In the next two patches, we'll temporarily decrypt the pages containing
the initial SMRAM save state map, for SMBASE relocation. (Unlike the
separate, relocated SMRAM save state map of each VCPU, the original,
shared map behaves similarly to a "common buffer" between guest and host.)
The decryption will occur near the beginning of the DXE phase, in
AmdSevDxe, and the re-encryption will occur in PiSmmCpuDxeSmm, via OVMF's
SmmCpuFeaturesLib instance.

There is a non-trivial time gap between these two points, and the DXE
phase might use the pages overlapping the initial SMRAM save state map for
arbitrary purposes meanwhile. In order to prevent any information leak
towards the hypervisor, make sure the DXE phase puts nothing in those
pages until re-encryption is done.

Creating a memalloc HOB for the area in question is safe:

- the temporary SEC/PEI RAM (stack and heap) is based at
  PcdOvmfSecPeiTempRamBase, which is above 8MB,

- the permanent PEI RAM (installed in PlatformPei's PublishPeiMemory()
  function) never starts below PcdOvmfDxeMemFvBase, which is also above
  8MB.

The allocated pages can be released to the DXE phase after SMBASE
relocation and re-encryption are complete.

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>
---
 OvmfPkg/PlatformPei/AmdSev.c | 29 ++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/OvmfPkg/PlatformPei/AmdSev.c b/OvmfPkg/PlatformPei/AmdSev.c
index 1509f260fb0b..2e14eaf8c3cc 100644
--- a/OvmfPkg/PlatformPei/AmdSev.c
+++ b/OvmfPkg/PlatformPei/AmdSev.c
@@ -1,38 +1,39 @@
 /**@file
   Initialize Secure Encrypted Virtualization (SEV) support
 
   Copyright (c) 2017, Advanced Micro Devices. All rights reserved.<BR>
 
   This program and the accompanying materials
   are licensed and made available under the terms and conditions of the BSD
   License which accompanies this distribution.  The full text of the license
   may be found at http://opensource.org/licenses/bsd-license.php
 
   THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
   WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
 
 **/
 //
 // The package level header files this module uses
 //
 #include <Library/DebugLib.h>
+#include <Library/HobLib.h>
 #include <Library/MemEncryptSevLib.h>
 #include <Library/PcdLib.h>
 #include <PiPei.h>
 #include <Register/Amd/Cpuid.h>
 #include <Register/Cpuid.h>
 
 #include "Platform.h"
 
 /**
 
   Function checks if SEV support is available, if present then it sets
   the dynamic PcdPteMemoryEncryptionAddressOrMask with memory encryption mask.
 
   **/
 VOID
 AmdSevInitialize (
   VOID
   )
 {
   CPUID_MEMORY_ENCRYPTION_INFO_EBX  Ebx;
@@ -49,21 +50,49 @@ AmdSevInitialize (
   //
   // CPUID Fn8000_001F[EBX] Bit 0:5 (memory encryption bit position)
   //
   AsmCpuid (CPUID_MEMORY_ENCRYPTION_INFO, NULL, &Ebx.Uint32, NULL, NULL);
   EncryptionMask = LShiftU64 (1, Ebx.Bits.PtePosBits);
 
   //
   // Set Memory Encryption Mask PCD
   //
   PcdStatus = PcdSet64S (PcdPteMemoryEncryptionAddressOrMask, EncryptionMask);
   ASSERT_RETURN_ERROR (PcdStatus);
 
   DEBUG ((DEBUG_INFO, "SEV is enabled (mask 0x%lx)\n", EncryptionMask));
 
   //
   // Set Pcd to Deny the execution of option ROM when security
   // violation.
   //
   PcdStatus = PcdSet32S (PcdOptionRomImageVerificationPolicy, 0x4);
   ASSERT_RETURN_ERROR (PcdStatus);
+
+  //
+  // When SMM is required, cover the pages containing the initial SMRAM Save
+  // State Map with a memory allocation HOB:
+  //
+  // There's going to be a time interval between our decrypting those pages for
+  // SMBASE relocation and re-encrypting the same pages after SMBASE
+  // relocation. We shall ensure that the DXE phase stay away from those pages
+  // until after re-encryption, in order to prevent an information leak to the
+  // hypervisor.
+  //
+  if (FeaturePcdGet (PcdSmmSmramRequire) && (mBootMode != BOOT_ON_S3_RESUME)) {
+    RETURN_STATUS LocateMapStatus;
+    UINTN         MapPagesBase;
+    UINTN         MapPagesCount;
+
+    LocateMapStatus = MemEncryptSevLocateInitialSmramSaveStateMapPages (
+                        &MapPagesBase,
+                        &MapPagesCount
+                        );
+    ASSERT_RETURN_ERROR (LocateMapStatus);
+
+    BuildMemoryAllocationHob (
+      MapPagesBase,                      // BaseAddress
+      EFI_PAGES_TO_SIZE (MapPagesCount), // Length
+      EfiBootServicesData                // MemoryType
+      );
+  }
 }
-- 
2.14.1.3.gb7cf6e02401b


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