OvmfPkg/AmdSevDxe: decrypt the pages of the initial SMRAM save state map

Based on the following patch from Brijesh Singh <brijesh.singh@amd.com>:

  [PATCH v2 1/2] OvmfPkg/AmdSevDxe: Clear the C-bit from SMM Saved State
  http://mid.mail-archive.com/20180228161415.28723-2-brijesh.singh@amd.com
  https://lists.01.org/pipermail/edk2-devel/2018-February/022016.html

Original commit message from Brijesh:

> When OVMF is built with SMM, SMMSaved State area (SMM_DEFAULT_SMBASE +
> SMRAM_SAVE_STATE_MAP_OFFSET) contains data which need to be accessed by
> both guest and hypervisor. Since the data need to be accessed by both
> hence we must map the SMMSaved State area as unencrypted (i.e C-bit
> cleared).
>
> This patch clears the SavedStateArea address before SMBASE relocation.
> Currently, we do not clear the SavedStateArea address after SMBASE is
> relocated due to the following reasons:
>
> 1) Guest BIOS never access the relocated SavedStateArea.
>
> 2) The C-bit works on page-aligned address, but the SavedStateArea
> address is not a page-aligned. Theoretically, we could roundup the
> address and clear the C-bit of aligned address but looking carefully we
> found that some portion of the page contains code -- which will causes a
> bigger issue for the SEV guest. When SEV is enabled, all the code must
> be encrypted otherwise hardware will cause trap.

Changes by Laszlo:

- separate AmdSevDxe bits from SmmCpuFeaturesLib bits;

- spell out PcdLib dependency with #include and in LibraryClasses;

- replace (SMM_DEFAULT_SMBASE + SMRAM_SAVE_STATE_MAP_OFFSET) calculation
  with call to new MemEncryptSevLocateInitialSmramSaveStateMapPages()
  function;

- consequently, pass page-aligned BaseAddress to
  MemEncryptSevClearPageEncMask();

- zero the pages before clearing the C-bit;

- pass Flush=TRUE to MemEncryptSevClearPageEncMask();

- harden the treatment of MemEncryptSevClearPageEncMask() failure.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Brijesh Singh <brijesh.singh@amd.com>

OvmfPkg/AmdSevDxe/AmdSevDxe.c

@@ -16,10 +16,13 @@
 
 **/
 
+#include <Library/BaseLib.h>
+#include <Library/BaseMemoryLib.h>
 #include <Library/DebugLib.h>
 #include <Library/DxeServicesTableLib.h>
 #include <Library/MemEncryptSevLib.h>
 #include <Library/MemoryAllocationLib.h>
+#include <Library/PcdLib.h>
 
 EFI_STATUS
 EFIAPI
@@ -68,5 +71,55 @@ AmdSevDxeEntryPoint (
     FreePool (AllDescMap);
   }
 
+  //
+  // When SMM is enabled, clear the C-bit from SMM Saved State Area
+  //
+  // NOTES: The SavedStateArea address cleared here is before SMBASE
+  // relocation. Currently, we do not clear the SavedStateArea address after
+  // SMBASE is relocated due to the following reasons:
+  //
+  // 1) Guest BIOS never access the relocated SavedStateArea.
+  //
+  // 2) The C-bit works on page-aligned address, but the SavedStateArea
+  // address is not a page-aligned. Theoretically, we could roundup the address
+  // and clear the C-bit of aligned address but looking carefully we found
+  // that some portion of the page contains code -- which will causes a bigger
+  // issues for SEV guest. When SEV is enabled, all the code must be encrypted
+  // otherwise hardware will cause trap.
+  //
+  // We restore the C-bit for this SMM Saved State Area after SMBASE relocation
+  // is completed (See OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c).
+  //
+  if (FeaturePcdGet (PcdSmmSmramRequire)) {
+    UINTN MapPagesBase;
+    UINTN MapPagesCount;
+
+    Status = MemEncryptSevLocateInitialSmramSaveStateMapPages (
+               &MapPagesBase,
+               &MapPagesCount
+               );
+    ASSERT_EFI_ERROR (Status);
+
+    //
+    // Although these pages were set aside (i.e., allocated) by PlatformPei, we
+    // could be after a warm reboot from the OS. Don't leak any stale OS data
+    // to the hypervisor.
+    //
+    ZeroMem ((VOID *)MapPagesBase, EFI_PAGES_TO_SIZE (MapPagesCount));
+
+    Status = MemEncryptSevClearPageEncMask (
+               0,             // Cr3BaseAddress -- use current CR3
+               MapPagesBase,  // BaseAddress
+               MapPagesCount, // NumPages
+               TRUE           // Flush
+               );
+    if (EFI_ERROR (Status)) {
+      DEBUG ((DEBUG_ERROR, "%a: MemEncryptSevClearPageEncMask(): %r\n",
+        __FUNCTION__, Status));
+      ASSERT (FALSE);
+      CpuDeadLoop ();
+    }
+  }
+
   return EFI_SUCCESS;
 }

OvmfPkg/AmdSevDxe/AmdSevDxe.inf

@@ -32,11 +32,17 @@
   OvmfPkg/OvmfPkg.dec
 
 [LibraryClasses]
+  BaseLib
+  BaseMemoryLib
   DebugLib
   DxeServicesTableLib
   MemEncryptSevLib
   MemoryAllocationLib
+  PcdLib
   UefiDriverEntryPoint
 
 [Depex]
   TRUE
+
+[FeaturePcd]
+  gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire