forked from OSchip/llvm-project
344 lines
15 KiB
ReStructuredText
344 lines
15 KiB
ReStructuredText
======================
|
|
Control Flow Integrity
|
|
======================
|
|
|
|
.. toctree::
|
|
:hidden:
|
|
|
|
ControlFlowIntegrityDesign
|
|
|
|
.. contents::
|
|
:local:
|
|
|
|
Introduction
|
|
============
|
|
|
|
Clang includes an implementation of a number of control flow integrity (CFI)
|
|
schemes, which are designed to abort the program upon detecting certain forms
|
|
of undefined behavior that can potentially allow attackers to subvert the
|
|
program's control flow. These schemes have been optimized for performance,
|
|
allowing developers to enable them in release builds.
|
|
|
|
To enable Clang's available CFI schemes, use the flag ``-fsanitize=cfi``.
|
|
You can also enable a subset of available :ref:`schemes <cfi-schemes>`.
|
|
As currently implemented, all schemes rely on link-time optimization (LTO);
|
|
so it is required to specify ``-flto``, and the linker used must support LTO,
|
|
for example via the `gold plugin`_.
|
|
|
|
To allow the checks to be implemented efficiently, the program must
|
|
be structured such that certain object files are compiled with CFI
|
|
enabled, and are statically linked into the program. This may preclude
|
|
the use of shared libraries in some cases.
|
|
|
|
The compiler will only produce CFI checks for a class if it can infer hidden
|
|
LTO visibility for that class. LTO visibility is a property of a class that
|
|
is inferred from flags and attributes. For more details, see the documentation
|
|
for :doc:`LTO visibility <LTOVisibility>`.
|
|
|
|
The ``-fsanitize=cfi-{vcall,nvcall,derived-cast,unrelated-cast}`` flags
|
|
require that a ``-fvisibility=`` flag also be specified. This is because the
|
|
default visibility setting is ``-fvisibility=default``, which would disable
|
|
CFI checks for classes without visibility attributes. Most users will want
|
|
to specify ``-fvisibility=hidden``, which enables CFI checks for such classes.
|
|
|
|
Experimental support for :ref:`cross-DSO control flow integrity
|
|
<cfi-cross-dso>` exists that does not require classes to have hidden LTO
|
|
visibility. This cross-DSO support has unstable ABI at this time.
|
|
|
|
.. _gold plugin: https://llvm.org/docs/GoldPlugin.html
|
|
|
|
.. _cfi-schemes:
|
|
|
|
Available schemes
|
|
=================
|
|
|
|
Available schemes are:
|
|
|
|
- ``-fsanitize=cfi-cast-strict``: Enables :ref:`strict cast checks
|
|
<cfi-strictness>`.
|
|
- ``-fsanitize=cfi-derived-cast``: Base-to-derived cast to the wrong
|
|
dynamic type.
|
|
- ``-fsanitize=cfi-unrelated-cast``: Cast from ``void*`` or another
|
|
unrelated type to the wrong dynamic type.
|
|
- ``-fsanitize=cfi-nvcall``: Non-virtual call via an object whose vptr is of
|
|
the wrong dynamic type.
|
|
- ``-fsanitize=cfi-vcall``: Virtual call via an object whose vptr is of the
|
|
wrong dynamic type.
|
|
- ``-fsanitize=cfi-icall``: Indirect call of a function with wrong dynamic
|
|
type.
|
|
- ``-fsanitize=cfi-mfcall``: Indirect call via a member function pointer with
|
|
wrong dynamic type.
|
|
|
|
You can use ``-fsanitize=cfi`` to enable all the schemes and use
|
|
``-fno-sanitize`` flag to narrow down the set of schemes as desired.
|
|
For example, you can build your program with
|
|
``-fsanitize=cfi -fno-sanitize=cfi-nvcall,cfi-icall``
|
|
to use all schemes except for non-virtual member function call and indirect call
|
|
checking.
|
|
|
|
Remember that you have to provide ``-flto`` if at least one CFI scheme is
|
|
enabled.
|
|
|
|
Trapping and Diagnostics
|
|
========================
|
|
|
|
By default, CFI will abort the program immediately upon detecting a control
|
|
flow integrity violation. You can use the :ref:`-fno-sanitize-trap=
|
|
<controlling-code-generation>` flag to cause CFI to print a diagnostic
|
|
similar to the one below before the program aborts.
|
|
|
|
.. code-block:: console
|
|
|
|
bad-cast.cpp:109:7: runtime error: control flow integrity check for type 'B' failed during base-to-derived cast (vtable address 0x000000425a50)
|
|
0x000000425a50: note: vtable is of type 'A'
|
|
00 00 00 00 f0 f1 41 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 20 5a 42 00
|
|
^
|
|
|
|
If diagnostics are enabled, you can also configure CFI to continue program
|
|
execution instead of aborting by using the :ref:`-fsanitize-recover=
|
|
<controlling-code-generation>` flag.
|
|
|
|
Forward-Edge CFI for Virtual Calls
|
|
==================================
|
|
|
|
This scheme checks that virtual calls take place using a vptr of the correct
|
|
dynamic type; that is, the dynamic type of the called object must be a
|
|
derived class of the static type of the object used to make the call.
|
|
This CFI scheme can be enabled on its own using ``-fsanitize=cfi-vcall``.
|
|
|
|
For this scheme to work, all translation units containing the definition
|
|
of a virtual member function (whether inline or not), other than members
|
|
of :ref:`blacklisted <cfi-blacklist>` types or types with public :doc:`LTO
|
|
visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
|
|
enabled and be statically linked into the program.
|
|
|
|
Performance
|
|
-----------
|
|
|
|
A performance overhead of less than 1% has been measured by running the
|
|
Dromaeo benchmark suite against an instrumented version of the Chromium
|
|
web browser. Another good performance benchmark for this mechanism is the
|
|
virtual-call-heavy SPEC 2006 xalancbmk.
|
|
|
|
Note that this scheme has not yet been optimized for binary size; an increase
|
|
of up to 15% has been observed for Chromium.
|
|
|
|
Bad Cast Checking
|
|
=================
|
|
|
|
This scheme checks that pointer casts are made to an object of the correct
|
|
dynamic type; that is, the dynamic type of the object must be a derived class
|
|
of the pointee type of the cast. The checks are currently only introduced
|
|
where the class being casted to is a polymorphic class.
|
|
|
|
Bad casts are not in themselves control flow integrity violations, but they
|
|
can also create security vulnerabilities, and the implementation uses many
|
|
of the same mechanisms.
|
|
|
|
There are two types of bad cast that may be forbidden: bad casts
|
|
from a base class to a derived class (which can be checked with
|
|
``-fsanitize=cfi-derived-cast``), and bad casts from a pointer of
|
|
type ``void*`` or another unrelated type (which can be checked with
|
|
``-fsanitize=cfi-unrelated-cast``).
|
|
|
|
The difference between these two types of casts is that the first is defined
|
|
by the C++ standard to produce an undefined value, while the second is not
|
|
in itself undefined behavior (it is well defined to cast the pointer back
|
|
to its original type) unless the object is uninitialized and the cast is a
|
|
``static_cast`` (see C++14 [basic.life]p5).
|
|
|
|
If a program as a matter of policy forbids the second type of cast, that
|
|
restriction can normally be enforced. However it may in some cases be necessary
|
|
for a function to perform a forbidden cast to conform with an external API
|
|
(e.g. the ``allocate`` member function of a standard library allocator). Such
|
|
functions may be :ref:`blacklisted <cfi-blacklist>`.
|
|
|
|
For this scheme to work, all translation units containing the definition
|
|
of a virtual member function (whether inline or not), other than members
|
|
of :ref:`blacklisted <cfi-blacklist>` types or types with public :doc:`LTO
|
|
visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
|
|
enabled and be statically linked into the program.
|
|
|
|
Non-Virtual Member Function Call Checking
|
|
=========================================
|
|
|
|
This scheme checks that non-virtual calls take place using an object of
|
|
the correct dynamic type; that is, the dynamic type of the called object
|
|
must be a derived class of the static type of the object used to make the
|
|
call. The checks are currently only introduced where the object is of a
|
|
polymorphic class type. This CFI scheme can be enabled on its own using
|
|
``-fsanitize=cfi-nvcall``.
|
|
|
|
For this scheme to work, all translation units containing the definition
|
|
of a virtual member function (whether inline or not), other than members
|
|
of :ref:`blacklisted <cfi-blacklist>` types or types with public :doc:`LTO
|
|
visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
|
|
enabled and be statically linked into the program.
|
|
|
|
.. _cfi-strictness:
|
|
|
|
Strictness
|
|
----------
|
|
|
|
If a class has a single non-virtual base and does not introduce or override
|
|
virtual member functions or fields other than an implicitly defined virtual
|
|
destructor, it will have the same layout and virtual function semantics as
|
|
its base. By default, casts to such classes are checked as if they were made
|
|
to the least derived such class.
|
|
|
|
Casting an instance of a base class to such a derived class is technically
|
|
undefined behavior, but it is a relatively common hack for introducing
|
|
member functions on class instances with specific properties that works under
|
|
most compilers and should not have security implications, so we allow it by
|
|
default. It can be disabled with ``-fsanitize=cfi-cast-strict``.
|
|
|
|
Indirect Function Call Checking
|
|
===============================
|
|
|
|
This scheme checks that function calls take place using a function of the
|
|
correct dynamic type; that is, the dynamic type of the function must match
|
|
the static type used at the call. This CFI scheme can be enabled on its own
|
|
using ``-fsanitize=cfi-icall``.
|
|
|
|
For this scheme to work, each indirect function call in the program, other
|
|
than calls in :ref:`blacklisted <cfi-blacklist>` functions, must call a
|
|
function which was either compiled with ``-fsanitize=cfi-icall`` enabled,
|
|
or whose address was taken by a function in a translation unit compiled with
|
|
``-fsanitize=cfi-icall``.
|
|
|
|
If a function in a translation unit compiled with ``-fsanitize=cfi-icall``
|
|
takes the address of a function not compiled with ``-fsanitize=cfi-icall``,
|
|
that address may differ from the address taken by a function in a translation
|
|
unit not compiled with ``-fsanitize=cfi-icall``. This is technically a
|
|
violation of the C and C++ standards, but it should not affect most programs.
|
|
|
|
Each translation unit compiled with ``-fsanitize=cfi-icall`` must be
|
|
statically linked into the program or shared library, and calls across
|
|
shared library boundaries are handled as if the callee was not compiled with
|
|
``-fsanitize=cfi-icall``.
|
|
|
|
This scheme is currently only supported on the x86 and x86_64 architectures.
|
|
|
|
``-fsanitize-cfi-icall-generalize-pointers``
|
|
--------------------------------------------
|
|
|
|
Mismatched pointer types are a common cause of cfi-icall check failures.
|
|
Translation units compiled with the ``-fsanitize-cfi-icall-generalize-pointers``
|
|
flag relax pointer type checking for call sites in that translation unit,
|
|
applied across all functions compiled with ``-fsanitize=cfi-icall``.
|
|
|
|
Specifically, pointers in return and argument types are treated as equivalent as
|
|
long as the qualifiers for the type they point to match. For example, ``char*``,
|
|
``char**``, and ``int*`` are considered equivalent types. However, ``char*`` and
|
|
``const char*`` are considered separate types.
|
|
|
|
``-fsanitize-cfi-icall-generalize-pointers`` is not compatible with
|
|
``-fsanitize-cfi-cross-dso``.
|
|
|
|
|
|
``-fsanitize=cfi-icall`` and ``-fsanitize=function``
|
|
----------------------------------------------------
|
|
|
|
This tool is similar to ``-fsanitize=function`` in that both tools check
|
|
the types of function calls. However, the two tools occupy different points
|
|
on the design space; ``-fsanitize=function`` is a developer tool designed
|
|
to find bugs in local development builds, whereas ``-fsanitize=cfi-icall``
|
|
is a security hardening mechanism designed to be deployed in release builds.
|
|
|
|
``-fsanitize=function`` has a higher space and time overhead due to a more
|
|
complex type check at indirect call sites, as well as a need for run-time
|
|
type information (RTTI), which may make it unsuitable for deployment. Because
|
|
of the need for RTTI, ``-fsanitize=function`` can only be used with C++
|
|
programs, whereas ``-fsanitize=cfi-icall`` can protect both C and C++ programs.
|
|
|
|
On the other hand, ``-fsanitize=function`` conforms more closely with the C++
|
|
standard and user expectations around interaction with shared libraries;
|
|
the identity of function pointers is maintained, and calls across shared
|
|
library boundaries are no different from calls within a single program or
|
|
shared library.
|
|
|
|
Member Function Pointer Call Checking
|
|
=====================================
|
|
|
|
This scheme checks that indirect calls via a member function pointer
|
|
take place using an object of the correct dynamic type. Specifically, we
|
|
check that the dynamic type of the member function referenced by the member
|
|
function pointer matches the "function pointer" part of the member function
|
|
pointer, and that the member function's class type is related to the base
|
|
type of the member function. This CFI scheme can be enabled on its own using
|
|
``-fsanitize=cfi-mfcall``.
|
|
|
|
The compiler will only emit a full CFI check if the member function pointer's
|
|
base type is complete. This is because the complete definition of the base
|
|
type contains information that is necessary to correctly compile the CFI
|
|
check. To ensure that the compiler always emits a full CFI check, it is
|
|
recommended to also pass the flag ``-fcomplete-member-pointers``, which
|
|
enables a non-conforming language extension that requires member pointer
|
|
base types to be complete if they may be used for a call.
|
|
|
|
For this scheme to work, all translation units containing the definition
|
|
of a virtual member function (whether inline or not), other than members
|
|
of :ref:`blacklisted <cfi-blacklist>` types or types with public :doc:`LTO
|
|
visibility <LTOVisibility>`, must be compiled with ``-flto`` or ``-flto=thin``
|
|
enabled and be statically linked into the program.
|
|
|
|
This scheme is currently not compatible with cross-DSO CFI or the
|
|
Microsoft ABI.
|
|
|
|
.. _cfi-blacklist:
|
|
|
|
Blacklist
|
|
=========
|
|
|
|
A :doc:`SanitizerSpecialCaseList` can be used to relax CFI checks for certain
|
|
source files, functions and types using the ``src``, ``fun`` and ``type``
|
|
entity types. Specific CFI modes can be be specified using ``[section]``
|
|
headers.
|
|
|
|
.. code-block:: bash
|
|
|
|
# Suppress all CFI checking for code in a file.
|
|
src:bad_file.cpp
|
|
src:bad_header.h
|
|
# Ignore all functions with names containing MyFooBar.
|
|
fun:*MyFooBar*
|
|
# Ignore all types in the standard library.
|
|
type:std::*
|
|
# Disable only unrelated cast checks for this function
|
|
[cfi-unrelated-cast]
|
|
fun:*UnrelatedCast*
|
|
# Disable CFI call checks for this function without affecting cast checks
|
|
[cfi-vcall|cfi-nvcall|cfi-icall]
|
|
fun:*BadCall*
|
|
|
|
|
|
.. _cfi-cross-dso:
|
|
|
|
Shared library support
|
|
======================
|
|
|
|
Use **-f[no-]sanitize-cfi-cross-dso** to enable the cross-DSO control
|
|
flow integrity mode, which allows all CFI schemes listed above to
|
|
apply across DSO boundaries. As in the regular CFI, each DSO must be
|
|
built with ``-flto``.
|
|
|
|
Normally, CFI checks will only be performed for classes that have hidden LTO
|
|
visibility. With this flag enabled, the compiler will emit cross-DSO CFI
|
|
checks for all classes, except for those which appear in the CFI blacklist
|
|
or which use a ``no_sanitize`` attribute.
|
|
|
|
Design
|
|
======
|
|
|
|
Please refer to the :doc:`design document<ControlFlowIntegrityDesign>`.
|
|
|
|
Publications
|
|
============
|
|
|
|
`Control-Flow Integrity: Principles, Implementations, and Applications <https://research.microsoft.com/pubs/64250/ccs05.pdf>`_.
|
|
Martin Abadi, Mihai Budiu, Úlfar Erlingsson, Jay Ligatti.
|
|
|
|
`Enforcing Forward-Edge Control-Flow Integrity in GCC & LLVM <http://www.pcc.me.uk/~peter/acad/usenix14.pdf>`_.
|
|
Caroline Tice, Tom Roeder, Peter Collingbourne, Stephen Checkoway,
|
|
Úlfar Erlingsson, Luis Lozano, Geoff Pike.
|