On Fri, 9 Dec 2022 at 07:41, Adam Dunlap via groups.io
<acdunlap=google....@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 <acdun...@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?
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