[PATCH 2] UefiCpuPkg: restore strict page attributes via #DB in nonstop mode only

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

> v2: Per Laszlo's comments, repack origianl two patches into one with
>     title changed and relevant commits added

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1576

The root cause of this issue is that non-stop mode of Heap Guard and
NULL Detection set TF bit (single-step) in EFLAG unconditionally in
the common handler in CpuExceptionLib.

If PcdCpuSmmStaticPageTable is FALSE, the SMM will only create page
table for memory below 4G. If SMM tries to access memory beyond 4G,
a page fault exception will be triggered and the memory to access
will be added to page table so that SMM code can continue the access.

Because of above issue, the TF bit is set after the page fault is
handled and then fall into another DEBUG exception. Since non-stop
mode of Heap Guard and NULL Detection are not enabled, no special
DEBUG exception handler is registered. The default handler just
prints exception context and go into dead loop.

Actually EFLAGS can be changed in any standard exception handler.
There's no need to do single-step setup in assembly code. So the fix
is to move the logic to C code part of page fault exception handler
so that we can fully validate the configuration and prevent TF bit
from being set unexpectedly.

Fixes: dcc026217fdc363f55c217039fc43d344f69fed6
       16b918bbaf51211a32ae04d9d8a5ba6ccca25a6a
Test:
 - Pass special test of accessing memory beyond 4G in SMM mode
 - Boot to OS with Qemu emulator platform (Fedora27, Ubuntu18.04,
   Windows7, Windows10)

Cc: Eric Dong <eric.dong@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jian J Wang <jian.j.wang@intel.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
---
 UefiCpuPkg/CpuDxe/CpuPageTable.c                      | 11 ++++++++++-
 .../Ia32/ExceptionHandlerAsm.nasm                     |  7 -------
 .../X64/ExceptionHandlerAsm.nasm                      |  4 ----
 3 files changed, 10 insertions(+), 12 deletions(-)

diff --git a/UefiCpuPkg/CpuDxe/CpuPageTable.c b/UefiCpuPkg/CpuDxe/CpuPageTable.c
index 4bee8c7772..812537417d 100644
--- a/UefiCpuPkg/CpuDxe/CpuPageTable.c
+++ b/UefiCpuPkg/CpuDxe/CpuPageTable.c
@@ -1300,7 +1300,16 @@ PageFaultExceptionHandler (
   // Display ExceptionType, CPU information and Image information
   //
   DumpCpuContext (ExceptionType, SystemContext);
-  if (!NonStopMode) {
+  if (NonStopMode) {
+    //
+    // Set TF in EFLAGS
+    //
+    if (mPagingContext.MachineType == IMAGE_FILE_MACHINE_I386) {
+      SystemContext.SystemContextIa32->Eflags |= (UINT32)BIT8;
+    } else {
+      SystemContext.SystemContextX64->Rflags |= (UINT64)BIT8;
+    }
+  } else {
     CpuDeadLoop ();
   }
 }
diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionHandlerAsm.nasm b/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionHandlerAsm.nasm
index 6fcf5fb23f..45d6474091 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionHandlerAsm.nasm
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/Ia32/ExceptionHandlerAsm.nasm
@@ -383,13 +383,6 @@ ErrorCodeAndVectorOnStack:
     pop     dword [ebp - 4]
     mov     esp, ebp
     pop     ebp
-
-; Enable TF bit after page fault handler runs
-    cmp     dword [esp], 14       ; #PF?
-    jne     .5
-    bts     dword [esp + 16], 8   ; EFLAGS
-
-.5:
     add     esp, 8
     cmp     dword [esp - 16], 0   ; check EXCEPTION_HANDLER_CONTEXT.OldIdtHandler
     jz      DoReturn
diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
index f842af2336..7b97810d10 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/ExceptionHandlerAsm.nasm
@@ -336,10 +336,6 @@ HasErrorCode:
     pop     r15
 
     mov     rsp, rbp
-    cmp     qword [rbp + 8], 14 ; #PF?
-    jne     .1
-    bts     qword [rsp + 40], 8 ; RFLAGS.TF
-.1:
     pop     rbp
     add     rsp, 16
     cmp     qword [rsp - 32], 0  ; check EXCEPTION_HANDLER_CONTEXT.OldIdtHandler
-- 
2.17.1.windows.2