SEV Command Privilege Separation

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.

Re: SEV Command Privilege Separation

[Singh, Brijesh]

On 2/14/19 3:08 PM, Nathaniel McCallum wrote:
> 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.
> 


We had somewhat a similar discussion on libvirt ML, currently the
udev rules for the /dev/sev is 0600. As you have noticed there
is no privilege separation, if admin grants 0644 then user/group
will able to issue a command which can modify  the SEV certificate
chain. At the very minimal we should perform the mode check
before issuing the commands to PSP. If user does not have 'write'
access then any commands which modifies the cert-chain should
fail. Its in my TODO list, the patches are always welcome :)

-Brijesh