Re: [PATCH Part2 v6 17/49] crypto: ccp: Add the SNP_{SET,GET}_EXT_CONFIG command

Dionna Amalie Glaze
2022-08-08T19:28:04Z

To preface, I don't want to delay this patch set, only have the
conversation at the most appropriate place.

>
> > The SEV-SNP firmware provides the SNP_CONFIG command used to set the
> > system-wide configuration value for SNP guests. The information includes
> > the TCB version string to be reported in guest attestation reports.
>

The system-wide aspect of this makes me wonder if we can also have a
VM instance-specific extension. This is important for the use case
that we may see secure boot variables included in the launch
measurement, making offline signing of the UEFI image impossible. We
can't sign the cross-product of all UEFI builds and every user's EFI
variables. We'd like to include an instance-specific certificate that
specifies the platform-endorsed golden measurement of the UEFI.

An alternative that doesn't require a change to the kernel is to just
make this certificate fetchable from a FAMILY_ID-keyed, predetermined
URL prefix + IMAGE_ID + '.crt', but this requires a download (and
continuous hosting) to do something as routine as collecting an
attestation report. It's up to the upstream community to determine if
that is an acceptable cost to keep the complexity of a certificate
table merge operation out of the kernel.

The SNP API specification gives an interpretation to the data blob
here as a table of GUID/offset pairs followed by data blobs that
presumably are at the appropriate offsets into the data pages. The
spec allows for the host to add any number of GUID/offset pairs it
wants, with 3 specific GUIDs recommended for the AMD PSP certificate
chain.

The snp_guest_ext_guest_request function in ccp is what passes back
the certificate data that was previously stored, so I'm wondering if
it can take an extra (pointer,len) pair of VM instance certificate
data to merge with the host certificate data before returning to the
guest. The new required length is the sum total of both the header
certs and instance certs. The operation to copy the data is no longer
a memcpy but a header merge that tracks the offset shifts caused by a
larger header and other certificates in the remaining data pages.

I can propose my own patch on top of this v6 patch set that adds a KVM
ioctl like KVM_{GET,SET}_INSTANCE_SNP_EXT_CONFIG and then pass along
the stored certificate blob in the request call. I'd prefer to have
the design agreed upon upfront though.

--
-Dionna Glaze, PhD (she/her)