OvmfPkg/QemuVideoDxe: Bypass NULL pointer detection during VBE SHIM installing

QemuVideoDxe driver will link VBE SHIM into page 0. If NULL pointer
detection is enabled, this driver will fail to load. NULL pointer detection
bypassing code is added to prevent such problem during boot.

Please note that Windows 7 will try to access VBE SHIM during boot if it's
installed, and then cause boot failure. This can be fixed by setting BIT7
of PcdNullPointerDetectionPropertyMask to disable NULL pointer detection
after EndOfDxe. As far as we know, there's no other OSs has such issue.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Ayellet Wolman <ayellet.wolman@intel.com>
Suggested-by: Ayellet Wolman <ayellet.wolman@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf

@@ -77,3 +77,4 @@
 [Pcd]
   gOptionRomPkgTokenSpaceGuid.PcdDriverSupportedEfiVersion
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfHostBridgePciDevId
+  gEfiMdeModulePkgTokenSpaceGuid.PcdNullPointerDetectionPropertyMask

OvmfPkg/QemuVideoDxe/VbeShim.c

@@ -75,6 +75,20 @@ InstallVbeShim (
   UINTN                Printed;
   VBE_MODE_INFO        *VbeModeInfo;
 
+  if ((PcdGet8 (PcdNullPointerDetectionPropertyMask) & (BIT0|BIT7)) == BIT0) {
+    DEBUG ((
+      DEBUG_WARN,
+      "%a: page 0 protected, not installing VBE shim\n",
+      __FUNCTION__
+      ));
+    DEBUG ((
+      DEBUG_WARN,
+      "%a: page 0 protection prevents Windows 7 from booting anyway\n",
+      __FUNCTION__
+      ));
+    return;
+  }
+
   Segment0 = 0x00000;
   SegmentC = 0xC0000;
   SegmentF = 0xF0000;