audk/MdePkg/Library/StackCheckLib/Readme.md

StackCheckLib Overview

Table of Contents

• StackCheckLib
  • Table of Contents
  • Introduction and Library Instances
    • StackCheckLib
    • DynamicStackCookieEntryPointLib
    • StackCheckLibNull
  • How Failures are Handled
  • Debugging Stack Cookie Check Failures
  • Usage

Introduction and Library Instances

StackCheckLib contains the required functionality for initializing the stack cookie value (based on a randomly generated value during build time), 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.

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.

The entry point libraries under MdePkg/DynamicStackCookieEntryPointLib/ update the stack cookie value at runtime 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 temporary RAM phase of the boot process. It is always preferable to use one of the dynamic stack cookie entry points when possible.

StackCheckLib

StackCheckLib provides the stack cookie checking functionality per architecture and toolchain. The currently supported pairs are IA32{GCC,MSVC}, X64{GCC, MSVC}, ARM{GCC, MSVC}, and AARCH64{GCC, MSVC}. StackCheckLib is agnostic as to whether the stack cookie was updated during build time or run time; it simply checks the cookie in the MSVC case and in both GCC and MSVC responds to stack cookie checking failures.

To add support for other architectures/toolchains, additional assembly files should be added to StackCheckLib.inf and scoped to that architecture/toolchain.

Note: Stack cookie failures generate exceptions and SEC and PEI_CORE may not have exception handlers registered. In order to safely use stack cookie checking in these phases, a platform should implement exception handlers because unhandled exceptions may lead to a hung system state. If a platform does not implement exception handlers in SEC and PEI_CORE, it is recommended to use StackCheckLibNull for these phases, except for development purposes.

DynamicStackCookieEntryPointLib

Each EntryPoint lib under MdePkg/DynamicStackCookieEntryPointLib/ is an instance of that module type entry point lib 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 in _ModuleEntryPoint by calling rdrand on x86 and RNDR on AARCH64. If the random number generator is not supported on that platform or otherwise returns an error, then the value will still have the build-time randomized value to fall back on.

Typically, dynamic cookies cannot be used for SEC, PEI_CORE, and PEIM modules, due to the lack of the ability to write to globals for many architectures. If a given platform can write to globals during these phases, it is recommended to use the provided dynamic stack cookie entry point lib for those types. Note that SEC does not have a universal entry point, so there is no dynamic stack cookie entry point lib there.

The dynamic stack cookie entry point lib is used in place of the standard entry point lib, e.g. for UefiDriverEntryPoint to have dynamic stack cookies, a platform would remove MdePkg/Library/UefiDriverEntryPoint/UefiDriverEntryPoint.inf from its DSC and instead include MdePkg/Library/DynamicStackCookieEntryPointLib/UefiDriverEntryPoint.inf.

See the Usage section for other ways of including these libraries.

Note: Standalone MM Core support for dynamic cookies for AARCH64 is currently not supported, due to the unique entry point mechanism there. This support will be added at a future date.

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 outside of SEC and PEI_CORE.

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 is added as a dependency for each entry point lib. This means that custom entry point libs need to have StackCheckLib added as a dependency. 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 and StackCheckFailureLibNull for all types.

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

Custom Stack Check Library Configuration

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

Static Stack Check Cookie Configuration

[Defines]
  DEFINE CUSTOM_STACK_CHECK_LIB = STATIC

This will cause MdeLibs.dsc.inc to not link StackCheckLibNull and instead link StackCheckLib to perform stack cookie checking on the static stack cookies, but not update any of the stack cookies at runtime.

Because edk2 does not implement exception handling for SEC and PEI_CORE, MdeLibs.dsc.inc uses StackCheckLibNull for these phases always. If a platform wishes to use StackCheckLib for these phases, it should override this in its DSC, e.g.:

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

It is recommended that a platform only do this for debugging or if they have implemented exception handlers for these phases.

Dynamic Stack Cookie Configuration

[Defines]
  DEFINE CUSTOM_STACK_CHECK_LIB = DYNAMIC

This will cause MdeLibs.dsc.inc to not link StackCheckLibNull and instead link StackCheckLib to perform stack cookie checking. It will also link the dynamic stack cookie updating versions of DxeCoreEntryPoint, StandaloneMmDriverEntryPoint, UefiApplicationEntryPoint, and UefiDriverEntryPoint.

Because edk2 does not implement exception handling for SEC and PEI_CORE, MdeLibs.dsc.inc uses StackCheckLibNull for these phases always. If a platform wishes to use StackCheckLib for these phases, it can enable static stack cookie checking, as documented in the previous section. Due to the fact that writable global variables are not supported in the SEC or PEI phases of execution, dynamic stack cookie checking is not supported here.

It is recommended that a platform only do this for debugging or if they have implemented exception handlers for these phases.

Note: StandaloneMmCoreEntryPoint is recommended to use the dynamic stack cookie if possible, but as it is not supported on AARCH64 today, it is not included in MdeLibs.dsc.inc. Platforms should include this separately, e.g.:

[LibraryClasses.X64]
  StandaloneMmCoreEntryPoint|MdePkg/Library/DynamicStackCookieEntryPointLib/StandaloneMmCoreEntryPoint.inf

Platforms then must remove any references to these entry point libs in their DSC, so that the MdeLibs.dsc.inc versions are chosen. Alternatively, for better DSC readability, a platform can directly reference the dynamic stack cookie entry points.

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.

Alternatively, a module can have the stack cookies inserted but checking disabled by including the following in a DSC:

SomePkg/SomeDirectory/SomeModule.inf {
  <LibraryClasses>
    StackCheckLib|MdePkg/Library/StackCheckLibNull/StackCheckLibNull.inf
}

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