Merge "Update RKP VM documentation for RKP" into main

### Support for Android Virtualization Framework

The Android Virtualization Framwork (AVF) relies on RKP to provision keys for VMs. A

privileged vm, the RKP VM, is reponsible for generating and managing the keys for client

VMs that run virtualized workloads. See the following for more background information on the

RKP VM:

*    [rkp-vm](https://android.googlesource.com/platform/packages/modules/Virtualization/+/main/service_vm/README.md#rkp-vm-remote-key-provisioning-virtual-machine)

*    [rkp-service](https://source.android.com/docs/core/ota/modular-system/remote-key-provisioning#stack-architecture)

It is important to distinquish the RKP VM from other components, such as KeyMint. An

[RKP VM marker](https://pigweed.googlesource.com/open-dice/+/HEAD/docs/android.md#configuration-descriptor)

(key `-70006`) is used for this purpose. The existence or absence of this marker is used to

identify the type of component decribed by a given DICE chain.

The following describes which certificate types may be request based on the RKP VM marker:

1. "rkp-vm": If a DICE chain has zero or more certificates without the RKP VM

   marker followed by one or more certificates with the marker, then that chain

   describes an RKP VM. If there are further certificates without the RKP VM

   marker, then the chain does not describe an RKP VM.

   Implementations must include the first RKP VM marker as early as possible

   after the point of divergence between TEE and non-TEE components in the DICE

   chain, prior to loading the Android Bootloader (ABL).

2. "widevine" or "keymint": If there are no certificates with the RKP VM

   marker then it describes a TEE component.

3. None: Any component described by a DICE chain that does not match the above

   two categories.

The Android Virtualization Framework (AVF) relies on RKP to provision keys for

VMs. There are a privileged set of VMs that RKP will recognise and provision

keys to for specific applications, like Widevine, and for services, like

[VM attestation][vm-attestation]. These privileged VMs are identified by their

DICE chain through a combination of the [RKP VM marker][rkp-vm-marker]

(key `-70006`) and the component name.

[vm-attestation]: http://android.googlesource.com/platform/packages/modules/Virtualization/+/main/docs/vm_remote_attestation.md

[rkp-vm-marker]: https://pigweed.googlesource.com/open-dice/+/HEAD/docs/android.md#configuration-descriptor

If a DICE chain begins from the root with zero or more certificates without

the RKP VM marker, followed by only certificates with the marker up to and

including the leaf certificate, then that chain describes a VM that RKP might

provision keys to. Implementations must include the first RKP VM marker as early

as possible after the point of divergence between TEE and non-TEE components in

the DICE chain, prior to loading the Android Bootloader (ABL).

The component name of the leaf certificate then identifies the kind of keys for

RKP to provision:

*   "rkp-vm": for VM attestation keys managed by the [service VM][service-vm]

*   "keymint": for Android attestation keys

*   "widevine": for Widevine keys

[service-vm]: https://android.googlesource.com/platform/packages/modules/Virtualization/+/main/service_vm/README.md#rkp-vm-remote-key-provisioning-virtual-machine

If there are no certificates with the RKP VM marker in the DICE chain then it

describes a TEE component that can be provisioned with Widevine and Android

attestation keys.

Any remaining DICE chains describe a component to which RKP will not provision

keys.

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