[PATCH] MdeModulePkg/Core: fix feature conflict between NX and NULL detection

[Jian J Wang <jian.j.wang@intel.com>]

If enabled, NX memory protection feature will mark all free memory as
NX (non-executable), including page 0. This will overwrite the attributes
of page 0 if NULL pointer detection feature is also enabled and then
compromise the functionality of it. The solution is skipping the NX
attributes setting to page 0 if NULL pointer detection feature is enabled.

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
---
 MdeModulePkg/Core/Dxe/Misc/MemoryProtection.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/MdeModulePkg/Core/Dxe/Misc/MemoryProtection.c b/MdeModulePkg/Core/Dxe/Misc/MemoryProtection.c
index 862593f562..150167bf66 100644
--- a/MdeModulePkg/Core/Dxe/Misc/MemoryProtection.c
+++ b/MdeModulePkg/Core/Dxe/Misc/MemoryProtection.c
@@ -845,10 +845,22 @@ InitializeDxeNxMemoryProtectionPolicy (
 
     Attributes = GetPermissionAttributeForMemoryType (MemoryMapEntry->Type);
     if (Attributes != 0) {
-      SetUefiImageMemoryAttributes (
-        MemoryMapEntry->PhysicalStart,
-        LShiftU64 (MemoryMapEntry->NumberOfPages, EFI_PAGE_SHIFT),
-        Attributes);
+      if (MemoryMapEntry->PhysicalStart == 0 &&
+          PcdGet8 (PcdNullPointerDetectionPropertyMask) != 0) {
+        //
+        // Skip page 0 if NULL pointer detection is enabled to avoid attributes
+        // overwritten.
+        //
+        SetUefiImageMemoryAttributes (
+          MemoryMapEntry->PhysicalStart + EFI_PAGE_SIZE,
+          LShiftU64 (MemoryMapEntry->NumberOfPages - 1, EFI_PAGE_SHIFT),
+          Attributes);
+      } else {
+        SetUefiImageMemoryAttributes (
+          MemoryMapEntry->PhysicalStart,
+          LShiftU64 (MemoryMapEntry->NumberOfPages, EFI_PAGE_SHIFT),
+          Attributes);
+      }
     }
     MemoryMapEntry = NEXT_MEMORY_DESCRIPTOR (MemoryMapEntry, DescriptorSize);
   }
-- 
2.14.1.windows.1