[PATCH] IntelSiliconPkg IntelVTdDxe: Fix potential NULL pointer dereference

[Star Zeng <star.zeng@intel.com>]

The implementation of MdeModulePkg\Universal\Acpi\AcpiTableDxe reserves
first entry of RSDT/XSDT to FADT, the first entry value is 0 when FADT
is installed. So the RSDT/XSDT parsing code should check the entry value
first before checking the table signature.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
---
 IntelSiliconPkg/Feature/VTd/IntelVTdDxe/DmarAcpiTable.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/IntelSiliconPkg/Feature/VTd/IntelVTdDxe/DmarAcpiTable.c b/IntelSiliconPkg/Feature/VTd/IntelVTdDxe/DmarAcpiTable.c
index b981bcdb3aa0..342830a01fb0 100644
--- a/IntelSiliconPkg/Feature/VTd/IntelVTdDxe/DmarAcpiTable.c
+++ b/IntelSiliconPkg/Feature/VTd/IntelVTdDxe/DmarAcpiTable.c
@@ -1,6 +1,6 @@
 /** @file
 
-  Copyright (c) 2017, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2017 - 2018, Intel Corporation. All rights reserved.<BR>
   This program and the accompanying materials
   are licensed and made available under the terms and conditions of the BSD License
   which accompanies this distribution.  The full text of the license may be found at
@@ -889,7 +889,7 @@ ScanTableInRSDT (
   EntryPtr = &Rsdt->Entry;
   for (Index = 0; Index < EntryCount; Index ++, EntryPtr ++) {
     Table = (EFI_ACPI_DESCRIPTION_HEADER*)((UINTN)(*EntryPtr));
-    if (Table->Signature == Signature) {
+    if ((Table != NULL) && (Table->Signature == Signature)) {
       return Table;
     }
   }
@@ -923,7 +923,7 @@ ScanTableInXSDT (
   for (Index = 0; Index < EntryCount; Index ++) {
     CopyMem (&EntryPtr, (VOID *)(BasePtr + Index * sizeof(UINT64)), sizeof(UINT64));
     Table = (EFI_ACPI_DESCRIPTION_HEADER*)((UINTN)(EntryPtr));
-    if (Table->Signature == Signature) {
+    if ((Table != NULL) && (Table->Signature == Signature)) {
       return Table;
     }
   }
-- 
2.7.0.windows.1