* [PATCH v3 1/1] UefiCpuPkg/CpuExceptionHandlerLib: Clear CET shadow stack token busy bit
2021-02-07 7:07 [PATCH v3 0/1] Fix CET shadow stack token busy bit clear issue Sheng Wei
@ 2021-02-07 7:07 ` Sheng Wei
0 siblings, 0 replies; 2+ messages in thread
From: Sheng Wei @ 2021-02-07 7:07 UTC (permalink / raw)
To: devel; +Cc: Eric Dong, Ray Ni, Laszlo Ersek, Rahul Kumar, Jiewen Yao,
Roger Feng
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.
So, the busy bit should be cleared when return back form SMM exception to
SMM handler. Otherwise, keeping busy bit 1 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>
Cc: Roger Feng <roger.feng@intel.com>
---
.../DxeCpuExceptionHandlerLib.inf | 3 ++
.../PeiCpuExceptionHandlerLib.inf | 3 ++
.../SecPeiCpuExceptionHandlerLib.inf | 4 ++
.../SmmCpuExceptionHandlerLib.inf | 3 ++
.../X64/Xcode5ExceptionHandlerAsm.nasm | 48 ++++++++++++++++++++--
.../Xcode5SecPeiCpuExceptionHandlerLib.inf | 4 ++
UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmFuncsArch.c | 15 ++++++-
7 files changed, 76 insertions(+), 4 deletions(-)
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..05a802a633 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
@@ -371,8 +373,48 @@ DoReturn:
push qword [rax + 0x18] ; save EFLAGS in new location
mov rax, [rax] ; restore rax
popfq ; restore EFLAGS
- DB 0x48 ; prefix to composite "retq" with next "retf"
- retf ; far return
+
+ ; The follow algorithm is used for clear shadow stack token busy bit.
+ ; The comment is based on the sample shadow stack.
+ ; The sample shadow stack layout :
+ ; Address | Context
+ ; +-------------------------+
+ ; 0xFD0 | FREE | it is 0xFD8|0x02|(LMA & CS.L), after SAVEPREVSSP.
+ ; +-------------------------+
+ ; 0xFD8 | Prev SSP |
+ ; +-------------------------+
+ ; 0xFE0 | RIP |
+ ; +-------------------------+
+ ; 0xFE8 | CS |
+ ; +-------------------------+
+ ; 0xFF0 | 0xFF0 | BUSY | BUSY flag cleared after CLRSSBSY
+ ; +-------------------------+
+ ; 0xFF8 | 0xFD8|0x02|(LMA & CS.L) |
+ ; +-------------------------+
+ ; Instructions for Intel Control Flow Enforcement Technology (CET) are supported since NASM version 2.15.01.
+ push rax ; SSP should be 0xFD8 at this point
+ cmp byte [dword ASM_PFX(FeaturePcdGet (PcdCpuSmmStackGuard))], 0
+ jz CetDone
+ mov rax, cr4
+ and rax, 0x800000 ; check if CET is enabled
+ jz CetDone
+ mov rax, 0x04 ; advance past cs:lip:prevssp;supervisor shadow stack token
+ INCSSP_RAX ; After this SSP should be 0xFF8
+ DB 0xF3, 0x0F, 0x01, 0xEA ; SAVEPREVSSP ; now the shadow stack restore token will be created at 0xFD0
+ READSSP_RAX ; Read new SSP, SSP should be 0x1000
+ push rax
+ sub rax, 0x10
+ DB 0xF3, 0x0F, 0xAE, 0x30 ; CLRSSBSY RAX ; Clear token at 0xFF0 ; SSP should be 0 after this
+ sub rax, 0x20
+ DB 0xF3, 0x0F, 0x01, 0x28 ; RSTORSSP RAX ; Restore to token at 0xFD0, new SSP will be 0xFD0
+ pop rax
+ mov rax, 0x01 ; Pop off the new save token created
+ INCSSP_RAX ; SSP should be 0xFD8 now
+CetDone:
+ pop rax ; restore rax
+
+ DB 0x48 ; prefix to composite "retq" with next "retf"
+ retf ; far return
DoIret:
iretq
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
+
diff --git a/UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmFuncsArch.c b/UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmFuncsArch.c
index 28f8e8e133..7ef3b1d488 100644
--- a/UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmFuncsArch.c
+++ b/UefiCpuPkg/PiSmmCpuDxeSmm/X64/SmmFuncsArch.c
@@ -173,6 +173,7 @@ InitShadowStack (
{
UINTN SmmShadowStackSize;
UINT64 *InterruptSspTable;
+ UINT32 InterruptSsp;
if ((PcdGet32 (PcdControlFlowEnforcementPropertyMask) != 0) && mCetSupported) {
SmmShadowStackSize = EFI_PAGES_TO_SIZE (EFI_SIZE_TO_PAGES (PcdGet32 (PcdCpuSmmShadowStackSize)));
@@ -191,7 +192,19 @@ InitShadowStack (
ASSERT (mSmmInterruptSspTables != 0);
DEBUG ((DEBUG_INFO, "mSmmInterruptSspTables - 0x%x\n", mSmmInterruptSspTables));
}
- mCetInterruptSsp = (UINT32)((UINTN)ShadowStack + EFI_PAGES_TO_SIZE(1) - sizeof(UINT64));
+
+ //
+ // The highest address on the stack (0xFF8) is a save-previous-ssp token pointing to a location that is 40 bytes away - 0xFD0.
+ // The supervisor shadow stack token is just above it at address 0xFF0. This is where the interrupt SSP table points.
+ // So when an interrupt of exception occurs, we can use SAVESSP/RESTORESSP/CLEARSSBUSY for the supervisor shadow stack,
+ // due to the reason the RETF in SMM exception handler cannot clear the BUSY flag with same CPL.
+ // (only IRET or RETF with different CPL can clear BUSY flag)
+ // Please refer to UefiCpuPkg/Library/CpuExceptionHandlerLib/X64 for the full stack frame at runtime.
+ //
+ InterruptSsp = (UINT32)((UINTN)ShadowStack + EFI_PAGES_TO_SIZE(1) - sizeof(UINT64));
+ *(UINT32 *)(UINTN)InterruptSsp = (InterruptSsp - sizeof(UINT64) * 4) | 0x2;
+ mCetInterruptSsp = InterruptSsp - sizeof(UINT64);
+
mCetInterruptSspTable = (UINT32)(UINTN)(mSmmInterruptSspTables + sizeof(UINT64) * 8 * CpuIndex);
InterruptSspTable = (UINT64 *)(UINTN)mCetInterruptSspTable;
InterruptSspTable[1] = mCetInterruptSsp;
--
2.16.2.windows.1
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