[RFC PATCH 2/3] MdeModulePkg: Enable forward edge CFI in mem attributes table

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

The memory attributes table has been extended with a flag that indicates
whether or not the OS is permitted to map the EFI runtime code regions
with strict enforcement for IBT/BTI landing pad instructions.

This is generally a property of the firmware build, and so we can permit
a platform to set this property using a PCD, and put the burden on the
platform definition to set the toolchain options accordingly.

There is one snag, however: PE/COFF does not expose whether or not the
code was generated with landing pads, so if any runtime DXE drivers were
loaded from storage other than the firmware volumes, we must assume that
setting the CFI flag in the memory attributes table is unsafe.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 MdeModulePkg/Core/Dxe/DxeMain.h                    |  2 ++
 MdeModulePkg/Core/Dxe/DxeMain.inf                  |  1 +
 MdeModulePkg/Core/Dxe/Image/Image.c                | 11 +++++++++++
 MdeModulePkg/Core/Dxe/Misc/MemoryAttributesTable.c |  7 ++++++-
 MdeModulePkg/MdeModulePkg.dec                      |  8 ++++++++
 5 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/MdeModulePkg/Core/Dxe/DxeMain.h b/MdeModulePkg/Core/Dxe/DxeMain.h
index 815a6b4bd844..427a5fc78f72 100644
--- a/MdeModulePkg/Core/Dxe/DxeMain.h
+++ b/MdeModulePkg/Core/Dxe/DxeMain.h
@@ -280,6 +280,8 @@ extern EFI_MEMORY_TYPE_INFORMATION  gMemoryTypeInformation[EfiMaxMemoryType + 1]
 extern BOOLEAN                    gDispatcherRunning;
 extern EFI_RUNTIME_ARCH_PROTOCOL  gRuntimeTemplate;
 
+extern BOOLEAN                    gMemoryAttributesTableForwardCfi;
+
 extern EFI_LOAD_FIXED_ADDRESS_CONFIGURATION_TABLE  gLoadModuleAtFixAddressConfigurationTable;
 extern BOOLEAN                                     gLoadFixedAddressCodeMemoryReady;
 //
diff --git a/MdeModulePkg/Core/Dxe/DxeMain.inf b/MdeModulePkg/Core/Dxe/DxeMain.inf
index 35d5bf0dee6f..e6ff67773a69 100644
--- a/MdeModulePkg/Core/Dxe/DxeMain.inf
+++ b/MdeModulePkg/Core/Dxe/DxeMain.inf
@@ -187,6 +187,7 @@ [Pcd]
   gEfiMdeModulePkgTokenSpaceGuid.PcdHeapGuardPropertyMask                   ## CONSUMES
   gEfiMdeModulePkgTokenSpaceGuid.PcdCpuStackGuard                           ## CONSUMES
   gEfiMdeModulePkgTokenSpaceGuid.PcdFwVolDxeMaxEncapsulationDepth           ## CONSUMES
+  gEfiMdeModulePkgTokenSpaceGuid.PcdMemoryAttributesTableForwardCfi         ## CONSUMES
 
 # [Hob]
 # RESOURCE_DESCRIPTOR   ## CONSUMES
diff --git a/MdeModulePkg/Core/Dxe/Image/Image.c b/MdeModulePkg/Core/Dxe/Image/Image.c
index 06cc6744b8c6..181fefdb6657 100644
--- a/MdeModulePkg/Core/Dxe/Image/Image.c
+++ b/MdeModulePkg/Core/Dxe/Image/Image.c
@@ -1398,6 +1398,17 @@ CoreLoadImageCommon (
     CoreNewDebugImageInfoEntry (EFI_DEBUG_IMAGE_INFO_TYPE_NORMAL, &Image->Info, Image->Handle);
   }
 
+  //
+  // If we loaded a runtime DXE driver from something other than a FV, it
+  // was not built as part of the firmware image, and so we cannot assume
+  // that it was built with IBT/BTI landing pads for forward edge control
+  // flow integrity.
+  //
+  if (!ImageIsFromFv &&
+      (Image->ImageContext.ImageCodeMemoryType == EfiRuntimeServicesCode)) {
+    gMemoryAttributesTableForwardCfi = FALSE;
+  }
+
   //
   // Reinstall loaded image protocol to fire any notifications
   //
diff --git a/MdeModulePkg/Core/Dxe/Misc/MemoryAttributesTable.c b/MdeModulePkg/Core/Dxe/Misc/MemoryAttributesTable.c
index e07921371187..cdd35ade0a8a 100644
--- a/MdeModulePkg/Core/Dxe/Misc/MemoryAttributesTable.c
+++ b/MdeModulePkg/Core/Dxe/Misc/MemoryAttributesTable.c
@@ -89,6 +89,7 @@ BOOLEAN                      mMemoryAttributesTableEnable      = TRUE;
 BOOLEAN                      mMemoryAttributesTableEndOfDxe    = FALSE;
 EFI_MEMORY_ATTRIBUTES_TABLE  *mMemoryAttributesTable           = NULL;
 BOOLEAN                      mMemoryAttributesTableReadyToBoot = FALSE;
+BOOLEAN                      gMemoryAttributesTableForwardCfi  = FixedPcdGetBool (PcdMemoryAttributesTableForwardCfi);
 
 /**
   Install MemoryAttributesTable.
@@ -182,7 +183,11 @@ InstallMemoryAttributesTable (
   MemoryAttributesTable->Version         = EFI_MEMORY_ATTRIBUTES_TABLE_VERSION;
   MemoryAttributesTable->NumberOfEntries = RuntimeEntryCount;
   MemoryAttributesTable->DescriptorSize  = (UINT32)DescriptorSize;
-  MemoryAttributesTable->Reserved        = 0;
+  if (gMemoryAttributesTableForwardCfi) {
+    MemoryAttributesTable->Flags         = EFI_MEMORY_ATTRIBUTES_FLAGS_RT_FORWARD_CONTROL_FLOW_GUARD;
+  } else {
+    MemoryAttributesTable->Flags         = 0;
+  }
   DEBUG ((DEBUG_VERBOSE, "MemoryAttributesTable:\n"));
   DEBUG ((DEBUG_VERBOSE, "  Version              - 0x%08x\n", MemoryAttributesTable->Version));
   DEBUG ((DEBUG_VERBOSE, "  NumberOfEntries      - 0x%08x\n", MemoryAttributesTable->NumberOfEntries));
diff --git a/MdeModulePkg/MdeModulePkg.dec b/MdeModulePkg/MdeModulePkg.dec
index 9605c617b7a8..d336a38655a3 100644
--- a/MdeModulePkg/MdeModulePkg.dec
+++ b/MdeModulePkg/MdeModulePkg.dec
@@ -1093,6 +1093,14 @@ [PcdsFixedAtBuild]
   # @Prompt Enable UEFI Stack Guard.
   gEfiMdeModulePkgTokenSpaceGuid.PcdCpuStackGuard|FALSE|BOOLEAN|0x30001055
 
+  ## Indicates whether the EFI memory attributes table will inform the OS that
+  #  forward edge control flow guards have been inserted into the runtime services
+  #  code regions.
+  #   TRUE  - Runtime code has forward control flow guards.<BR>
+  #   FALSE - Runtime code does not have forward control flow guards.<BR>
+  # @Prompt Enable forward control flow guards in EFI memory attributes table
+  gEfiMdeModulePkgTokenSpaceGuid.PcdMemoryAttributesTableForwardCfi|FALSE|BOOLEAN|0x30001056
+
 [PcdsFixedAtBuild, PcdsPatchableInModule]
   ## Dynamic type PCD can be registered callback function for Pcd setting action.
   #  PcdMaxPeiPcdCallBackNumberPerPcdEntry indicates the maximum number of callback function
-- 
2.39.1