SafeStack — Clang 8 documentation

# Clang 8 documentation

## SafeStack

- Introduction
  - Performance
  - Compatibility
    - Known compatibility limitations
  - Security
    - Known security limitations
- Usage
  - Supported Platforms
  - Low-level API
    - `__has_feature(safe_stack)`
    - `__attribute__((no_sanitize("safe-stack")))`
    - `__builtin___get_unsafe_stack_ptr()`
    - `__builtin___get_unsafe_stack_bottom()`
    - `__builtin___get_unsafe_stack_top()`
    - `__builtin___get_unsafe_stack_start()`
- Design
  - setjmp and exception handling
  - Publications

## Introduction

SafeStack is an instrumentation pass that protects programs against attacks based on stack buffer overflows, without introducing any measurable performance overhead. It works by separating the program stack into two distinct regions: the safe stack and the unsafe stack. The safe stack stores return addresses, register spills, and local variables that are always accessed in a safe way, while the unsafe stack stores everything else. This separation ensures that buffer overflows on the unsafe stack cannot be used to overwrite anything on the safe stack.

SafeStack is a part of the Code-Pointer Integrity (CPI) Project.

### Performance

The performance overhead of the SafeStack instrumentation is less than 0.1% on average across a variety of benchmarks (see the Code-Pointer Integrity paper for details). This is mainly because most small functions do not have any variables that require the unsafe stack and, hence, do not need unsafe stack frames to be created. The cost of creating unsafe stack frames for large functions is amortized by the cost of executing the function.

In some cases, SafeStack actually improves the performance. Objects that end up being moved to the unsafe stack are usually large arrays or variables that are used through multiple stack frames. Moving such objects away from the safe stack increases the locality of frequently accessed values on the stack, such as register spills, return addresses, and small local variables.

### Compatibility

Most programs, static libraries, or individual files can be compiled with SafeStack as is. SafeStack requires basic runtime support, which, on most platforms, is implemented as a compiler-rt library that is automatically linked in when the program is compiled with SafeStack.

Linking a DSO with SafeStack is not currently supported.

#### Known compatibility limitations

Certain code that relies on low-level stack manipulations requires adaption to work with SafeStack. One example is mark-and-sweep garbage collection implementations for C/C++ (e.g., Oilpan in chromium/blink), which must be changed to look for the live pointers on both safe and unsafe stacks.

SafeStack supports linking statically modules that are compiled with and without SafeStack. An executable compiled with SafeStack can load dynamic libraries that are not compiled with SafeStack. At the moment, compiling dynamic libraries with SafeStack is not supported.

Signal handlers that use `sigaltstack()` must not use the unsafe stack (see `__attribute__((no_sanitize("safe-stack")))` below).

Programs that use APIs from `ucontext.h` are not supported yet.

### Security

SafeStack protects return addresses, spilled registers and local variables that are always accessed in a safe way by separating them in a dedicated safe stack region. The safe stack is automatically protected against stack-based buffer overflows, since it is disjoint from the unsafe stack in memory, and it itself is always accessed in a safe way. In the current implementation, the safe stack is protected against arbitrary memory write vulnerabilities though randomization and information hiding: the safe stack is allocated at a random address and the instrumentation ensures that no pointers to the safe stack are ever stored outside of the safe stack itself (see limitations below).

#### Known security limitations

A complete protection against control-flow hijack attacks requires combining SafeStack with another mechanism that enforces the integrity of code pointers that are stored on the heap or the unsafe stack, such as CPI, or a forward-edge control flow integrity mechanism that enforces correct calling conventions at indirect call sites, such as IFCC with arity checks. Clang has control-flow integrity protection scheme for C++ virtual calls, but not non-virtual indirect calls. With SafeStack alone, an attacker can overwrite a function pointer on the heap or the unsafe stack and cause a program to call arbitrary location, which in turn might enable stack pivoting and return-oriented programming.

In its current implementation, SafeStack provides precise protection against stack-based buffer overflows, but protection against arbitrary memory write vulnerabilities is probabilistic and relies on randomization and information hiding. The randomization is currently based on system-enforced ASLR and shares its known security limitations. The safe stack pointer hiding is not perfect yet either: system library functions such as `swapcontext`, exception handling mechanisms, intrinsics such as `__builtin_frame_address`, or low-level bugs in runtime support could leak the safe stack pointer. In the future, such leaks could be detected by static or dynamic analysis tools and prevented by adjusting such functions to either encrypt the stack pointer when storing it in the heap (as already done e.g., by `setjmp`/`longjmp` implementation in glibc), or store it in a safe region instead.

The CPI paper describes two alternative, stronger safe stack protection mechanisms, that rely on software fault isolation, or hardware segmentation (as available on x86-32 and some x86-64 CPUs).

At the moment, SafeStack assumes that the compiler’s implementation is correct. This has not been verified except through manual code inspection, and could always regress in the future. It’s therefore desirable to have a separate static or dynamic binary verification tool that would check the correctness of the SafeStack instrumentation in final binaries.

## Usage

To enable SafeStack, just pass `-fsanitize=safe-stack` flag to both compile and link command lines.

### Supported Platforms

SafeStack was tested on Linux, NetBSD, FreeBSD and MacOSX.

### Low-level API

#### `__has_feature(safe_stack)`

In some rare cases one may need to execute different code depending on whether SafeStack is enabled. The macro `__has_feature(safe_stack)` can be used for this purpose.

    #if __has_feature(safe_stack)
    // code that builds only under SafeStack
    #endif

#### `__attribute__((no_sanitize("safe-stack")))`

Use `__attribute__((no_sanitize("safe-stack")))` on a function declaration to specify that the safe stack instrumentation should not be applied to that function, even if enabled globally (see `-fsanitize=safe-stack` flag). This attribute may be required for functions that make assumptions about the exact layout of their stack frames.

All local variables in functions with this attribute will be stored on the safe stack. The safe stack remains unprotected against memory errors when accessing these variables, so extra care must be taken to manually ensure that all such accesses are safe. Furthermore, the addresses of such local variables should never be stored on the heap, as it would leak the location of the SafeStack.

#### `__builtin___get_unsafe_stack_ptr()`

This builtin function returns current unsafe stack pointer of the current thread.

#### `__builtin___get_unsafe_stack_bottom()`

This builtin function returns a pointer to the bottom of the unsafe stack of the current thread.

#### `__builtin___get_unsafe_stack_top()`

This builtin function returns a pointer to the top of the unsafe stack of the current thread.

#### `__builtin___get_unsafe_stack_start()`

Deprecated: This builtin function is an alias for `__builtin___get_unsafe_stack_bottom()`.

## Design

Please refer to the Code-Pointer Integrity project page for more information about the design of the SafeStack and its related technologies.

### setjmp and exception handling

The OSDI‘14 paper mentions that on Linux the instrumentation pass finds calls to setjmp or functions that may throw an exception, and inserts required instrumentation at their call sites. Specifically, the instrumentation pass saves the shadow stack pointer on the safe stack before the call site, and restores it either after the call to setjmp or after an exception has been caught. This is implemented in the function `SafeStack::createStackRestorePoints`.

### Publications

Code-Pointer Integrity. Volodymyr Kuznetsov, Laszlo Szekeres, Mathias Payer, George Candea, R. Sekar, Dawn Song. USENIX Symposium on Operating Systems Design and Implementation (OSDI), Broomfield, CO, October 2014