========= SafeStack ========= .. contents:: :local: 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, which includes return addresses. 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. Limitations ----------- SafeStack has not been subjected to a comprehensive security review, and there exist known weaknesses, including but not limited to the following. In its current state, the separation of local variables provides protection against stack buffer overflows, but the safe stack itself is not protected from being corrupted through a pointer dereference. The Code-Pointer Integrity paper describes two ways in which we may protect the safe stack: hardware segmentation on the 32-bit x86 architecture or information hiding on other architectures. Even with information hiding, the safe stack would merely be hidden from attackers by being somewhere in the address space. Depending on the application, the address could be predictable even on 64-bit address spaces because not all the bits are addressable, multiple threads each have their stack, the application could leak the safe stack address to memory via ``__builtin_frame_address``, bugs in the low-level runtime support etc. Safe stack leaks could be mitigated by writing and deploying a static binary analysis or a dynamic binary instrumentation based tool to find leaks. This approach doesn't prevent an attacker from "imbalancing" the safe stack by say having just one call, and doing two rets (thereby returning to an address that wasn't meant as a return address). This can be at least partially mitigated by deploying SafeStack alongside a forward control-flow integrity mechanism to ensure that calls are made using the correct calling convention. Clang does not currently implement a comprehensive forward control-flow integrity protection scheme; there exists one that protects :doc:`virtual calls ` but not non-virtual indirect calls. 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 together modules that are compiled with and without SafeStack, both statically and dynamically. One corner case that is not supported is using ``dlopen()`` to load a dynamic library that uses SafeStack into a program that is not compiled with SafeStack but uses threads. 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. Usage ===== To enable SafeStack, just pass ``-fsanitize=safe-stack`` flag to both compile and link command lines. Supported Platforms ------------------- SafeStack was tested on Linux, 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. .. code-block:: c #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. Care should be taken when using this attribute. The return address is not protected against stack buffer overflows, and it is easier to leak the address of the safe stack to memory by taking the address of a local variable. ``__builtin___get_unsafe_stack_ptr()`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This builtin function returns current unsafe stack pointer of the current thread. ``__builtin___get_unsafe_stack_start()`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This builtin function returns a pointer to the start of the unsafe stack of the current thread. Design ====== Please refer to `http://dslab.epfl.ch/proj/cpi/ `_ for more information about the design of the SafeStack and its related technologies. 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