ShellPkg: acpiview: Prevent infinite loop if structure length is 0

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2534

Extend validation of ACPI structure lengths which are read from the
ACPI table being parsed. Additionally check if the structure 'Length'
field value is positive. If not, stop parsing the faulting table.

Some ACPI tables define internal structures of variable size. The
'Length' field inside the substructure is used to update a pointer used
for table traversal. If the byte-length of the structure is equal to 0,
acpiview can enter an infinite loop. This condition can occur if, for
example, the zero-allocated ACPI table buffer is not fully populated.
This is typically a bug on the ACPI table writer side.

In short, this method helps acpiview recover gracefully from a
zero-valued ACPI structure length.

Signed-off-by: Krzysztof Koch <krzysztof.koch@arm.com>
Reviewed-by: Sami Mujawar <sami.mujawar@arm.com>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>

ShellPkg/Library/UefiShellAcpiViewCommandLib/Parsers/Dbg2/Dbg2Parser.c

@@ -1,7 +1,7 @@
 /** @file
   DBG2 table parser
 
-  Copyright (c) 2016 - 2019, ARM Limited. All rights reserved.
+  Copyright (c) 2016 - 2020, ARM Limited. All rights reserved.
   SPDX-License-Identifier: BSD-2-Clause-Patent
 
   @par Reference(s):
@@ -282,15 +282,16 @@ ParseAcpiDbg2 (
       return;
     }
 
-    // Make sure the Debug Device Information structure lies inside the table.
-    if ((Offset + *DbgDevInfoLen) > AcpiTableLength) {
+    // Validate Debug Device Information Structure length
+    if ((*DbgDevInfoLen == 0) ||
+        ((Offset + (*DbgDevInfoLen)) > AcpiTableLength)) {
       IncrementErrorCount ();
       Print (
-        L"ERROR: Invalid Debug Device Information structure length. " \
-          L"DbgDevInfoLen = %d. RemainingTableBufferLength = %d. " \
-          L"DBG2 parsing aborted.\n",
+        L"ERROR: Invalid Debug Device Information Structure length. " \
+          L"Length = %d. Offset = %d. AcpiTableLength = %d.\n",
         *DbgDevInfoLen,
-        AcpiTableLength - Offset
+        Offset,
+        AcpiTableLength
         );
       return;
     }