The mechanism behind is the same as NULL pointer detection enabled in EDK-II
core. SMM has its own page table and we have to disable page 0 again in SMM
mode.

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>
---
 UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/PageTbl.c     | 12 ++++++++++++
 UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c        | 25 ++++++++++++++++++++++++-
 UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.inf |  1 +
 UefiCpuPkg/PiSmmCpuDxeSmm/X64/PageTbl.c      | 12 ++++++++++++
 4 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/PageTbl.c b/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/PageTbl.c
index f295c2ebf2..1c9e239a34 100644
--- a/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/PageTbl.c
+++ b/UefiCpuPkg/PiSmmCpuDxeSmm/Ia32/PageTbl.c
@@ -155,6 +155,18 @@ SmiPFHandler (
     }
   }
 
+  //
+  // If NULL pointer was just accessed
+  //
+  if ((PcdGet8 (PcdNullPointerDetectionPropertyMask) & BIT1) != 0 &&
+      (PFAddress < EFI_PAGE_SIZE)) {
+    DEBUG ((DEBUG_ERROR, "!!! NULL pointer access !!!\n"));
+    DEBUG_CODE (
+      DumpModuleInfoByIp ((UINTN)SystemContext.SystemContextIa32->Rip);
+    );
+    CpuDeadLoop ();
+  }
+
   if (FeaturePcdGet (PcdCpuSmmProfileEnable)) {
     SmmProfilePFHandler (
       SystemContext.SystemContextIa32->Eip,
diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
index f086b97c30..ed2afadb21 100644
--- a/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
+++ b/UefiCpuPkg/PiSmmCpuDxeSmm/MpService.c
@@ -855,10 +855,10 @@ Gen4GPageTable (
     Pte[Index] = (Index << 21) | mAddressEncMask | IA32_PG_PS | PAGE_ATTRIBUTE_BITS;
   }
 
+  Pdpte = (UINT64*)PageTable;
   if (FeaturePcdGet (PcdCpuSmmStackGuard)) {
     Pages = (UINTN)PageTable + EFI_PAGES_TO_SIZE (5);
     GuardPage = mSmmStackArrayBase + EFI_PAGE_SIZE;
-    Pdpte = (UINT64*)PageTable;
     for (PageIndex = Low2MBoundary; PageIndex <= High2MBoundary; PageIndex += SIZE_2MB) {
       Pte = (UINT64*)(UINTN)(Pdpte[BitFieldRead32 ((UINT32)PageIndex, 30, 31)] & ~mAddressEncMask & ~(EFI_PAGE_SIZE - 1));
       Pte[BitFieldRead32 ((UINT32)PageIndex, 21, 29)] = (UINT64)Pages | mAddressEncMask | PAGE_ATTRIBUTE_BITS;
@@ -886,6 +886,29 @@ Gen4GPageTable (
     }
   }
 
+  if ((PcdGet8 (PcdNullPointerDetectionPropertyMask) & BIT1) != 0) {
+    Pte = (UINT64*)(UINTN)(Pdpte[0] & ~mAddressEncMask & ~(EFI_PAGE_SIZE - 1));
+    if ((Pte[0] & IA32_PG_PS) == 0) {
+      // 4K-page entries are already mapped. Just hide the first one anyway.
+      Pte = (UINT64*)(UINTN)(Pte[0] & ~mAddressEncMask & ~(EFI_PAGE_SIZE - 1));
+      Pte[0] &= ~1; // Hide page 0
+    } else {
+      // Create 4K-page entries
+      Pages = (UINTN)AllocatePageTableMemory (1);
+      ASSERT (Pages != 0);
+
+      Pte[0] = (UINT64)(Pages | mAddressEncMask | PAGE_ATTRIBUTE_BITS);
+
+      Pte = (UINT64*)Pages;
+      PageAddress = 0;
+      Pte[0] = PageAddress | mAddressEncMask; // Hide page 0 but present left
+      for (Index = 1; Index < EFI_PAGE_SIZE / sizeof (*Pte); Index++) {
+        PageAddress += EFI_PAGE_SIZE;
+        Pte[Index] = PageAddress | mAddressEncMask | PAGE_ATTRIBUTE_BITS;
+      }
+    }
+  }
+
   return (UINT32)(UINTN)PageTable;
 }
 
diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.inf b/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.inf
index 099792e6ce..31cb215342 100644
--- a/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.inf
+++ b/UefiCpuPkg/PiSmmCpuDxeSmm/PiSmmCpuDxeSmm.inf
@@ -159,6 +159,7 @@
   gUefiCpuPkgTokenSpaceGuid.PcdCpuSmmStaticPageTable               ## CONSUMES
   gEfiMdeModulePkgTokenSpaceGuid.PcdAcpiS3Enable                   ## CONSUMES
   gEfiMdeModulePkgTokenSpaceGuid.PcdPteMemoryEncryptionAddressOrMask    ## CONSUMES
+  gEfiMdeModulePkgTokenSpaceGuid.PcdNullPointerDetectionPropertyMask    ## CONSUMES
 
 [Depex]
   gEfiMpServiceProtocolGuid
diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/X64/PageTbl.c b/UefiCpuPkg/PiSmmCpuDxeSmm/X64/PageTbl.c
index 3dde80f9ba..f3791ce897 100644
--- a/UefiCpuPkg/PiSmmCpuDxeSmm/X64/PageTbl.c
+++ b/UefiCpuPkg/PiSmmCpuDxeSmm/X64/PageTbl.c
@@ -872,6 +872,18 @@ SmiPFHandler (
     }
   }
 
+  //
+  // If NULL pointer was just accessed
+  //
+  if ((PcdGet8 (PcdNullPointerDetectionPropertyMask) & BIT1) != 0 &&
+      (PFAddress < EFI_PAGE_SIZE)) {
+    DEBUG ((DEBUG_ERROR, "!!! NULL pointer access !!!\n"));
+    DEBUG_CODE (
+      DumpModuleInfoByIp ((UINTN)SystemContext.SystemContextX64->Rip);
+    );
+    CpuDeadLoop ();
+  }
+
   if (FeaturePcdGet (PcdCpuSmmProfileEnable)) {
     SmmProfilePFHandler (
       SystemContext.SystemContextX64->Rip,
-- 
2.14.1.windows.1

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