Layers: Better checks on PAE page tables

This checks that the very top level table points to the next four pages,
as we'd expected in general. This relies on the same assumptions as the
existing PAE detection did, ie that the PAE page_map maps the next four
pages immediately.

Previously we didn't check that the top page was valid, once we found
the self-referential pointer.  This adds in an appropriate check to
reduce false positives.

Closes #631.

volatility3/framework/automagic/windows.py

@@ -28,9 +28,9 @@ The self-referential indices for older versions of windows are listed below:
 """
 import logging
 import struct
-from typing import Generator, List, Optional, Tuple, Type, Iterable
+from typing import Generator, Iterable, List, Optional, Tuple, Type
 
-from volatility3.framework import interfaces, layers, constants
+from volatility3.framework import constants, interfaces, layers
 from volatility3.framework.configuration import requirements
 from volatility3.framework.layers import intel
 
@@ -116,10 +116,27 @@ class DtbSelfRefPae(DtbSelfReferential):
                          mask = 0x3FFFFFFFFFF000,
                          reserved_bits = 0x0)
 
-    def __call__(self, *args, **kwargs):
-        dtb = super().__call__(*args, **kwargs)
+    @staticmethod
+    def _and_bytes(abytes, bbytes):
+        return bytes([a & b for a, b in zip(abytes[::-1], bbytes[::-1])][::-1])
+
+    def __call__(self, data: bytes, data_offset: int, page_offset: int) -> Optional[Tuple[int, int]]:
+        dtb = super().__call__(data, data_offset, page_offset)
         if dtb:
-            return dtb[0] - 0x4000, dtb[1]
+            # Find the top page
+            top_pae_page = dtb[0] - 0x4000
+            # The top page should map to the next four pages after it
+            # Build what we expect the page table to be
+            expected_table = b''.join([struct.pack(self.ptr_struct, top_pae_page + (i * 0x1000)) for i in range(1, 5)])
+            # Mask off the page bits of top level page map
+            page_table_mask = b"\x00\xf0\xff\xff\xff\xff\xff\xff" * 4
+            page_table = data[top_pae_page - data_offset: top_pae_page - data_offset + (4 * self.ptr_size)]
+            # Compare them
+            anded_bytes = self._and_bytes(page_table, page_table_mask)
+            if (anded_bytes == expected_table):
+                return top_pae_page, dtb[1]
+            # Return None since the dtb value *isn't* None
+            return None
         return dtb