When setting new page table pool to RO, only disable/enable WP when
Cr0.WP has been set to 1 to fix potential PF caused by b822be1a20
(UefiCpuPkg/PiSmmCpuDxeSmm: Introduce page table pool mechanism).
With previous code, if someone want to modify the page table and
Cr0.WP has been cleared before modify page table, Cr0.WP may be set
to 1 again since new pool may be generated during this process
Then PF fault may happens.

Signed-off-by: Dun Tan <dun.tan@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
---
 UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c | 40 ++++++++++++++++++++++++++++------------
 1 file changed, 28 insertions(+), 12 deletions(-)

diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c b/UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c
index 4bb23f6920..9519f12654 100644
--- a/UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c
+++ b/UefiCpuPkg/PiSmmCpuDxeSmm/SmmCpuMemoryManagement.c
@@ -67,8 +67,10 @@ InitializePageTablePool (
   IN UINTN  PoolPages
   )
 {
-  VOID     *Buffer;
-  BOOLEAN  CetEnabled;
+  VOID      *Buffer;
+  BOOLEAN   CetEnabled;
+  BOOLEAN   WpEnabled;
+  IA32_CR0  Cr0;
 
   //
   // Always reserve at least PAGE_TABLE_POOL_UNIT_PAGES, including one page for
@@ -106,21 +108,35 @@ InitializePageTablePool (
   //
   if (mIsReadOnlyPageTable) {
     CetEnabled = ((AsmReadCr4 () & CR4_CET_ENABLE) != 0) ? TRUE : FALSE;
-    if (CetEnabled) {
+    Cr0.UintN  = AsmReadCr0 ();
+    WpEnabled  = (Cr0.Bits.WP != 0) ? TRUE : FALSE;
+    if (WpEnabled) {
+      if (CetEnabled) {
+        //
+        // CET must be disabled if WP is disabled. Disable CET before clearing CR0.WP. 
+        //
+        DisableCet ();
+      }
+
       //
-      // CET must be disabled if WP is disabled.
+      // Only disable/enable WP when Cr0.Bits.WP has been set to 1.
       //
-      DisableCet ();
+      Cr0.Bits.WP = 0;
+      AsmWriteCr0 (Cr0.UintN);
     }
 
-    AsmWriteCr0 (AsmReadCr0 () & ~CR0_WP);
     SmmSetMemoryAttributes ((EFI_PHYSICAL_ADDRESS)(UINTN)Buffer, EFI_PAGES_TO_SIZE (PoolPages), EFI_MEMORY_RO);
-    AsmWriteCr0 (AsmReadCr0 () | CR0_WP);
-    if (CetEnabled) {
-      //
-      // re-enable CET.
-      //
-      EnableCet ();
+    if (WpEnabled) {
+      Cr0.UintN   = AsmReadCr0 ();
+      Cr0.Bits.WP = 1;
+      AsmWriteCr0 (Cr0.UintN);
+
+      if (CetEnabled) {
+        //
+        // re-enable CET.
+        //
+        EnableCet ();
+      }
     }
   }
 
-- 
2.31.1.windows.1



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