SEV Command Privilege Separation

[Nathaniel McCallum]

I've been working on wrapping various SEV kernel APIs for userspace
consumption. There does not appear to be any privilege separation for
these commands: you can run them all or none of them. This is less
than ideal because it means that a compromise of the code which
launches VMs could make permanent changes to the SEV certificate
chain.

These commands are required to attest the VM environment:
  SEV_PDH_CERT_EXPORT
  SEV_PLATFORM_STATUS
  SEV_GET_ID

These commands manage the SEV certificate chain:
  SEV_PEK_CERT_IMPORT
  SEV_FACTORY_RESET
  SEV_PEK_GEN
  SEV_PEK_CSR
  SEV_PDH_GEN

I would expect the first group of commands to be able to be called by
whatever actor launches VMs. The latter group of commands should be
able to be called by whatever actor manages the SEV environment.

I don't have strong opinions on how this privilege separation should
happen. It might be sufficient to distinguish these based on the mode
of the open() call. This could then be managed with filesystem
permissions. But I'm open to other ideas.