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audk/MdePkg/Library/StackCheckLib/Readme.md

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StackCheckLib

Table of Contents

Introduction and Library Instances

StackCheckLib contains the required functionality for initializing the stack cookie value, checking the value, and triggering an interrupt when a mismatch occurs. The stack cookie is a random value placed on the stack between the stack variables and the return address so that continuously writing past the stack variables will cause the stack cookie to be overwritten. Before the function returns, the stack cookie value will be checked and if there is a mismatch then StackCheckLib handles the failure.

Because UEFI doesn't use the C runtime libraries provided by MSVC, the stack check code is written in assembly within this library. GCC and Clang compilers have built-in support for stack cookie checking, so this library only handles failures.

StackCheckLibStaticInit

StackCheckLibStaticInit is an instance of StackCheckLib which does not update the stack cookie value for the module at runtime. It's always preferable to use StackCheckLibDynamicInit for improved security but there are cases where the stack cookie global cannot be written to such as in execute-in-place (XIP) modules and during the Cache-as-RAM (CAR) phase of the boot process. The stack cookie value is initialized at compile time via updates to the AutoGen process. Each module will define STACK_COOKIE_VALUE which is used for the module stack cookie value.

StackCheckLibDynamicInit

This section is future work. The below is the proposed instance.

StackCheckLibDynamicInit is an instance of StackCheckLib which updates the stack cookie value for the module at runtime. This is the preferred method for stack cookie initialization as it provides improved security. The stack cookie value is initialized at runtime by calling GetRandomNumber32() or GetRandomNumber64() to generate a random value via the platform's random number generator protocol. If the random number generator returns an error, then the value will still have the build-time randomized value to fall back on.

StackCheckLibNull

StackCheckLibNull is an instance of StackCheckLib which does not perform any stack cookie checks. This is useful for modules which will fail if stack cookie checks are inserted. Of course, this is not recommended for production code.

How Failures are Handled

When a stack cookie check fails, the StackCheckLib library will first call into a hook function StackCheckFailureHook() which only has a NULL implementation in edk2. The NULL implementation will simply print the failure address and return, but a platform can implement their own instance of this library which can perform additional actions before the system triggers an interrupt.

After StackCheckFailureHook() returns, the library will trigger an interrupt with PcdStackCookieExceptionVector.

  • On IA32 and X64 platforms, PcdStackCookieExceptionVector is used as an index into the Interrupt Descriptor Table.
  • On ARM platforms, a software interrupt (SWI) is called with the value of PcdStackCookieExceptionVector. The value can be retrieved by the handler by reading bits [7:0] of the instruction opcode which will allow the handler to determine if the interrupt was triggered by the stack cookie check. Reference: Arm A64 Instruction Set Architecture Version 2024-3
  • On AARCH64 platforms, a supervisor call (SVC) is called with the value of PcdStackCookieExceptionVector. This value can similarly be retrieved by the handler to determine if the interrupt was triggered by the stack cookie check. Reference: Arm A64 Instruction Set Architecture Version 2024-3

Debugging Stack Cookie Check Failures

Tracking down the origin of stack cookie failures can be difficult. Programmers may attempt printf debugging to determine which function has an overflow only to find that the failure disappears on the next boot. This curiosity is usually due to the black-box heuristic used by compilers to determine where to put stack cookie checks or compiler optimization features removing the failing check. The address where the failed stack cookie check occurred will be printed using DebugLib. If .map files are available, the address combined with the image offset can be used to determine the function which failed.

GNU-based compilers have the -fstack-protector-all flag to force stack cookie checks on all functions which could create a more consistent environment for debugging assuming an earlier failure doesn't mask the targeted one and the flash space can accommodate the increased size.

The Visual Studio (MSVC) toolchain has the ability to generate .cod files during compilation which interleave C and the generated assembly code. These files will contain the stack cookie checks and are useful for determining where the checks are placed. To generate these files, append /FAcs to the build options for each target module. The easiest way to do this is to update the tools_def file so the <TARGET>_<TOOLCHAIN>_<ARCH>_CC_FLAGS includes /FAcs.

Usage

edk2 updated the tools_def to add /GS to VS2022 and VS2019 IA32/X64 builds and -fstack-protector to GCC builds. This will cause stack cookie references to be inserted throughout the code. Every module should have a StackCheckLib instance linked to satisfy these references. So every module doesn't need to add StackCheckLib to the LibraryClasses section of the INF file, StackCheckLib instances should be linked as NULL in the platform DSC files. The only exception to this is MSVC built host-based unit tests as they will be compiled with the runtime libraries which already contain the stack cookie definitions and will collide with StackCheckLib. A StackCheckLibHostApplication.inf is linked by UnitTestFrameworkPkg/UnitTestFrameworkPkgHost.dsc.inc that provides the stack cookie functions for GCC HOST_APPLICATIONS but not for MSVC HOST_APPLICATIONS.

Default Stack Check Library Configuration

MdePkg/MdeLibs.dsc.inc links StackCheckLibNull for all types except SEC, HOST_APPLICATION, and USER_DEFINED in order to not break existing DSCs. SEC cannot be generically linked to because there are some SEC modules which do not link against the standard entry point libraries and thus do not get stack cookies inserted by the compiler. USER_DEFINED modules are by nature different from other modules, so we do not make any assumptions about their state.

As stated above, all HOST_APPLICATIONS will link against a HOST_APPLICATION specific implementation provided in UnitTestFrameworkPkg/UnitTestFrameworkPkgHost.dsc.inc.

To link the rest of a platform's modules to StackCheckLibNull, a platform would needs to link it for all SEC and USER_DEFINED modules. If all of the DSC's SEC and USER_DEFINED modules link against the entry point libs, it would look like the following:

[LibraryClasses.common.SEC, LibraryClasses.common.USER_DEFINED]
  NULL|MdePkg/Library/StackCheckLibNull/StackCheckLibNull.inf

If some do not, then the individual SEC/USER_DEFINED modules that do link against the entry point libs will need to be linked to StackCheckLibNull, such as below. This case is identifiable if a DSC is built and the linker complains the stack check functions are not found for a module.

UefiCpuPkg/SecCore/SecCore.inf {
    <LibraryClasses>
      NULL|MdePkg/Library/StackCheckLibNull/StackCheckLibNull.inf
  }

Custom Stack Check Library Configuration

In order to use a different instance of StackCheckLib than MdeLibs.dsc.inc provides, a DSC should add the following:

[Defines]
  DEFINE CUSTOM_STACK_CHECK_LIB = TRUE

This will cause MdeLibs.dsc.inc to not link StackCheckLibNull and rely on a DSC to link whichever version(s) of StackCheckLib it desires.

It is recommended that SEC and PEI_CORE modules use StackCheckLibNull and pre-memory modules should use StackCheckLibStaticInit. All other modules should use StackCheckLibDynamicInit.

Below is an example of how to link the StackCheckLib instances in the platform DSC file but it may need customization based on the platform's requirements:

[LibraryClasses.common.SEC, LibraryClasses.common.PEI_CORE]
  NULL|MdePkg/Library/StackCheckLibNull/StackCheckLibNull.inf

[LibraryClasses.common.PEIM]
  NULL|MdePkg/Library/StackCheckLib/StackCheckLibStaticInit.inf

[LibraryClasses.common.MM_CORE_STANDALONE, LibraryClasses.common.MM_STANDALONE, LibraryClasses.common.DXE_CORE,
LibraryClasses.common.SMM_CORE, LibraryClasses.common.DXE_SMM_DRIVER, LibraryClasses.common.DXE_DRIVER,
LibraryClasses.common.DXE_RUNTIME_DRIVER, LibraryClasses.common.DXE_SAL_DRIVER, LibraryClasses.common.UEFI_DRIVER,
LibraryClasses.common.UEFI_APPLICATION]
  NULL|MdePkg/Library/StackCheckLib/StackCheckLibDynamicInit.inf

Disable Stack Check Library

If a platform would like to disable stack cookies (say for debugging purposes), they can add the following to their DSC:

[BuildOptions]
  MSVC:*_*_*_CC_FLAGS = /GS-
  GCC:*_*_*_CC_FLAGS = -fno-stack-protector

The same build options can be put in a module's INF to only disable stack cookies for that module.

It is not recommended to disable stack cookie checking in production scenarios.