Re: [edk2-devel] [PATCH] OvmfPkg/PlatformPei: Validate SEC's GHCB page

["Ard Biesheuvel" <ardb@kernel.org>]

On Fri, 9 Dec 2022 at 07:41, Adam Dunlap via groups.io
<acdunlap=google.com@groups.io> wrote:
>
> When running under SEV-ES, a page of shared memory is allocated for the
> GHCB during the SEC phase at address 0x809000. This page of memory is
> eventually passed to the OS as EfiConventionalMemory. When running
> SEV-SNP, this page is not PVALIDATE'd in the RMP table, meaning that if
> the guest OS tries to access the page, it will think that the host has
> voilated the security guarantees and will likely crash.
>
> This patch validates this page immediately after EDK2 switches to using
> the GHCB page allocated for the PEI phase.
>
> This was tested by writing a UEFI application that reads to and writes
> from one bytes of each page of memory and checks to see if a #VC
> exception is generated indicating that the page was not validated.
>
> Signed-off-by: Adam Dunlap <acdunlap@google.com>
> ---
>  OvmfPkg/PlatformPei/AmdSev.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/OvmfPkg/PlatformPei/AmdSev.c b/OvmfPkg/PlatformPei/AmdSev.c
> index e1b9fd9b7f..c465732068 100644
> --- a/OvmfPkg/PlatformPei/AmdSev.c
> +++ b/OvmfPkg/PlatformPei/AmdSev.c
> @@ -206,6 +206,7 @@ AmdSevEsInitialize (
>  {
>    UINT8                *GhcbBase;
>    PHYSICAL_ADDRESS     GhcbBasePa;
> +  PHYSICAL_ADDRESS     PrevGhcbPa;
>    UINTN                GhcbPageCount;
>    UINT8                *GhcbBackupBase;
>    UINT8                *GhcbBackupPages;
> @@ -293,8 +294,24 @@ AmdSevEsInitialize (
>      GhcbRegister (GhcbBasePa);
>    }
>
> +  PrevGhcbPa = AsmReadMsr64 (MSR_SEV_ES_GHCB);
> +
>    AsmWriteMsr64 (MSR_SEV_ES_GHCB, GhcbBasePa);
>
> +  //
> +  // Now that the PEI GHCB is set up, the SEC GHCB page is no longer necessary
> +  // to keep shared. Later, it is exposed to the OS as EfiConventionalMemory, so
> +  // it needs to be marked private. The size of the region is hardcoded in
> +  // OvmfPkg/ResetVector/ResetVector.nasmb in the definition of
> +  // SNP_SEC_MEM_BASE_DESC_2.
> +  //
> +  ASSERT (PrevGhcbPa == FixedPcdGet32(PcdOvmfSecGhcbBase));
> +
> +  ASSERT_RETURN_ERROR(MemEncryptSevSetPageEncMask(
> +    0 /*Cr3 -- use system Cr3*/,
> +    PrevGhcbPa,
> +    1 /*Number of pages*/));
> +

What happens to this call when ASSERT()s are compiled out?