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199 lines
8.5 KiB
199 lines
8.5 KiB
=========
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SafeStack
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=========
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.. contents::
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:local:
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Introduction
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============
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SafeStack is an instrumentation pass that protects programs against attacks
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based on stack buffer overflows, without introducing any measurable performance
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overhead. It works by separating the program stack into two distinct regions:
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the safe stack and the unsafe stack. The safe stack stores return addresses,
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register spills, and local variables that are always accessed in a safe way,
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while the unsafe stack stores everything else. This separation ensures that
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buffer overflows on the unsafe stack cannot be used to overwrite anything
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on the safe stack.
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SafeStack is a part of the `Code-Pointer Integrity (CPI) Project
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<http://dslab.epfl.ch/proj/cpi/>`_.
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Performance
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-----------
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The performance overhead of the SafeStack instrumentation is less than 0.1% on
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average across a variety of benchmarks (see the `Code-Pointer Integrity
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<http://dslab.epfl.ch/pubs/cpi.pdf>`__ paper for details). This is mainly
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because most small functions do not have any variables that require the unsafe
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stack and, hence, do not need unsafe stack frames to be created. The cost of
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creating unsafe stack frames for large functions is amortized by the cost of
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executing the function.
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In some cases, SafeStack actually improves the performance. Objects that end up
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being moved to the unsafe stack are usually large arrays or variables that are
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used through multiple stack frames. Moving such objects away from the safe
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stack increases the locality of frequently accessed values on the stack, such
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as register spills, return addresses, and small local variables.
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Compatibility
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-------------
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Most programs, static libraries, or individual files can be compiled
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with SafeStack as is. SafeStack requires basic runtime support, which, on most
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platforms, is implemented as a compiler-rt library that is automatically linked
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in when the program is compiled with SafeStack.
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Linking a DSO with SafeStack is not currently supported.
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Known compatibility limitations
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Certain code that relies on low-level stack manipulations requires adaption to
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work with SafeStack. One example is mark-and-sweep garbage collection
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implementations for C/C++ (e.g., Oilpan in chromium/blink), which must be
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changed to look for the live pointers on both safe and unsafe stacks.
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SafeStack supports linking statically modules that are compiled with and
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without SafeStack. An executable compiled with SafeStack can load dynamic
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libraries that are not compiled with SafeStack. At the moment, compiling
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dynamic libraries with SafeStack is not supported.
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Signal handlers that use ``sigaltstack()`` must not use the unsafe stack (see
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``__attribute__((no_sanitize("safe-stack")))`` below).
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Programs that use APIs from ``ucontext.h`` are not supported yet.
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Security
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--------
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SafeStack protects return addresses, spilled registers and local variables that
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are always accessed in a safe way by separating them in a dedicated safe stack
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region. The safe stack is automatically protected against stack-based buffer
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overflows, since it is disjoint from the unsafe stack in memory, and it itself
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is always accessed in a safe way. In the current implementation, the safe stack
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is protected against arbitrary memory write vulnerabilities though
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randomization and information hiding: the safe stack is allocated at a random
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address and the instrumentation ensures that no pointers to the safe stack are
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ever stored outside of the safe stack itself (see limitations below).
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Known security limitations
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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A complete protection against control-flow hijack attacks requires combining
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SafeStack with another mechanism that enforces the integrity of code pointers
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that are stored on the heap or the unsafe stack, such as `CPI
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<http://dslab.epfl.ch/proj/cpi/>`_, or a forward-edge control flow integrity
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mechanism that enforces correct calling conventions at indirect call sites,
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such as `IFCC <http://research.google.com/pubs/archive/42808.pdf>`_ with arity
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checks. Clang has control-flow integrity protection scheme for :doc:`C++ virtual
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calls <ControlFlowIntegrity>`, but not non-virtual indirect calls. With
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SafeStack alone, an attacker can overwrite a function pointer on the heap or
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the unsafe stack and cause a program to call arbitrary location, which in turn
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might enable stack pivoting and return-oriented programming.
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In its current implementation, SafeStack provides precise protection against
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stack-based buffer overflows, but protection against arbitrary memory write
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vulnerabilities is probabilistic and relies on randomization and information
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hiding. The randomization is currently based on system-enforced ASLR and shares
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its known security limitations. The safe stack pointer hiding is not perfect
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yet either: system library functions such as ``swapcontext``, exception
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handling mechanisms, intrinsics such as ``__builtin_frame_address``, or
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low-level bugs in runtime support could leak the safe stack pointer. In the
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future, such leaks could be detected by static or dynamic analysis tools and
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prevented by adjusting such functions to either encrypt the stack pointer when
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storing it in the heap (as already done e.g., by ``setjmp``/``longjmp``
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implementation in glibc), or store it in a safe region instead.
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The `CPI paper <http://dslab.epfl.ch/pubs/cpi.pdf>`_ describes two alternative,
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stronger safe stack protection mechanisms, that rely on software fault
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isolation, or hardware segmentation (as available on x86-32 and some x86-64
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CPUs).
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At the moment, SafeStack assumes that the compiler's implementation is correct.
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This has not been verified except through manual code inspection, and could
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always regress in the future. It's therefore desirable to have a separate
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static or dynamic binary verification tool that would check the correctness of
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the SafeStack instrumentation in final binaries.
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Usage
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=====
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To enable SafeStack, just pass ``-fsanitize=safe-stack`` flag to both compile
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and link command lines.
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Supported Platforms
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-------------------
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SafeStack was tested on Linux, FreeBSD and MacOSX.
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Low-level API
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-------------
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``__has_feature(safe_stack)``
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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In some rare cases one may need to execute different code depending on
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whether SafeStack is enabled. The macro ``__has_feature(safe_stack)`` can
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be used for this purpose.
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.. code-block:: c
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#if __has_feature(safe_stack)
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// code that builds only under SafeStack
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#endif
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``__attribute__((no_sanitize("safe-stack")))``
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Use ``__attribute__((no_sanitize("safe-stack")))`` on a function declaration
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to specify that the safe stack instrumentation should not be applied to that
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function, even if enabled globally (see ``-fsanitize=safe-stack`` flag). This
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attribute may be required for functions that make assumptions about the
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exact layout of their stack frames.
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All local variables in functions with this attribute will be stored on the safe
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stack. The safe stack remains unprotected against memory errors when accessing
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these variables, so extra care must be taken to manually ensure that all such
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accesses are safe. Furthermore, the addresses of such local variables should
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never be stored on the heap, as it would leak the location of the SafeStack.
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``__builtin___get_unsafe_stack_ptr()``
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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This builtin function returns current unsafe stack pointer of the current
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thread.
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``__builtin___get_unsafe_stack_start()``
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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This builtin function returns a pointer to the start of the unsafe stack of the
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current thread.
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Design
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======
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Please refer to the `Code-Pointer Integrity <http://dslab.epfl.ch/proj/cpi/>`__
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project page for more information about the design of the SafeStack and its
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related technologies.
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setjmp and exception handling
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-----------------------------
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The `OSDI'14 paper <http://dslab.epfl.ch/pubs/cpi.pdf>`_ mentions that
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on Linux the instrumentation pass finds calls to setjmp or functions that
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may throw an exception, and inserts required instrumentation at their call
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sites. Specifically, the instrumentation pass saves the shadow stack pointer
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on the safe stack before the call site, and restores it either after the
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call to setjmp or after an exception has been caught. This is implemented
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in the function ``SafeStack::createStackRestorePoints``.
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Publications
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------------
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`Code-Pointer Integrity <http://dslab.epfl.ch/pubs/cpi.pdf>`__.
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Volodymyr Kuznetsov, Laszlo Szekeres, Mathias Payer, George Candea, R. Sekar, Dawn Song.
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USENIX Symposium on Operating Systems Design and Implementation
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(`OSDI <https://www.usenix.org/conference/osdi14>`_), Broomfield, CO, October 2014
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