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

["Dov Murik" <dovmurik@linux.ibm.com>]

On 21/02/2023 11:38, Gerd Hoffmann wrote:
> On Mon, Feb 20, 2023 at 08:44:23AM -0600, Tom Lendacky wrote:
>> 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.
> 
> Explicitly describing things sounds better to me too.
> 

Thanks for the feedback Tom and Gerd.


I can define a new section type OVMF_SECTION_TYPE_KERNEL_HASHES. In the AmdSev
target it'll cover the single MEMFD page at 00F000 (after the CPUID page).  
Now there's a question for the QEMU side -- should QEMU then fill the page
and encrypt it (launch_update type=NORMAL)? (currently the whole hashes table
creation and encryption is done elsewhere there)

And on regular OvmfX64 builds - should that area should be with type
OVMF_SECTION_TYPE_SNP_SEC_MEM which is accepted as a type=ZERO page ?


Playing with this idea, the metadata list will add:


; Kernel hashes section for measured direct boot
%define OVMF_SECTION_TYPE_KERNEL_HASHES   0x5

...

; Kernel hashes for measured direct boot, or zero page if
; there are no kernel hashes / SEV secrets
SevSnpKernelHashes:
  DD  SEV_SNP_KERNEL_HASHES_BASE
  DD  SEV_SNP_KERNEL_HASHES_SIZE
  DD  SEV_SNP_KERNEL_HASHES_TYPE



and the base/size/type of that region are defined in an
%if statement in ResetVector.nasmb:


%if (FixedPcdGet32 (PcdSevLaunchSecretBase) > 0)
  ; There's a reserved page for SEV secrets and hashes; the VMM will fill and
  ; validate the page
  %define SEV_SNP_KERNEL_HASHES_TYPE    OVMF_SECTION_TYPE_KERNEL_HASHES
  %define SEV_SNP_KERNEL_HASHES_BASE    (FixedPcdGet32 (PcdSevLaunchSecretBase))
%else
  ; No SEV secrets and hashes page; the VMM will validate it as another zero page
  %define SEV_SNP_KERNEL_HASHES_TYPE    OVMF_SECTION_TYPE_SNP_SEC_MEM
  %define SEV_SNP_KERNEL_HASHES_BASE    (CPUID_BASE + CPUID_SIZE)
%endif
%define SEV_SNP_KERNEL_HASHES_SIZE    (FixedPcdGet32 (PcdOvmfSecPeiTempRamBase) - SEV_SNP_KERNEL_HASHES_BASE)


(I still need to figure out the point about QEMU above.)


Is that what you had in mind?

-Dov