[PATCH v3 4/4] 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.

Given that the PE/COFF spec now defines a DllCharacteristicsEx flag that
indicates whether or not a loaded image is compatible with this, we can
wire this up to the flag in the memory attributes table, and set it if
all loaded runtime image are compatible with it.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
Reviewed-by: Oliver Smith-Denny <osde@linux.microsoft.com>
---
 MdeModulePkg/Core/Dxe/DxeMain.h                    |  2 ++
 MdeModulePkg/Core/Dxe/Image/Image.c                | 10 ++++++++++
 MdeModulePkg/Core/Dxe/Misc/MemoryAttributesTable.c |  8 +++++++-
 3 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/MdeModulePkg/Core/Dxe/DxeMain.h b/MdeModulePkg/Core/Dxe/DxeMain.h
index 815a6b4bd844a452..43daa037be441150 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/Image/Image.c b/MdeModulePkg/Core/Dxe/Image/Image.c
index 8704ebea9a7c88c0..9dbfb2a1fad22ced 100644
--- a/MdeModulePkg/Core/Dxe/Image/Image.c
+++ b/MdeModulePkg/Core/Dxe/Image/Image.c
@@ -1399,6 +1399,16 @@ CoreLoadImageCommon (
     CoreNewDebugImageInfoEntry (EFI_DEBUG_IMAGE_INFO_TYPE_NORMAL, &Image->Info, Image->Handle);
   }
 
+  //
+  // Check whether we are loading a runtime image that lacks support for
+  // IBT/BTI landing pads.
+  //
+  if ((Image->ImageContext.ImageCodeMemoryType == EfiRuntimeServicesCode) &&
+      ((Image->ImageContext.DllCharacteristicsEx & EFI_IMAGE_DLLCHARACTERISTICS_EX_FORWARD_CFI_COMPAT) == 0))
+  {
+    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 82fa026bceb990e5..fd127ee167e1ac9a 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  = TRUE;
 
 /**
   Install MemoryAttributesTable.
@@ -182,7 +183,12 @@ InstallMemoryAttributesTable (
   MemoryAttributesTable->Version         = EFI_MEMORY_ATTRIBUTES_TABLE_VERSION;
   MemoryAttributesTable->NumberOfEntries = RuntimeEntryCount;
   MemoryAttributesTable->DescriptorSize  = (UINT32)DescriptorSize;
-  MemoryAttributesTable->Flags           = 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));
-- 
2.39.2