[PATCH v3 6/6] OvmfPkg/QemuVideoDxe: Bypass NULL pointer detection during VBE SHIM installing

[Jian J Wang <jian.j.wang@intel.com>]

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.

> According to Laszlo, remove the code disabling/enabling the NULL pointer
> detection but just ignore the installing if it's enabled

Cc: Star Zeng <star.zeng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Cc: Jordan Justen <jordan.l.justen@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>
---
 OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf |  1 +
 OvmfPkg/QemuVideoDxe/VbeShim.c        | 14 ++++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf b/OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf
index 577e07b0a8..ff68c99e96 100644
--- a/OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf
+++ b/OvmfPkg/QemuVideoDxe/QemuVideoDxe.inf
@@ -77,3 +77,4 @@
 [Pcd]
   gOptionRomPkgTokenSpaceGuid.PcdDriverSupportedEfiVersion
   gUefiOvmfPkgTokenSpaceGuid.PcdOvmfHostBridgePciDevId
+  gEfiMdeModulePkgTokenSpaceGuid.PcdNullPointerDetectionPropertyMask
diff --git a/OvmfPkg/QemuVideoDxe/VbeShim.c b/OvmfPkg/QemuVideoDxe/VbeShim.c
index e45a08e887..8ba5522cde 100644
--- a/OvmfPkg/QemuVideoDxe/VbeShim.c
+++ b/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;
-- 
2.14.1.windows.1