or attestation can initiate a series of remediation steps that are managed and controlled by the policies in the data center. Barring any hardware or configuration issues, then, a failed attestation would mean one of following two conditions:

• Someone or something has tampered with one or more launch components.

• A wrong version (compared to the known-good or whitelist) of BIOS, OS, drivers, and so on has been installed and attempted to launch at the server/host.

Security tools like security information and event management (SIEMs), compliance tools, and configuration checkers would flag these alerts to drive the appropriate remediation actions. In short, having the ability to assert the integrity of a platform is both valuable and necessary. With a set of platforms that have demonstrated integrity, they can be aggregated to do interesting things. This aggregation of platforms is what we refer to as a trusted compute pool (TCP).

 
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