Re: [PATCH v9 14/32] OvmfPkg/MemEncryptSevLib: add support to validate system RAM

["Gerd Hoffmann" <kraxel@redhat.com>]

On Wed, Oct 13, 2021 at 11:56:55AM -0500, Brijesh Singh wrote:
> BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3275
> 
> Many of the integrity guarantees of SEV-SNP are enforced through the
> Reverse Map Table (RMP). Each RMP entry contains the GPA at which a
> particular page of DRAM should be mapped. The guest can request the
> hypervisor to add pages in the RMP table via the Page State Change VMGEXIT
> defined in the GHCB specification section 2.5.1 and 4.1.6. Inside each RMP
> entry is a Validated flag; this flag is automatically cleared to 0 by the
> CPU hardware when a new RMP entry is created for a guest. Each VM page
> can be either validated or invalidated, as indicated by the Validated
> flag in the RMP entry. Memory access to a private page that is not
> validated generates a #VC. A VM can use the PVALIDATE instruction to
> validate the private page before using it.
> 
> During the guest creation, the boot ROM memory is pre-validated by the
> AMD-SEV firmware. The MemEncryptSevSnpValidateSystemRam() can be called
> during the SEC and PEI phase to validate the detected system RAM.
> 
> One of the fields in the Page State Change NAE is the RMP page size. The
> page size input parameter indicates that either a 4KB or 2MB page should
> be used while adding the RMP entry. During the validation, when possible,
> the MemEncryptSevSnpValidateSystemRam() will use the 2MB entry. A
> hypervisor backing the memory may choose to use the different page size
> in the RMP entry. In those cases, the PVALIDATE instruction should return
> SIZEMISMATCH. If a SIZEMISMATCH is detected, then validate all 512-pages
> constituting a 2MB region.
> 
> Upon completion, the PVALIDATE instruction sets the rFLAGS.CF to 0 if
> instruction changed the RMP entry and to 1 if the instruction did not
> change the RMP entry. The rFlags.CF will be 1 only when a memory region
> is already validated. We should not double validate a memory
> as it could lead to a security compromise. If double validation is
> detected, terminate the boot.

Acked-by: Gerd Hoffmann <kraxel@redhat.com>