CET in SMM

Background

Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP), have been the prevalent attack methodology for stealth exploit writers targeting vulnerabilities in programs.

Control-flow Enforcement Technology (CET) provides the following capabilities to defend against ROP/JOP style control-flow subversion attacks:

1. Shadow Stack – return address protection to defend against Return Oriented Programming.
2. Indirect branch tracking – free branch protection to defend against Jump/Call Oriented Programming.

For detail of CET, please refer to Control-flow Enforcement Technology whitepaper (https://software.intel.com/sites/default/files/managed/4d/2a/control-flow-enforcement-technology-preview.pdf)

Introduction

EDKII has enabled different technology for security, such as memory level protection (https://legacy.gitbook.com/book/edk2-docs/a-tour-beyond-bios-memory-protection-in-uefi-bios/details), or buffer overflow mitigation (https://legacy.gitbook.com/book/edk2-docs/a-tour-beyond-bios-mitigate-buffer-overflow-in-ue/details).

Now EDKII can use CET to enforce the control-flow as well. The current status is that EDKII enabled ShadowStatck in SMM.

This wiki page focuses on how to enable CET in SMM.

PCD

The PCD - PcdControlFlowEnforcementPropertyMask defined in MdePkg.dec(https://github.com/tianocore/edk2/blob/master/MdePkg/MdePkg.dec) to indicates the control flow enforcement enabling state.

  ## Indicates the control flow enforcement enabling state.
  #  If enabled, it uses control flow enforcement technology to prevent ROP or JOP.<BR><BR>
  #   BIT0 - SMM CET Shadow Stack is enabled.<BR>
  #   Other - reserved
  # @Prompt Enable control flow enforcement.
  gEfiMdePkgTokenSpaceGuid.PcdControlFlowEnforcementPropertyMask|0x0|UINT32|0x30001017

How to enable

1. A real platform may configure gEfiMdePkgTokenSpaceGuid.PcdControlFlowEnforcementPropertyMask in platform dsc to enable CET in SMM. A emulation platform might not be able to enable this PCD, because enabling CET required supervisor priviledge.

[PcdsFixedAtBuild.common]
  gEfiMdePkgTokenSpaceGuid.PcdControlFlowEnforcementPropertyMask|0x1

NOTE: If a platform wants to enable CET, it MUST enable memory protection such as ReadOnly Code Region to ReadOnly, NonExecutable Data Region.

2. If a control flow violation is detected, the system will generate an exception.

!!!! X64 Exception Type - 15(#CP - Control Protection)  CPU Apic ID - 00000000 !!!!
ExceptionData - 0000000000000001
RIP  - 000000007FDD6326, CS  - 0000000000000038, RFLAGS - 0000000000010006
RAX  - 000000007FE3EB00, RCX - 0000000000000000, RDX - 000000007FD07140
RBX  - 000000007EC7D018, RSP - 000000007FE3EB28, RBP - 000000007FE3EC79
RSI  - 000000007FFE2A01, RDI - 000000007AF3C518
R8   - 0000000000000000, R9  - 000000007FDD7179, R10 - 0000000000000002
R11  - 000000007FE3E9A0, R12 - 000000007B2614B8, R13 - 0000000000000003
R14  - 000000007FFFB3B8, R15 - 0000000076726473
DS   - 0000000000000020, ES  - 0000000000000020, FS  - 0000000000000020
GS   - 0000000000000020, SS  - 0000000000000020
CR0  - 0000000080010033, CR2 - 0000000000000000, CR3 - 000000007FE14000
CR4  - 0000000000800668, CR8 - 0000000000000000
DR0  - 0000000000000000, DR1 - 0000000000000000, DR2 - 0000000000000000
DR3  - 0000000000000000, DR6 - 00000000FFFF0FF0, DR7 - 0000000000000400
GDTR - 000000007FE12000 000000000000004F, LDTR - 0000000000000000
IDTR - 000000007FE1C000 00000000000001FF,   TR - 0000000000000040
FXSAVE_STATE - 000000007FE3E780
!!!! Find image based on IP(0x%x) c:\edk\Build\PlatformPkg\DEBUG_VS2015x86\X64\TestPkg\StackCookieTest\StackCookieTestSmm\DEBUG\StackCookieTestSmm.pdb (ImageBase=000000007FDCF000, EntryPoint=000000007FDD0310) !!!!