[PATCH 2/2] UefiCpuPkg/CpuExceptionHandlerLib: Clear CET shadow stack token busy bit

["Sheng Wei" <w.sheng@intel.com>]

If CET shadows stack feature enabled in SMM and stack switch is enabled.
When code execute from SMM handler to SMM exception, CPU will check SMM
exception shadow stack token busy bit if it is cleared or not. 
If it is set, it will trigger #DF exception.
If it is not set, CPU will set the busy bit when enter SMM exception.
The busy bit should be cleared when return back form SMM exception to SMM
handler. Otherwise, keeping busy bit in set state will cause to trigger
#DF exception when enter SMM exception next time.
So, we use instruction SAVEPREVSSP, CLRSSBSY and RSTORSSP to clear the
shadow stack token busy bit before RETF instruction in SMM exception.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3192

Signed-off-by: Sheng Wei <w.sheng@intel.com>
Cc: Eric Dong <eric.dong@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Rahul Kumar <rahul1.kumar@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
---
 .../DxeCpuExceptionHandlerLib.inf                  |  3 +++
 .../PeiCpuExceptionHandlerLib.inf                  |  3 +++
 .../SecPeiCpuExceptionHandlerLib.inf               |  4 ++++
 .../SmmCpuExceptionHandlerLib.inf                  |  3 +++
 .../X64/Xcode5ExceptionHandlerAsm.nasm             | 28 +++++++++++++++++++++-
 .../Xcode5SecPeiCpuExceptionHandlerLib.inf         |  4 ++++
 6 files changed, 44 insertions(+), 1 deletion(-)

diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeCpuExceptionHandlerLib.inf b/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeCpuExceptionHandlerLib.inf
index 07b34c92a8..e7a81bebdb 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeCpuExceptionHandlerLib.inf
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/DxeCpuExceptionHandlerLib.inf
@@ -43,6 +43,9 @@
   gUefiCpuPkgTokenSpaceGuid.PcdCpuStackSwitchExceptionList
   gUefiCpuPkgTokenSpaceGuid.PcdCpuKnownGoodStackSize
 
+[FeaturePcd]
+  gUefiCpuPkgTokenSpaceGuid.PcdCpuSmmStackGuard                    ## CONSUMES
+
 [Packages]
   MdePkg/MdePkg.dec
   MdeModulePkg/MdeModulePkg.dec
diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/PeiCpuExceptionHandlerLib.inf b/UefiCpuPkg/Library/CpuExceptionHandlerLib/PeiCpuExceptionHandlerLib.inf
index feae7b3e06..cf5bfe4083 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/PeiCpuExceptionHandlerLib.inf
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/PeiCpuExceptionHandlerLib.inf
@@ -57,3 +57,6 @@
 [Pcd]
   gEfiMdeModulePkgTokenSpaceGuid.PcdCpuStackGuard    # CONSUMES
 
+[FeaturePcd]
+  gUefiCpuPkgTokenSpaceGuid.PcdCpuSmmStackGuard                    ## CONSUMES
+
diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/SecPeiCpuExceptionHandlerLib.inf b/UefiCpuPkg/Library/CpuExceptionHandlerLib/SecPeiCpuExceptionHandlerLib.inf
index 967cb61ba6..8ae4feae62 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/SecPeiCpuExceptionHandlerLib.inf
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/SecPeiCpuExceptionHandlerLib.inf
@@ -49,3 +49,7 @@
   LocalApicLib
   PeCoffGetEntryPointLib
   VmgExitLib
+
+[FeaturePcd]
+  gUefiCpuPkgTokenSpaceGuid.PcdCpuSmmStackGuard                    ## CONSUMES
+
diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/SmmCpuExceptionHandlerLib.inf b/UefiCpuPkg/Library/CpuExceptionHandlerLib/SmmCpuExceptionHandlerLib.inf
index 4cdb11c04e..5c3d1f7cfd 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/SmmCpuExceptionHandlerLib.inf
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/SmmCpuExceptionHandlerLib.inf
@@ -53,3 +53,6 @@
   DebugLib
   VmgExitLib
 
+[FeaturePcd]
+  gUefiCpuPkgTokenSpaceGuid.PcdCpuSmmStackGuard                    ## CONSUMES
+
diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/Xcode5ExceptionHandlerAsm.nasm b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/Xcode5ExceptionHandlerAsm.nasm
index 26cae56cc5..13fd147f11 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/Xcode5ExceptionHandlerAsm.nasm
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/X64/Xcode5ExceptionHandlerAsm.nasm
@@ -1,5 +1,5 @@
 ;------------------------------------------------------------------------------ ;
-; Copyright (c) 2012 - 2018, Intel Corporation. All rights reserved.<BR>
+; Copyright (c) 2012 - 2021, Intel Corporation. All rights reserved.<BR>
 ; SPDX-License-Identifier: BSD-2-Clause-Patent
 ;
 ; Module Name:
@@ -13,6 +13,7 @@
 ; Notes:
 ;
 ;------------------------------------------------------------------------------
+%include "Nasm.inc"
 
 ;
 ; CommonExceptionHandler()
@@ -23,6 +24,7 @@
 extern ASM_PFX(mErrorCodeFlag)    ; Error code flags for exceptions
 extern ASM_PFX(mDoFarReturnFlag)  ; Do far return flag
 extern ASM_PFX(CommonExceptionHandler)
+extern ASM_PFX(FeaturePcdGet (PcdCpuSmmStackGuard))
 
 SECTION .data
 
@@ -371,6 +373,30 @@ DoReturn:
     push    qword [rax + 0x18]       ; save EFLAGS in new location
     mov     rax, [rax]        ; restore rax
     popfq                     ; restore EFLAGS
+
+    push    rax
+    cmp     byte [dword ASM_PFX(FeaturePcdGet (PcdCpuSmmStackGuard))], 0
+    jz      CetDone
+    mov     rax, cr4
+    and     rax, 0x800000     ; check if CET is enabled
+    jz      CetDone
+    push    rbx
+    mov     rax, 0x04
+    INCSSP_RAX
+    SAVEPREVSSP
+    READSSP_RAX
+    mov     rbx, rax
+    sub     rax, 0x10
+    CLRSSBSY_RAX
+    mov     rax, rbx
+    sub     rax, 0x30
+    RSTORSSP_RAX
+    mov     rax, 0x01
+    INCSSP_RAX
+    pop     rbx
+CetDone:
+    pop     rax
+
     DB      0x48               ; prefix to composite "retq" with next "retf"
     retf                      ; far return
 DoIret:
diff --git a/UefiCpuPkg/Library/CpuExceptionHandlerLib/Xcode5SecPeiCpuExceptionHandlerLib.inf b/UefiCpuPkg/Library/CpuExceptionHandlerLib/Xcode5SecPeiCpuExceptionHandlerLib.inf
index 743c2aa766..a15f125d5b 100644
--- a/UefiCpuPkg/Library/CpuExceptionHandlerLib/Xcode5SecPeiCpuExceptionHandlerLib.inf
+++ b/UefiCpuPkg/Library/CpuExceptionHandlerLib/Xcode5SecPeiCpuExceptionHandlerLib.inf
@@ -54,3 +54,7 @@
   LocalApicLib
   PeCoffGetEntryPointLib
   VmgExitLib
+
+[FeaturePcd]
+  gUefiCpuPkgTokenSpaceGuid.PcdCpuSmmStackGuard                    ## CONSUMES
+
-- 
2.16.2.windows.1