Re: [RESEND] [PATCH v2 2/2] OvmfPkg/ResetVector: Exclude SEV launch secrets page from pre-validation

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

On 2/20/23 02:49, Dov Murik wrote:
> In order to allow the VMM (such as QEMU) to add a page with hashes of
> kernel/initrd/cmdline for measured direct boot on SNP, this page must
> not be part of the SNP metadata list reported to the VMM.
> 
> Check if that page is defined; if it is, skip it in the metadata list.
> In such case, VMM should fill the page with the hashes content, or
> explicitly update it as a zero page (if kernel hashes are not used).

Would it be better to define a new section type (similar to what I did in 
the SVSM PoC)? This way, it remains listed in the metadata and allows the 
VMM to detect it and decide how to handle it.

Thanks,
Tom

> 
> Note that for SNP, the launch secret part of the page (lower 3KB) are
> not relevant and will stay zero.  The last 1KB is used for the hashes.
> 
> This should have no effect on OvmfPkgX64 targets (which don't define
> PcdSevLaunchSecretBase).
> 
> Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
> ---
>   OvmfPkg/ResetVector/ResetVector.nasmb | 14 +++++++++++++-
>   1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/OvmfPkg/ResetVector/ResetVector.nasmb b/OvmfPkg/ResetVector/ResetVector.nasmb
> index 94fbb0a87b37..16f3daf49d82 100644
> --- a/OvmfPkg/ResetVector/ResetVector.nasmb
> +++ b/OvmfPkg/ResetVector/ResetVector.nasmb
> @@ -75,7 +75,19 @@
>   ;
>   %define SNP_SEC_MEM_BASE_DESC_2       (GHCB_BASE + 0x1000)
>   %define SNP_SEC_MEM_SIZE_DESC_2       (SEV_SNP_SECRETS_BASE - SNP_SEC_MEM_BASE_DESC_2)
> -%define SNP_SEC_MEM_BASE_DESC_3       (CPUID_BASE + CPUID_SIZE)
> +%if (FixedPcdGet32 (PcdSevLaunchSecretBase) > 0)
> +  ; There's a reserved page for SEV secrets and hashes; the VMM will fill and
> +  ; validate the page, or mark it as a zero page.
> +  %define EXPECTED_END_OF_LAUNCH_SECRET_PAGE (FixedPcdGet32 (PcdSevLaunchSecretBase) + \
> +                                              FixedPcdGet32 (PcdSevLaunchSecretSize) + \
> +                                              FixedPcdGet32 (PcdQemuHashTableSize))
> +  %if (FixedPcdGet32 (PcdOvmfSecPeiTempRamBase) != EXPECTED_END_OF_LAUNCH_SECRET_PAGE)
> +    %error "PcdOvmfSecPeiTempRamBase must start directly after the SEV Launch Secret page"
> +  %endif
> +  %define SNP_SEC_MEM_BASE_DESC_3     (FixedPcdGet32 (PcdOvmfSecPeiTempRamBase))
> +%else
> +  %define SNP_SEC_MEM_BASE_DESC_3     (CPUID_BASE + CPUID_SIZE)
> +%endif
>   %define SNP_SEC_MEM_SIZE_DESC_3       (FixedPcdGet32 (PcdOvmfPeiMemFvBase) - SNP_SEC_MEM_BASE_DESC_3)
>   
>   %ifdef ARCH_X64