Re: [PATCH v4 6/7] MdeModulePkg/DxeCore: implement memory protection policy

["Gao, Liming" <liming.gao@intel.com>]

Reviewed-by: Liming Gao <liming.gao@intel.com>

-----Original Message-----
From: edk2-devel [mailto:edk2-devel-bounces@lists.01.org] On Behalf Of Ard Biesheuvel
Sent: Monday, February 27, 2017 10:38 PM
To: edk2-devel@lists.01.org; afish@apple.com; leif.lindholm@linaro.org; Kinney, Michael D <michael.d.kinney@intel.com>; Gao, Liming <liming.gao@intel.com>; Yao, Jiewen <jiewen.yao@intel.com>
Cc: Tian, Feng <feng.tian@intel.com>; lersek@redhat.com; Zeng, Star <star.zeng@intel.com>; Ard Biesheuvel <ard.biesheuvel@linaro.org>
Subject: [edk2] [PATCH v4 6/7] MdeModulePkg/DxeCore: implement memory protection policy

This implements a DXE memory protection policy that ensure that regions
that don't require executable permissions are mapped with the non-exec
attribute set.

First of all, it iterates over all entries in the UEFI memory map, and
removes executable permissions according to the configured DXE memory
protection policy, as recorded in PcdDxeMemoryProtectionPolicy.

Secondly, it sets or clears the non-executable attribute when allocating
or freeing pages, both for page based or pool based allocations.

Note that this complements the image protection facility, which applies
strict permissions to BootServicesCode/RuntimeServicesCode regions when
the section alignment allows it. The memory protection configured by this
patch operates on non-code regions only.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 MdeModulePkg/Core/Dxe/DxeMain.h               |  24 ++
 MdeModulePkg/Core/Dxe/DxeMain.inf             |   1 +
 MdeModulePkg/Core/Dxe/Mem/Page.c              |   7 +
 MdeModulePkg/Core/Dxe/Mem/Pool.c              |   7 +
 MdeModulePkg/Core/Dxe/Misc/MemoryProtection.c | 371 +++++++++++++++++++-
 5 files changed, 409 insertions(+), 1 deletion(-)

diff --git a/MdeModulePkg/Core/Dxe/DxeMain.h b/MdeModulePkg/Core/Dxe/DxeMain.h
index b14be9a74d8e..5668c1f2d648 100644
--- a/MdeModulePkg/Core/Dxe/DxeMain.h
+++ b/MdeModulePkg/Core/Dxe/DxeMain.h
@@ -2949,4 +2949,28 @@ MemoryProtectionExitBootServicesCallback (
   VOID
   );
 
+/**
+  Manage memory permission attributes on a memory range, according to the
+  configured DXE memory protection policy.
+
+  @param  OldType           The old memory type of the range
+  @param  NewType           The new memory type of the range
+  @param  Memory            The base address of the range
+  @param  Length            The size of the range (in bytes)
+
+  @return EFI_SUCCESS       If the the CPU arch protocol is not installed yet
+  @return EFI_SUCCESS       If no DXE memory protection policy has been configured
+  @return EFI_SUCCESS       If OldType and NewType use the same permission attributes
+  @return other             Return value of gCpu->SetMemoryAttributes()
+
+**/
+EFI_STATUS
+EFIAPI
+ApplyMemoryProtectionPolicy (
+  IN  EFI_MEMORY_TYPE       OldType,
+  IN  EFI_MEMORY_TYPE       NewType,
+  IN  EFI_PHYSICAL_ADDRESS  Memory,
+  IN  UINT64                Length
+  );
+
 #endif
diff --git a/MdeModulePkg/Core/Dxe/DxeMain.inf b/MdeModulePkg/Core/Dxe/DxeMain.inf
index 371e91cb0d7e..30d5984f7c1f 100644
--- a/MdeModulePkg/Core/Dxe/DxeMain.inf
+++ b/MdeModulePkg/Core/Dxe/DxeMain.inf
@@ -191,6 +191,7 @@ [Pcd]
   gEfiMdeModulePkgTokenSpaceGuid.PcdMemoryProfileDriverPath                 ## CONSUMES
   gEfiMdeModulePkgTokenSpaceGuid.PcdPropertiesTableEnable                   ## CONSUMES
   gEfiMdeModulePkgTokenSpaceGuid.PcdImageProtectionPolicy                   ## CONSUMES
+  gEfiMdeModulePkgTokenSpaceGuid.PcdDxeNxMemoryProtectionPolicy             ## CONSUMES
 
 # [Hob]
 # RESOURCE_DESCRIPTOR   ## CONSUMES
diff --git a/MdeModulePkg/Core/Dxe/Mem/Page.c b/MdeModulePkg/Core/Dxe/Mem/Page.c
index bda4f6397e91..d596db7ad427 100644
--- a/MdeModulePkg/Core/Dxe/Mem/Page.c
+++ b/MdeModulePkg/Core/Dxe/Mem/Page.c
@@ -553,6 +553,9 @@ CoreAddMemoryDescriptor (
   CoreFreeMemoryMapStack ();
   CoreReleaseMemoryLock ();
 
+  ApplyMemoryProtectionPolicy (EfiMaxMemoryType, Type, Start,
+    EFI_PAGES_TO_SIZE (NumberOfPages));
+
   //
   // If Loading Module At Fixed Address feature is enabled. try to allocate memory with Runtime code & Boot time code type
   //
@@ -1344,6 +1347,8 @@ CoreAllocatePages (
       NULL
       );
     InstallMemoryAttributesTableOnMemoryAllocation (MemoryType);
+    ApplyMemoryProtectionPolicy (EfiConventionalMemory, MemoryType, *Memory,
+      EFI_PAGES_TO_SIZE (NumberOfPages));
   }
   return Status;
 }
@@ -1460,6 +1465,8 @@ CoreFreePages (
       NULL
       );
     InstallMemoryAttributesTableOnMemoryAllocation (MemoryType);
+    ApplyMemoryProtectionPolicy (MemoryType, EfiConventionalMemory, Memory,
+      EFI_PAGES_TO_SIZE (NumberOfPages));
   }
   return Status;
 }
diff --git a/MdeModulePkg/Core/Dxe/Mem/Pool.c b/MdeModulePkg/Core/Dxe/Mem/Pool.c
index ebb2fceedd80..ced64443c77d 100644
--- a/MdeModulePkg/Core/Dxe/Mem/Pool.c
+++ b/MdeModulePkg/Core/Dxe/Mem/Pool.c
@@ -310,6 +310,10 @@ CoreAllocatePoolPagesI (
   Buffer = CoreAllocatePoolPages (PoolType, NoPages, Granularity);
   CoreReleaseMemoryLock ();
 
+  if (Buffer != NULL) {
+    ApplyMemoryProtectionPolicy (EfiConventionalMemory, PoolType,
+      (EFI_PHYSICAL_ADDRESS)(UINTN)Buffer, EFI_PAGES_TO_SIZE (NoPages));
+  }
   return Buffer;
 }
 
@@ -560,6 +564,9 @@ CoreFreePoolPagesI (
   CoreAcquireMemoryLock ();
   CoreFreePoolPages (Memory, NoPages);
   CoreReleaseMemoryLock ();
+
+  ApplyMemoryProtectionPolicy (PoolType, EfiConventionalMemory,
+    (EFI_PHYSICAL_ADDRESS)(UINTN)Memory, EFI_PAGES_TO_SIZE (NoPages));
 }
 
 /**
diff --git a/MdeModulePkg/Core/Dxe/Misc/MemoryProtection.c b/MdeModulePkg/Core/Dxe/Misc/MemoryProtection.c
index 46d88463d417..172d6679857a 100644
--- a/MdeModulePkg/Core/Dxe/Misc/MemoryProtection.c
+++ b/MdeModulePkg/Core/Dxe/Misc/MemoryProtection.c
@@ -64,8 +64,16 @@ WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
 #define DO_NOT_PROTECT                         0x00000000
 #define PROTECT_IF_ALIGNED_ELSE_ALLOW          0x00000001
 
+#define MEMORY_TYPE_OS_RESERVED_MIN            0x80000000
+#define MEMORY_TYPE_OEM_RESERVED_MIN           0x70000000
+
+#define PREVIOUS_MEMORY_DESCRIPTOR(MemoryDescriptor, Size) \
+  ((EFI_MEMORY_DESCRIPTOR *)((UINT8 *)(MemoryDescriptor) - (Size)))
+
 UINT32   mImageProtectionPolicy;
 
+extern LIST_ENTRY         mGcdMemorySpaceMap;
+
 /**
   Sort code section in image record, based upon CodeSegmentBase from low to high.
 
@@ -647,6 +655,251 @@ UnprotectUefiImage (
 }
 
 /**
+  Return the EFI memory permission attribute associated with memory
+  type 'MemoryType' under the configured DXE memory protection policy.
+**/
+STATIC
+UINT64
+GetPermissionAttributeForMemoryType (
+  IN EFI_MEMORY_TYPE    MemoryType
+  )
+{
+  UINT64 TestBit;
+
+  if ((UINT32)MemoryType >= MEMORY_TYPE_OS_RESERVED_MIN) {
+    TestBit = BIT63;
+  } else if ((UINT32)MemoryType >= MEMORY_TYPE_OEM_RESERVED_MIN) {
+    TestBit = BIT62;
+  } else {
+    TestBit = LShiftU64 (1, MemoryType);
+  }
+
+  if ((PcdGet64 (PcdDxeNxMemoryProtectionPolicy) & TestBit) != 0) {
+    return EFI_MEMORY_XP;
+  } else {
+    return 0;
+  }
+}
+
+/**
+  Sort memory map entries based upon PhysicalStart, from low to high.
+
+  @param  MemoryMap              A pointer to the buffer in which firmware places
+                                 the current memory map.
+  @param  MemoryMapSize          Size, in bytes, of the MemoryMap buffer.
+  @param  DescriptorSize         Size, in bytes, of an individual EFI_MEMORY_DESCRIPTOR.
+**/
+STATIC
+VOID
+SortMemoryMap (
+  IN OUT EFI_MEMORY_DESCRIPTOR  *MemoryMap,
+  IN UINTN                      MemoryMapSize,
+  IN UINTN                      DescriptorSize
+  )
+{
+  EFI_MEMORY_DESCRIPTOR       *MemoryMapEntry;
+  EFI_MEMORY_DESCRIPTOR       *NextMemoryMapEntry;
+  EFI_MEMORY_DESCRIPTOR       *MemoryMapEnd;
+  EFI_MEMORY_DESCRIPTOR       TempMemoryMap;
+
+  MemoryMapEntry = MemoryMap;
+  NextMemoryMapEntry = NEXT_MEMORY_DESCRIPTOR (MemoryMapEntry, DescriptorSize);
+  MemoryMapEnd = (EFI_MEMORY_DESCRIPTOR *) ((UINT8 *) MemoryMap + MemoryMapSize);
+  while (MemoryMapEntry < MemoryMapEnd) {
+    while (NextMemoryMapEntry < MemoryMapEnd) {
+      if (MemoryMapEntry->PhysicalStart > NextMemoryMapEntry->PhysicalStart) {
+        CopyMem (&TempMemoryMap, MemoryMapEntry, sizeof(EFI_MEMORY_DESCRIPTOR));
+        CopyMem (MemoryMapEntry, NextMemoryMapEntry, sizeof(EFI_MEMORY_DESCRIPTOR));
+        CopyMem (NextMemoryMapEntry, &TempMemoryMap, sizeof(EFI_MEMORY_DESCRIPTOR));
+      }
+
+      NextMemoryMapEntry = NEXT_MEMORY_DESCRIPTOR (NextMemoryMapEntry, DescriptorSize);
+    }
+
+    MemoryMapEntry      = NEXT_MEMORY_DESCRIPTOR (MemoryMapEntry, DescriptorSize);
+    NextMemoryMapEntry  = NEXT_MEMORY_DESCRIPTOR (MemoryMapEntry, DescriptorSize);
+  }
+}
+
+/**
+  Merge adjacent memory map entries if they use the same memory protection policy
+
+  @param[in, out]  MemoryMap              A pointer to the buffer in which firmware places
+                                          the current memory map.
+  @param[in, out]  MemoryMapSize          A pointer to the size, in bytes, of the
+                                          MemoryMap buffer. On input, this is the size of
+                                          the current memory map.  On output,
+                                          it is the size of new memory map after merge.
+  @param[in]       DescriptorSize         Size, in bytes, of an individual EFI_MEMORY_DESCRIPTOR.
+**/
+STATIC
+VOID
+MergeMemoryMapForProtectionPolicy (
+  IN OUT EFI_MEMORY_DESCRIPTOR  *MemoryMap,
+  IN OUT UINTN                  *MemoryMapSize,
+  IN UINTN                      DescriptorSize
+  )
+{
+  EFI_MEMORY_DESCRIPTOR       *MemoryMapEntry;
+  EFI_MEMORY_DESCRIPTOR       *MemoryMapEnd;
+  UINT64                      MemoryBlockLength;
+  EFI_MEMORY_DESCRIPTOR       *NewMemoryMapEntry;
+  EFI_MEMORY_DESCRIPTOR       *NextMemoryMapEntry;
+  UINT64                      Attributes;
+
+  SortMemoryMap (MemoryMap, *MemoryMapSize, DescriptorSize);
+
+  MemoryMapEntry = MemoryMap;
+  NewMemoryMapEntry = MemoryMap;
+  MemoryMapEnd = (EFI_MEMORY_DESCRIPTOR *) ((UINT8 *) MemoryMap + *MemoryMapSize);
+  while ((UINTN)MemoryMapEntry < (UINTN)MemoryMapEnd) {
+    CopyMem (NewMemoryMapEntry, MemoryMapEntry, sizeof(EFI_MEMORY_DESCRIPTOR));
+    NextMemoryMapEntry = NEXT_MEMORY_DESCRIPTOR (MemoryMapEntry, DescriptorSize);
+
+    do {
+      MemoryBlockLength = (UINT64) (EFI_PAGES_TO_SIZE((UINTN)MemoryMapEntry->NumberOfPages));
+      Attributes = GetPermissionAttributeForMemoryType (MemoryMapEntry->Type);
+
+      if (((UINTN)NextMemoryMapEntry < (UINTN)MemoryMapEnd) &&
+          Attributes == GetPermissionAttributeForMemoryType (NextMemoryMapEntry->Type) &&
+          ((MemoryMapEntry->PhysicalStart + MemoryBlockLength) == NextMemoryMapEntry->PhysicalStart)) {
+        MemoryMapEntry->NumberOfPages += NextMemoryMapEntry->NumberOfPages;
+        if (NewMemoryMapEntry != MemoryMapEntry) {
+          NewMemoryMapEntry->NumberOfPages += NextMemoryMapEntry->NumberOfPages;
+        }
+
+        NextMemoryMapEntry = NEXT_MEMORY_DESCRIPTOR (NextMemoryMapEntry, DescriptorSize);
+        continue;
+      } else {
+        MemoryMapEntry = PREVIOUS_MEMORY_DESCRIPTOR (NextMemoryMapEntry, DescriptorSize);
+        break;
+      }
+    } while (TRUE);
+
+    MemoryMapEntry = NEXT_MEMORY_DESCRIPTOR (MemoryMapEntry, DescriptorSize);
+    NewMemoryMapEntry = NEXT_MEMORY_DESCRIPTOR (NewMemoryMapEntry, DescriptorSize);
+  }
+
+  *MemoryMapSize = (UINTN)NewMemoryMapEntry - (UINTN)MemoryMap;
+
+  return ;
+}
+
+
+/**
+  Remove exec permissions from all regions whose type is identified by
+  PcdDxeNxMemoryProtectionPolicy
+**/
+STATIC
+VOID
+InitializeDxeNxMemoryProtectionPolicy (
+  VOID
+  )
+{
+  UINTN                             MemoryMapSize;
+  UINTN                             MapKey;
+  UINTN                             DescriptorSize;
+  UINT32                            DescriptorVersion;
+  EFI_MEMORY_DESCRIPTOR             *MemoryMap;
+  EFI_MEMORY_DESCRIPTOR             *MemoryMapEntry;
+  EFI_MEMORY_DESCRIPTOR             *MemoryMapEnd;
+  EFI_STATUS                        Status;
+  UINT64                            Attributes;
+  LIST_ENTRY                        *Link;
+  EFI_GCD_MAP_ENTRY                 *Entry;
+
+  //
+  // Get the EFI memory map.
+  //
+  MemoryMapSize = 0;
+  MemoryMap     = NULL;
+
+  Status = gBS->GetMemoryMap (
+                  &MemoryMapSize,
+                  MemoryMap,
+                  &MapKey,
+                  &DescriptorSize,
+                  &DescriptorVersion
+                  );
+  ASSERT (Status == EFI_BUFFER_TOO_SMALL);
+  do {
+    MemoryMap = (EFI_MEMORY_DESCRIPTOR *) AllocatePool (MemoryMapSize);
+    ASSERT (MemoryMap != NULL);
+    Status = gBS->GetMemoryMap (
+                    &MemoryMapSize,
+                    MemoryMap,
+                    &MapKey,
+                    &DescriptorSize,
+                    &DescriptorVersion
+                    );
+    if (EFI_ERROR (Status)) {
+      FreePool (MemoryMap);
+    }
+  } while (Status == EFI_BUFFER_TOO_SMALL);
+  ASSERT_EFI_ERROR (Status);
+
+  DEBUG((DEBUG_ERROR, "%a: applying strict permissions to active memory regions\n",
+    __FUNCTION__));
+
+  MergeMemoryMapForProtectionPolicy (MemoryMap, &MemoryMapSize, DescriptorSize);
+
+  MemoryMapEntry = MemoryMap;
+  MemoryMapEnd = (EFI_MEMORY_DESCRIPTOR *) ((UINT8 *) MemoryMap + MemoryMapSize);
+  while ((UINTN) MemoryMapEntry < (UINTN) MemoryMapEnd) {
+
+    Attributes = GetPermissionAttributeForMemoryType (MemoryMapEntry->Type);
+    if (Attributes != 0) {
+      SetUefiImageMemoryAttributes (
+        MemoryMapEntry->PhysicalStart,
+        EFI_PAGES_TO_SIZE (MemoryMapEntry->NumberOfPages),
+        Attributes);
+    }
+    MemoryMapEntry = NEXT_MEMORY_DESCRIPTOR (MemoryMapEntry, DescriptorSize);
+  }
+  FreePool (MemoryMap);
+
+  //
+  // Apply the policy for RAM regions that we know are present and
+  // accessible, but have not been added to the UEFI memory map (yet).
+  //
+  if (GetPermissionAttributeForMemoryType (EfiConventionalMemory) != 0) {
+    DEBUG((DEBUG_ERROR,
+      "%a: applying strict permissions to inactive memory regions\n",
+      __FUNCTION__));
+
+    CoreAcquireGcdMemoryLock ();
+
+    Link = mGcdMemorySpaceMap.ForwardLink;
+    while (Link != &mGcdMemorySpaceMap) {
+
+      Entry = CR (Link, EFI_GCD_MAP_ENTRY, Link, EFI_GCD_MAP_SIGNATURE);
+
+      if (Entry->GcdMemoryType == EfiGcdMemoryTypeReserved &&
+          Entry->EndAddress < MAX_ADDRESS &&
+          (Entry->Capabilities & (EFI_MEMORY_PRESENT | EFI_MEMORY_INITIALIZED | EFI_MEMORY_TESTED)) ==
+            (EFI_MEMORY_PRESENT | EFI_MEMORY_INITIALIZED)) {
+
+        Attributes = GetPermissionAttributeForMemoryType (EfiConventionalMemory) |
+                     (Entry->Attributes & CACHE_ATTRIBUTE_MASK);
+
+        DEBUG ((DEBUG_INFO,
+          "Untested GCD memory space region: - 0x%016lx - 0x%016lx (0x%016lx)\n",
+          Entry->BaseAddress, Entry->EndAddress - Entry->BaseAddress + 1,
+          Attributes));
+
+        ASSERT(gCpu != NULL);
+        gCpu->SetMemoryAttributes (gCpu, Entry->BaseAddress,
+          Entry->EndAddress - Entry->BaseAddress + 1, Attributes);
+      }
+
+      Link = Link->ForwardLink;
+    }
+    CoreReleaseGcdMemoryLock ();
+  }
+}
+
+
+/**
   A notification for CPU_ARCH protocol.
 
   @param[in]  Event                 Event whose notification function is being invoked.
@@ -674,6 +927,17 @@ MemoryProtectionCpuArchProtocolNotify (
     return;
   }
 
+  //
+  // Apply the memory protection policy on non-BScode/RTcode regions.
+  //
+  if (PcdGet64 (PcdDxeNxMemoryProtectionPolicy) != 0) {
+    InitializeDxeNxMemoryProtectionPolicy ();
+  }
+
+  if (mImageProtectionPolicy == 0) {
+    return;
+  }
+
   Status = gBS->LocateHandleBuffer (
                   ByProtocol,
                   &gEfiLoadedImageProtocolGuid,
@@ -753,7 +1017,19 @@ CoreInitializeMemoryProtection (
 
   mImageProtectionPolicy = PcdGet32(PcdImageProtectionPolicy);
 
-  if (mImageProtectionPolicy != 0) {
+  //
+  // Sanity check the PcdDxeNxMemoryProtectionPolicy setting:
+  // - code regions should have no EFI_MEMORY_XP attribute
+  // - EfiConventionalMemory and EfiBootServicesData should use the
+  //   same attribute
+  //
+  ASSERT ((GetPermissionAttributeForMemoryType (EfiBootServicesCode) & EFI_MEMORY_XP) == 0);
+  ASSERT ((GetPermissionAttributeForMemoryType (EfiRuntimeServicesCode) & EFI_MEMORY_XP) == 0);
+  ASSERT ((GetPermissionAttributeForMemoryType (EfiLoaderCode) & EFI_MEMORY_XP) == 0);
+  ASSERT (GetPermissionAttributeForMemoryType (EfiBootServicesData) ==
+          GetPermissionAttributeForMemoryType (EfiConventionalMemory));
+
+  if (mImageProtectionPolicy != 0 || PcdGet64 (PcdDxeNxMemoryProtectionPolicy) != 0) {
     Status = CoreCreateEvent (
                EVT_NOTIFY_SIGNAL,
                TPL_CALLBACK,
@@ -775,3 +1051,96 @@ CoreInitializeMemoryProtection (
   }
   return ;
 }
+
+/**
+  Returns whether we are currently executing in SMM mode
+**/
+STATIC
+BOOLEAN
+IsInSmm (
+  VOID
+  )
+{
+  BOOLEAN     InSmm;
+
+  InSmm = FALSE;
+  if (gSmmBase2 != NULL) {
+    gSmmBase2->InSmm (gSmmBase2, &InSmm);
+  }
+  return InSmm;
+}
+
+/**
+  Manage memory permission attributes on a memory range, according to the
+  configured DXE memory protection policy.
+
+  @param  OldType           The old memory type of the range
+  @param  NewType           The new memory type of the range
+  @param  Memory            The base address of the range
+  @param  Length            The size of the range (in bytes)
+
+  @return EFI_SUCCESS       If we are executing in SMM mode. No permission attributes
+                            are updated in this case
+  @return EFI_SUCCESS       If the the CPU arch protocol is not installed yet
+  @return EFI_SUCCESS       If no DXE memory protection policy has been configured
+  @return EFI_SUCCESS       If OldType and NewType use the same permission attributes
+  @return other             Return value of gCpu->SetMemoryAttributes()
+
+**/
+EFI_STATUS
+EFIAPI
+ApplyMemoryProtectionPolicy (
+  IN  EFI_MEMORY_TYPE       OldType,
+  IN  EFI_MEMORY_TYPE       NewType,
+  IN  EFI_PHYSICAL_ADDRESS  Memory,
+  IN  UINT64                Length
+  )
+{
+  UINT64  OldAttributes;
+  UINT64  NewAttributes;
+
+  //
+  // The policy configured in PcdDxeNxMemoryProtectionPolicy
+  // does not apply to allocations performed in SMM mode.
+  //
+  if (IsInSmm ()) {
+    return EFI_SUCCESS;
+  }
+
+  //
+  // If the CPU arch protocol is not installed yet, we cannot manage memory
+  // permission attributes, and it is the job of the driver that installs this
+  // protocol to set the permissions on existing allocations.
+  //
+  if (gCpu == NULL) {
+    return EFI_SUCCESS;
+  }
+
+  //
+  // Check if a DXE memory protection policy has been configured
+  //
+  if (PcdGet64 (PcdDxeNxMemoryProtectionPolicy) == 0) {
+    return EFI_SUCCESS;
+  }
+
+  //
+  // Update the executable permissions according to the DXE memory
+  // protection policy, but only if
+  // - the policy is different between the old and the new type, or
+  // - this is a newly added region (OldType == EfiMaxMemoryType)
+  //
+  NewAttributes = GetPermissionAttributeForMemoryType (NewType);
+
+  if (OldType != EfiMaxMemoryType) {
+    OldAttributes = GetPermissionAttributeForMemoryType (OldType);
+    if (OldAttributes == NewAttributes) {
+      // policy is the same between OldType and NewType
+      return EFI_SUCCESS;
+    }
+  } else if (NewAttributes == 0) {
+    // newly added region of a type that does not require protection
+    return EFI_SUCCESS;
+  }
+
+  return gCpu->SetMemoryAttributes (gCpu, Memory, Length, NewAttributes);
+}
-- 
2.7.4

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