Re: [PATCH Part1 v5 38/38] virt: sevguest: Add support to get extended report

[Dov Murik]

On 01/09/2021 0:11, Brijesh Singh wrote:
> 
> 
> On 8/31/21 3:22 PM, Dov Murik wrote:
>> Hi Brijesh,
>>
>> On 20/08/2021 18:19, Brijesh Singh wrote:
>>> Version 2 of GHCB specification defines NAE to get the extended guest
>>> request. It is similar to the SNP_GET_REPORT ioctl. The main difference
>>> is related to the additional data that be returned. The additional
>>> data returned is a certificate blob that can be used by the SNP guest
>>> user.
>>
>> It seems like the SNP_GET_EXT_REPORT ioctl does everything that the
>> SNP_GET_REPORT ioctl does, and more.  Why expose SNP_GET_REPORT to
>> userspace at all?
>>
>>
> 
> Since both of these options are provided by the GHCB protocol so I
> exposed it. Its possible that some applications may not care about the
> extended certificate blob. And in those case, if the hypervisor is
> programmed with the extended certificate blob and caller does not supply
> the enough number of pages to copy the blob then command should fail.
> This will enforce a new requirement on that guest application to
> allocate an extra memory. e.g:
> 
> 1. Hypervisor is programmed with a system wide certificate blob using
> the SNP_SET_EXT_CONFIG ioctl().
> 
> 2. Guest wants to get the report but does not care about the certificate
> blob.
> 
> 3. Guest issues a extended guest report with the npages = 0. The command
> will fail with invalid length and number of pages will be returned in
> the response.
> 
> 4. Guest will not need to allocate memory to hold the certificate and
> reissue the command.
> 
> The #4 is unnecessary for a guest which does not want to get. In this
> case, a guest can simply call the attestation report without asking for
> certificate blob. Please see the GHCB spec for more details.
> 

OK.  Originally I thought that by passing certs_address=NULL and
certs_len=0 the user program can say "I don't want this extra data"; but
now I understand that this will return an error (invalid length) with
number of pages needed.

-Dov