MdeModulePkg: Fix buffer overflow when merging guard pages in MergeMe…

…moryMap (microsoft#126)

Checks that the next map entry is valid before dereferencing to merge
the guard pages. If the final entry is at the end of a page with no
valid page following it, then this can cause an access violation.

Tested on Q35 platform boot.

N/A

MdeModulePkg/Core/Dxe/Misc/MemoryAttributesTable.c

@@ -395,11 +395,17 @@ MergeMemoryMap (
   NewMemoryMapEntry = MemoryMap;
   MemoryMapEnd      = (EFI_MEMORY_DESCRIPTOR *)((UINT8 *)MemoryMap + *MemoryMapSize);
   while ((UINTN)MemoryMapEntry < (UINTN)MemoryMapEnd) {
-    CopyMem (NewMemoryMapEntry, MemoryMapEntry, sizeof (EFI_MEMORY_DESCRIPTOR));
+    CopyMem (NewMemoryMapEntry, MemoryMapEntry, DescriptorSize); // MU_CHANGE Use size parameter for consistency.
     NextMemoryMapEntry = NEXT_MEMORY_DESCRIPTOR (MemoryMapEntry, DescriptorSize);
 
     do {
-      MergeGuardPages (NewMemoryMapEntry, NextMemoryMapEntry->PhysicalStart);
+      // MU_CHANGE START Fix overflow in the MergeGuardPages call.
+      if ((UINTN)NextMemoryMapEntry < (UINTN)MemoryMapEnd) {
+        MergeGuardPages (NewMemoryMapEntry, NextMemoryMapEntry->PhysicalStart);
+      }
+
+      // MU_CHANGE END
+
       MemoryBlockLength = LShiftU64 (NewMemoryMapEntry->NumberOfPages, EFI_PAGE_SHIFT);
       if (((UINTN)NextMemoryMapEntry < (UINTN)MemoryMapEnd) &&
           (NewMemoryMapEntry->Type == NextMemoryMapEntry->Type) &&