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docs: Update the ShadowCallStack documentation. 

- Remove most of the discussion of the x86_64 implementation;
  link to an older version of the documentation for details of
  that implementation.
- Add description of the compatibility and security issues discovered
  during the development of the aarch64 implementation for Android.

Differential Revision: https://reviews.llvm.org/D58105

llvm-svn: 353890
This commit is contained in:
Peter Collingbourne 2019-02-12 22:45:23 +00:00
parent acb231c8d8
commit 27aa8b62d3
1 changed files with 113 additions and 93 deletions
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206
clang/docs/ShadowCallStack.rst
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@ -8,28 +8,45 @@ ShadowCallStack
Introduction
============
ShadowCallStack is an **experimental** instrumentation pass, currently only
implemented for x86_64 and aarch64, that protects programs against return
address overwrites (e.g. stack buffer overflows.) It works by saving a
function's return address to a separately allocated 'shadow call stack'
in the function prolog and checking the return address on the stack against
the shadow call stack in the function epilog.
ShadowCallStack is an instrumentation pass, currently only implemented for
aarch64 and x86_64, that protects programs against return address overwrites
(e.g. stack buffer overflows.) It works by saving a function's return address
to a separately allocated 'shadow call stack' in the function prolog in
non-leaf functions and loading the return address from the shadow call stack
in the function epilog. The return address is also stored on the regular stack
for compatibility with unwinders, but is otherwise unused.
The aarch64 implementation is considered production ready, and
an `implementation of the runtime`_ has been added to Android's libc
(bionic). The x86_64 implementation was evaluated using Chromium and was
found to have critical performance and security deficiencies, and may be
removed in a future release of the compiler. This document only describes
the aarch64 implementation; details on the x86_64 implementation are found
in the `Clang 7.0.1 documentation`_.
.. _`implementation of the runtime`: https://android.googlesource.com/platform/bionic/+/808d176e7e0dd727c7f929622ec017f6e065c582/libc/bionic/pthread_create.cpp#128
.. _`Clang 7.0.1 documentation`: https://releases.llvm.org/7.0.1/tools/clang/docs/ShadowCallStack.html
Comparison
----------
To optimize for memory consumption and cache locality, the shadow call stack
stores an index followed by an array of return addresses. This is in contrast
to other schemes, like :doc:`SafeStack`, that mirror the entire stack and
trade-off consuming more memory for shorter function prologs and epilogs with
fewer memory accesses. Similarly, `Return Flow Guard`_ consumes more memory with
shorter function prologs and epilogs than ShadowCallStack but suffers from the
same race conditions (see `Security`_). Intel `Control-flow Enforcement Technology`_
(CET) is a proposed hardware extension that would add native support to
use a shadow stack to store/check return addresses at call/return time. It
would not suffer from race conditions at calls and returns and not incur the
overhead of function instrumentation, but it does require operating system
support.
To optimize for memory consumption and cache locality, the shadow call
stack stores only an array of return addresses. This is in contrast to other
schemes, like :doc:`SafeStack`, that mirror the entire stack and trade-off
consuming more memory for shorter function prologs and epilogs with fewer
memory accesses.
`Return Flow Guard`_ is a pure software implementation of shadow call stacks
on x86_64. It is similar to the ShadowCallStack x86_64 implementation but
trades off higher memory usage for a shorter prologue and epilogue. Like
x86_64 ShadowCallStack, it is inherently racy due to the architecture's use
of the stack for calls and returns.
Intel `Control-flow Enforcement Technology`_ (CET) is a proposed hardware
extension that would add native support to use a shadow stack to store/check
return addresses at call/return time. Being a hardware implementation, it
would not suffer from race conditions and would not incur the overhead of
function instrumentation, but it does require operating system support.
.. _`Return Flow Guard`: https://xlab.tencent.com/en/2016/11/02/return-flow-guard/
.. _`Control-flow Enforcement Technology`: https://software.intel.com/sites/default/files/managed/4d/2a/control-flow-enforcement-technology-preview.pdf
@ -37,57 +54,96 @@ support.
Compatibility
-------------
ShadowCallStack currently only supports x86_64 and aarch64. A runtime is not
currently provided in compiler-rt so one must be provided by the compiled
application.
A runtime is not provided in compiler-rt so one must be provided by the
compiled application or the operating system. Integrating the runtime into
the operating system should be preferred since otherwise all thread creation
and destruction would need to be intercepted by the application.
On aarch64, the instrumentation makes use of the platform register ``x18``.
On some platforms, ``x18`` is reserved, and on others, it is designated as
a scratch register. This generally means that any code that may run on the
same thread as code compiled with ShadowCallStack must either target one
of the platforms whose ABI reserves ``x18`` (currently Darwin, Fuchsia and
Windows) or be compiled with the flag ``-ffixed-x18``.
The instrumentation makes use of the platform register ``x18``. On some
platforms, ``x18`` is reserved, and on others, it is designated as a scratch
register. This generally means that any code that may run on the same thread
as code compiled with ShadowCallStack must either target one of the platforms
whose ABI reserves ``x18`` (currently Android, Darwin, Fuchsia and Windows)
or be compiled with the flag ``-ffixed-x18``. If absolutely necessary, code
compiled without ``-ffixed-x18`` may be run on the same thread as code that
uses ShadowCallStack by saving the register value temporarily on the stack
(`example in Android`_) but this should be done with care since it risks
leaking the shadow call stack address.
.. _`example in Android`: https://android-review.googlesource.com/c/platform/frameworks/base/+/803717
Because of the use of register ``x18``, the ShadowCallStack feature is
incompatible with any other feature that may use ``x18``. However, there
is no inherent reason why ShadowCallStack needs to use register ``x18``
specifically; in principle, a platform could choose to reserve and use another
register for ShadowCallStack, but this would be incompatible with the AAPCS64.
Special unwind information is required on functions that are compiled
with ShadowCallStack and that may be unwound, i.e. functions compiled with
``-fexceptions`` (which is the default in C++). Some unwinders (such as the
libgcc 4.9 unwinder) do not understand this unwind info and will segfault
when encountering it. LLVM libunwind processes this unwind info correctly,
however. This means that if exceptions are used together with ShadowCallStack,
the program must use a compatible unwinder.
Security
========
ShadowCallStack is intended to be a stronger alternative to
``-fstack-protector``. It protects from non-linear overflows and arbitrary
memory writes to the return address slot; however, similarly to
``-fstack-protector`` this protection suffers from race conditions because of
the call-return semantics on x86_64. There is a short race between the call
instruction and the first instruction in the function that reads the return
address where an attacker could overwrite the return address and bypass
ShadowCallStack. Similarly, there is a time-of-check-to-time-of-use race in the
function epilog where an attacker could overwrite the return address after it
has been checked and before it has been returned to. Modifying the call-return
semantics to fix this on x86_64 would incur an unacceptable performance overhead
due to return branch prediction.
memory writes to the return address slot.
The instrumentation makes use of the ``gs`` segment register on x86_64,
or the ``x18`` register on aarch64, to reference the shadow call stack
meaning that references to the shadow call stack do not have to be stored in
memory. This makes it possible to implement a runtime that avoids exposing
the address of the shadow call stack to attackers that can read arbitrary
memory. However, attackers could still try to exploit side channels exposed
by the operating system `[1]`_ `[2]`_ or processor `[3]`_ to discover the
address of the shadow call stack.
The instrumentation makes use of the ``x18`` register to reference the shadow
call stack, meaning that references to the shadow call stack do not have
to be stored in memory. This makes it possible to implement a runtime that
avoids exposing the address of the shadow call stack to attackers that can
read arbitrary memory. However, attackers could still try to exploit side
channels exposed by the operating system `[1]`_ `[2]`_ or processor `[3]`_
to discover the address of the shadow call stack.
.. _`[1]`: https://eyalitkin.wordpress.com/2017/09/01/cartography-lighting-up-the-shadows/
.. _`[2]`: https://www.blackhat.com/docs/eu-16/materials/eu-16-Goktas-Bypassing-Clangs-SafeStack.pdf
.. _`[3]`: https://www.vusec.net/projects/anc/
On x86_64, leaf functions are optimized to store the return address in a
free register and avoid writing to the shadow call stack if a register is
available. Very short leaf functions are uninstrumented if their execution
is judged to be shorter than the race condition window intrinsic to the
instrumentation.
Unless care is taken when allocating the shadow call stack, it may be
possible for an attacker to guess its address using the addresses of
other allocations. Therefore, the address should be chosen to make this
difficult. One way to do this is to allocate a large guard region without
read/write permissions, randomly select a small region within it to be
used as the address of the shadow call stack and mark only that region as
read/write. This also mitigates somewhat against processor side channels.
The intent is that the Android runtime `will do this`_, but the platform will
first need to be `changed`_ to avoid using ``setrlimit(RLIMIT_AS)`` to limit
memory allocations in certain processes, as this also limits the number of
guard regions that can be allocated.
On aarch64, the architecture's call and return instructions (``bl`` and
``ret``) operate on a register rather than the stack, which means that
leaf functions are generally protected from return address overwrites even
without ShadowCallStack. It also means that ShadowCallStack on aarch64 is not
vulnerable to the same types of time-of-check-to-time-of-use races as x86_64.
.. _`will do this`: https://android-review.googlesource.com/c/platform/bionic/+/891622
.. _`changed`: https://android-review.googlesource.com/c/platform/frameworks/av/+/837745
The runtime will need the address of the shadow call stack in order to
deallocate it when destroying the thread. If the entire program is compiled
with ``-ffixed-x18``, this is trivial: the address can be derived from the
value stored in ``x18`` (e.g. by masking out the lower bits). If a guard
region is used, the address of the start of the guard region could then be
stored at the start of the shadow call stack itself. But if it is possible
for code compiled without ``-ffixed-x18`` to run on a thread managed by the
runtime, which is the case on Android for example, the address must be stored
somewhere else instead. On Android we store the address of the start of the
guard region in TLS and deallocate the entire guard region including the
shadow call stack at thread exit. This is considered acceptable given that
the address of the start of the guard region is already somewhat guessable.
One way in which the address of the shadow call stack could leak is in the
``jmp_buf`` data structure used by ``setjmp`` and ``longjmp``. The Android
runtime `avoids this`_ by only storing the low bits of ``x18`` in the
``jmp_buf``, which requires the address of the shadow call stack to be
aligned to its size.
.. _`avoids this`: https://android.googlesource.com/platform/bionic/+/808d176e7e0dd727c7f929622ec017f6e065c582/libc/arch-arm64/bionic/setjmp.S#49
The architecture's call and return instructions (``bl`` and ``ret``) operate on
a register rather than the stack, which means that leaf functions are generally
protected from return address overwrites even without ShadowCallStack.
Usage
=====
@ -132,17 +188,7 @@ The following example code:
return bar() + 1;
}
Generates the following x86_64 assembly when compiled with ``-O2``:
.. code-block:: gas
push %rax
callq bar
add $0x1,%eax
pop %rcx
retq
or the following aarch64 assembly:
Generates the following aarch64 assembly when compiled with ``-O2``:
.. code-block:: none
@ -153,33 +199,7 @@ or the following aarch64 assembly:
ldp x29, x30, [sp], #16
ret
Adding ``-fsanitize=shadow-call-stack`` would output the following x86_64
assembly:
.. code-block:: gas
mov (%rsp),%r10
xor %r11,%r11
addq $0x8,%gs:(%r11)
mov %gs:(%r11),%r11
mov %r10,%gs:(%r11)
push %rax
callq bar
add $0x1,%eax
pop %rcx
xor %r11,%r11
mov %gs:(%r11),%r10
mov %gs:(%r10),%r10
subq $0x8,%gs:(%r11)
cmp %r10,(%rsp)
jne trap
retq
trap:
ud2
or the following aarch64 assembly:
Adding ``-fsanitize=shadow-call-stack`` would output the following assembly:
.. code-block:: none