Difference between revisions of "C-Based Toolchain Hardening"

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Revision as of 17:11, 17 February 2013

C-Based Toolchain Hardening is a treatment of project settings that will help you deliver reliable and secure code when using C, C++ and Objective C languages in a number of development environments. This article will examine Microsoft and GCC toolchains for the C, C++ and Objective C languages. It will guide you through the steps you should take to create executables with firmer defensive postures and increased integration with the available platform security. Effectively configuring the toolchain also means your project will enjoy a number of benefits during development, including enhanced warnings and static analysis, and self-debugging code.

There are four areas to be examined when hardening the toolchain: configuration, preprocessor, compiler, and linker. Nearly all areas are overlooked or neglected when setting up a project. The neglect appears to be pandemic, and it applies to nearly all projects including Auto-configured projects, Makefile-based, Eclipse-based, Visual Studio-based, and Xcode-based.

The article will also detail steps which quality assurance personnel can perform to ensure third party code meets organizational standards. Many organizations have Security Testing and Evaluation (ST&E) programs or operate in the US Federal arena where supply chain audits are necessary. If you audit a program with a lot of gaps, it could indicate the company providing the binaries does not have a mature engineering process or has gaps in its internal QA processes. For those who lack mature quality assurance or acceptance and testing criteria, then this article will also provide helpful suggestions.

Proper use of auditing tools such as checksec and readelf on Linux and BinScope on Windows means source code will be rarely needed for some portions of an audit. Lack of source code clears a number of legal obstacles in the acceptance testing process since NDAs or other agreements may not be required. For those who are not aware, the US's DMCA (PUBLIC LAW 105–304) has proper exceptions for reverse engineering and security testing and evaluation. The RE exemption is in Section 1205 (f) REVERSE ENGINEERING; and the ST&E exemption is in Section 1205 (i) SECURITY TESTING. If you don't need source code access, then you can decompile, re-engineer, and test without the need for consent or worry of reprisals.

This is a prescriptive article, and it will not debate semantics or speculate on behavior. As such, it will specify semantics, assign behaviors, and present a position. A secure toolchain is not a silver bullet. It is one piece of an overall strategy in the engineering process to help ensure success. It will compliment existing processes such as static analysis, dynamic analysis, secure coding, negative test suites, and the like. And a project will still require solid designs and architectures.

Finally, the OWASP ESAPI C++ project eats its own dog food. Many of the examples you will see in this article come directly from the ESAPI C++ project.

Contents

Wisdom

Code must be correct. It should be secure. It can be efficient.

Dr. Jon Bentley: "If it doesn't have to be correct, I can make it as fast as you'd like it to be".

Dr. Gary McGraw: "Thou shalt not rely solely on security features and functions to build secure software as security is an emergent property of the entire system and thus relies on building and integrating all parts properly".

Configuration

Configuration is the first opportunity to configure your project for success. Not only do you have to configure your project to meet reliability and security goals, you must also configure integrated libraries properly. You typically have has three choices. First, you can use auto-configuration utilities if on Linux or Unix. Second, you can write a makefile by hand. This is predominant on Linux, Mac OS X, and Unix, but it applies to Windows as well. Finally, you can use an integrated development environment or IDE.

At this stage in the process, you should concentrate on configuring for two builds: Debug and Release. Debug will be used for development and include full instrumentation. Release will be configured for production. The difference between the two settings is usually optimization level and debug level. A third build configuration is Test, and its usually a special case of Release.

For debug and release builds, the settings are typically diametrically opposed. Debug configurations have no optimizations and full debug information; while Release builds have optimizations and minimal to moderate debug information. The Test configuration is often a Release configuration that makes everything public for testing and builds a test harness. For example, all member functions public (a C++ class) and all interfaces (library or shared object) should be made available for testing.

Though many do not realize, debug code is more highly valued than release code because it holds additional instrumentation. The debug instrumentation will cause a program to become nearly "self-debugging". Self-debugging code reduces your time during trouble shooting and debugging. Reducing time under the debugger means you have more time for development and feature requests. The additional debug instrumentation will be removed in production code via preprocessor macros.

Auto Tools

Auto configuration tools are popular on many Linux and Unix based systems, and the tools include Autosetup, Autoconf, Automake, config, and Configure. The tools work together to produce project files from scripts and template files. After the process completes, your project should be setup and ready to be made with make.

When using auto configuration tools, there are a few files of interest worth mentioning. The files are part of the auto tools chain and include m4 and the various *.in, *.ac (autoconf), and *.am (automake) files. At times, you will have to open them, or the resulting makefiles, to tune the "stock" configuration.

There are three downsides to the command line configuration tools in the toolchain: (1) they often ignore user requests, (2) they cannot create configurations, and (3) security is often not a goal.

To demonstrate the first issue, confider your project with the following: configure CFLAGS="-Wall -fPIE" CXXFLAGS="-Wall -fPIE" LDFLAGS="-pie". You will probably find the auto tools ignored your request. You will have to open an m4 scripts, Makefile.in or Makefile.am and fix the configuration.

For the second point, you will probably be disappointed to learn Automake does not support the concept of configurations. Its not entirely Autoconf's or Automake's fault - Make and its inability to detect changes is the underlying problem. Specifically, Make only checks modification times of prerequisites and targets, and does not check things like CFLAGS and CXXFLAGS. The net effect is you will not receive expected results when you issue make debug and then make test or make release.

Finally, you will probably be disappointed to learn tools such as Autoconf and Automake miss many security related opportunities and ship insecure out of the box. There are a number of compiler switches and linker flags that improve the defensive posture of a program, but they are not 'on' by default. Tools like Autoconf - which are supposed to handle this situation - often provides setting to serve the lowest of all denominators.

A recent discussion on the Automake mailing list illuminates the issue: Enabling compiler warning flags. Attempts to improve default configurations were met with resistance and no action was taken. The resistance is often of the form, "<some useful warning> also produces false positives" or "<some obscure platform> does not support <established security feature>". Its noteworthy that David Wheeler, the author of Secure Programming for Linux and Unix HOWTO, was one of the folks trying to improve the posture.

Makefiles

Make is one of the earliest build systems dating back to the 1970s. Its available on Linux, Mac OS X and Unix, so you will frequently encounter projects using it. Unfortunately, Make has a number of short comings (Recursive Make Considered Harmful and What’s Wrong With GNU make?), and can cause some discomfort. Despite issues with Make, ESAPI C++ uses Make primarily for three reasons: first, its omnipresent; second, its easier to manage than the Auto Tools family; and third, libtool was out of the question.

Consider what happens when you: (1) type make debug, and then type make release. Each build would require different CFLAGS due to optimizations and level of debug support. In your makefile, you would extract the relevant target and set CFLAGS and CXXFLAGS similar to below (taken from ESAPI C++ Makefile):

# Makefile
DEBUG_GOALS = $(filter $(MAKECMDGOALS), debug)
ifneq ($(DEBUG_GOALS),)
  WANT_DEBUG := 1
  WANT_TEST := 0
  WANT_RELEASE := 0
endif
…

ifeq ($(WANT_DEBUG),1)
  ESAPI_CFLAGS += -DDEBUG=1 -UNDEBUG -g3 -ggdb -O0
  ESAPI_CXXFLAGS += -DDEBUG=1 -UNDEBUG -g3 -ggdb -O0
endif
…

# Merge ESAPI flags with user supplied flags. We perform the extra step to ensure 
# user options follow our options, which should give user option's a preference.
override CFLAGS := $(ESAPI_CFLAGS) $(CFLAGS)
override CXXFLAGS := $(ESAPI_CXXFLAGS) $(CXXFLAGS)
override LDFLAGS := $(ESAPI_LDFLAGS) $(LDFLAGS)
…

Make will first build the program in a debug configuration for a session under the debugger using a rule similar to:

%.cpp:%.o:
        $(CXX) $(CPPFLAGS) $(CXXFLAGS) -c $< -o $@

When you want the release build, Make will do nothing because it considers everything up to date despite the fact CFLAGS and CXXFLAGS have changed. Hence, your program will actually be in a debug configuration and risk a SIGABRT at runtime because debug instrumentation is present (recall assert calls abort() when NDEBUG is not defined). In essence, you have DoS'd yourself due to make.

In addition, many project do not honor the user's command line. ESAPI C++ does its best to ensure a user's flags are honored via override as shown above, but other projects do not. For example, consider a project that should be built with Position Independent Executable (PIE or ASLR) enabled and data execution prevention (DEP) enabled. Dismissing user settings combined with insecure out of the box settings (and not picking them up during auto-setup or auto-configure) means a program built with the following ail likely have neither defense:

$ make CFLAGS="-fPIE" CXXFLAGS="-fPIE" LDFLAGS="-pie -z,noexecstack, -z,noexecheap"

Defenses such as ASLR and DEP are especially important on Linux because Data Execution - not Prevention - is the norm.

Integration

Project level integration presents opportunities to harden your program or library with domain specific knowledge. For example, if the platform supports Position Independent Executables (PIE or ASLR) and data execution prevention (DEP), then you should integrate with it. The consequences of not doing so could result in exploitation. As a case in point, see KingCope's 0-days for MySQL in December, 2012 (CVE-2012-5579 and CVE-2012-5612, among others). Integration with platform security would have neutered a number of the 0-days.

In addition, its an opportunity to harden third party libraries you chose to include. Because you chose to include them, you and your users are responsible for them. If you or your users endure a SP800-53 audit, third party libraries will be in scope because the supply chain is included (specifically, item SA-12, Supply Chain Protection). The audits are not limited to those in the US Federal arena - financial institutions perform reviews too.

As an example, suppose you are including OpenSSL. You know (1) SSLv2 is insecure, (2) SSLv3 is insecure, and (3) compression is insecure (among others). In addition, suppose you don't use hardware and engines, and only allow static linking. Given the knowledge and specifications, you would configure the OpenSSL library as follows:

$ Configure darwin64-x86_64-cc -no-hw -no-engines -no-comp -no-shared -no-dso -no-sslv2 -no-sslv3 --openssldir=…

If you configure without the switches, you run the risk of failing an audit. Even more egregious is the answer given to auditors who specifically ask about configurations and protocols: "we don't use weak/wounded/broken ciphers" or "we follow best practices." nm or openssl s_client will reveal that, for example, compression is enabled. If the program is a remote server, then the following command will reveal if compression is active:

$ echo "GET / HTTP1.0" | openssl s_client -connect example.com:443

If the program is a client, the following will expose if compression is available:

$ nm /usr/local/ssl/iphoneos/lib/libcrypto.a 2>/dev/null | egrep -i "(COMP_CTX_new|COMP_CTX_free)"
0000000000000110 T COMP_CTX_free
0000000000000000 T COMP_CTX_new

In fact, any symbol within the OPENSSL_NO_COMP preprocessor macro will bear witness since -no-comp is translated into a CFLAGS define.

Preprocessor

The preprocessor is crucial to setting up a project for success. The C committee provided one macro - NDEBUG - and the macro can be used to derive a number of configurations and drive engineering processes. Unfortunately, the committee also left many related items to chance, which has resulted in programmers abusing builtin facilities. This section will help you set up you projects to integrate well with other projects and ensure reliability and security.

There are three topics to discuss when hardening the preprocessor. The first is well defined configurations which produce well defined behaviors, the second is useful behavior from assert, and the third is proper use of macros when integrating vendor code and third party libraries.

Configurations

To remove ambiguity, you should recognize two configurations: Release and Debug. Release is for production code on live servers, and its behavior is requested via the C/C++ NDEBUG macro. Its also the only macro observed by the C and C++ Committees and Posix. Diametrically opposed to release is Debug. While there is a compelling argument for !defined(NDEBUG), you should have an explicit macro for the configuration and that macro should be DEBUG. This is because vendors and outside libraries use DEBUG (or similar) macro for their configuration. For example, Carnegie Mellon's Mach kernel uses DEBUG, Microsoft's CRT uses _DEBUG, and Wind River Workbench uses DEBUG_MODE.

In addition to NDEBUG (Release) and DEBUG (Debug), you have two additional cross products: both are defined or neither are defined. Defining both should be an error, and defining neither should default to a release configuration. Below is from ESAPI C++ EsapiCommon.h, which is the configuration file used by all source files:

// Only one or the other, but not both
#if (defined(DEBUG) || defined(_DEBUG)) && (defined(NDEBUG) || defined(_NDEBUG))
# error Both DEBUG and NDEBUG are defined.
#endif

// The only time we switch to debug is when asked. NDEBUG or {nothing} results
// in release build (fewer surprises at runtime).
#if defined(DEBUG) || defined(_DEBUG)
# define ESAPI_BUILD_DEBUG 1
#else
# define ESAPI_BUILD_RELEASE 1
#endif

When DEBUG is in effect, your code should receive full debug instrumentation, including the full force of assertions.

ASSERT

Asserts will help you create self-debugging code. They help you find the point of first failure quickly and easily. Asserts should be used throughout your program, including parameter validation, return value checking and program state. If you have thorough code coverage, you will spend less time debugging and more time developing because f_DEBUG will debug themselves.

To use asserts effectively, you should assert everything. That includes parameters upon entering a function, return values from function calls, and any program state. Everywhere you place an if statement for validation or checking, you should have an assert. Everywhere you have an assert for validation or checking, you should have an if statement. They go hand-in-hand.

There is one problem with using asserts - Posix states assert should call abort() if NDEBUG is not defined. When debugging, NDEBUG will never be defined since you want the "program diagnostics" (quote from the Posix description). That makes assert and its accompanying abort() completely useless. The result of "program diagnostics" calling abort() due to standard C/C++ behavior is disuse - developers simply don't use them. Its incredibly bad for the development community because self-debugging programs can help eradicate so many stability problems.

Since self-debugging programs are so powerful, you will have to have to supply your own assert and signal handler with improved behavior. Your assert will exchange auto-aborting behavior for auto-debugging behavior. The auto-debugging facility will ensure the debugger snaps when a problem is detected, and you will find the point of first failure quickly and easily.

ESAPI C++ supplies its own assert with the behavior described above. In the code below, ASSERT raises SIGTRAP when in effect or it evaluates to void in other cases.

// A debug assert which should be sprinkled liberally. This assert fires and then continues rather
// than calling abort(). Useful when examining negative test cases from the command line.
#if (defined(ESAPI_BUILD_DEBUG) && defined(ESAPI_OS_STARNIX))
#  define ESAPI_ASSERT1(exp) {                                    \
    if(!(exp)) {                                                  \
      std::ostringstream oss;                                     \
      oss << "Assertion failed: " << (char*)(__FILE__) << "("     \
          << (int)__LINE__ << "): " << (char*)(__func__)          \
          << std::endl;                                           \
      std::cerr << oss.str();                                     \
      raise(SIGTRAP);                                             \
    }                                                             \
  }
#  define ESAPI_ASSERT2(exp, msg) {                               \
    if(!(exp)) {                                                  \
      std::ostringstream oss;                                     \
      oss << "Assertion failed: " << (char*)(__FILE__) << "("     \
          << (int)__LINE__ << "): " << (char*)(__func__)          \
          << ": \"" << (msg) << "\"" << std::endl;                \
      std::cerr << oss.str();                                     \
      raise(SIGTRAP);                                             \
    }                                                             \
  }
#elif (defined(ESAPI_BUILD_DEBUG) && defined(ESAPI_OS_WINDOWS))
#  define ESAPI_ASSERT1(exp)      assert(exp)
#  define ESAPI_ASSERT2(exp, msg) assert(exp)
#else
#  define ESAPI_ASSERT1(exp)      ((void)(exp))
#  define ESAPI_ASSERT2(exp, msg) ((void)(exp))
#endif

#if !defined(ASSERT)
#  define ASSERT(exp)     ESAPI_ASSERT1(exp)
#endif

At program startup, a SIGTRAP handler will be installed if one is not provided by another component:

struct DebugTrapHandler
{
  DebugTrapHandler()
  {
    struct sigaction new_handler, old_handler;

    do
      {
        int ret = 0;

        ret = sigaction (SIGTRAP, NULL, &old_handler);
        if (ret != 0) break; // Failed

        // Don't step on another's handler
        if (old_handler.sa_handler != NULL) break;

        new_handler.sa_handler = &DebugTrapHandler::NullHandler;
        new_handler.sa_flags = 0;

        ret = sigemptyset (&new_handler.sa_mask);
        if (ret != 0) break; // Failed

        ret = sigaction (SIGTRAP, &new_handler, NULL);
        if (ret != 0) break; // Failed

      } while(0);
  }

  static void NullHandler(int /*unused*/) { }

};

// We specify a relatively low priority, to make sure we run before other CTORs
// http://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html#C_002b_002b-Attributes
static const DebugTrapHandler g_dummyHandler __attribute__ ((init_priority (110)));

On a Windows platform, you would call _set_invalid_parameter_handler (and possibly set_unexpected or set_terminate) to install a new handler.

Live hosts running production code should always define NDEBUG (i.e., release configuration), which means they do not assert or auto-abort. Auto-abortion is not acceptable behavior, and anyone who asks for the behavior is completely abusing the functionality of "program diagnostics". If a program wants a core dump, then it should create the dump rather than crashing.

Additional Macros

Additional macros include any macros needed to integrate properly and securely. It includes integrating the program with the platform (for example MFC or Cocoa/CocoaTouch) and libraries (for example, Crypto++ or OpenSSL). It can be a challenge because you have to have proficiency with your platform and all included libraries and frameworks.

Its nearly impossible to enumerate all the possible combinations for all platforms and libraries, but the list below highlights the level of detail you will need when integrating.

In addition to what you should define, defining some macros and undefining others should trigger a security related defect. For example, -U_FORTIFY_SOURCES on Linux and _CRT_SECURE_NO_WARNINGS=1, _SCL_SECURE_NO_WARNINGS , STRSAFE_NO_DEPRECATE or _ATL_SECURE_NO_WARNINGS on Windows.

Platform/Library Debug Release
Table 1: Additional Platform/Library Macros
All DEBUG=1 NDEBUG=1
Linux _GLIBCXX_DEBUG=1a _FORTIFY_SOURCE=2
Android (4.2 and above) _FORTIFY_SOURCE=1
Cocoa/CocoaTouch NS_BLOCK_ASSERTIONS=1

#define NSLog(...) (define to nothing, preempt ASL)

SafeInt SAFEINT_DISALLOW_UNSIGNED_NEGATION=1 SAFEINT_DISALLOW_UNSIGNED_NEGATION=1
Microsoft _DEBUG=1, STRICT,

_SECURE_SCL=1, _HAS_ITERATOR_DEBUGGING=1
_CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES=1
_CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES_COUNT=1

STRICT

_CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES=1
_CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES_COUNT=1

Microsoft ATL _SECURE_ATL, _ATL_ALL_WARNINGS

_ATL_CSTRING_EXPLICIT_CONSTRUCTORS

_SECURE_ATL, _ATL_ALL_WARNINGS

_ATL_CSTRING_EXPLICIT_CONSTRUCTORS

STL Port _STLP_DEBUG=1, _STLP_USE_DEBUG_LIB=1

_STLP_DEBUG_ALLOC=1, _STLP_DEBUG_UNINITIALIZED=1

SQLite SQLITE_DEBUG, SQLITE_MEMDEBUG

SQLITE_OMIT_WAL, SQLITE_SECURE_DELETEb
SQLITE_DEFAULT_FILE_PERMISSIONS=Nc

SQLITE_OMIT_WAL, SQLITE_SECURE_DELETEb

SQLITE_DEFAULT_FILE_PERMISSIONS=Nc

SQLCipher Remove NDEBUG from Debug builds (Xcode)

SQLITE_HAS_CODEC=1

SQLITE_HAS_CODEC=1
SQLite/SQLCipher SQLITE_TEMP_STORE=3d SQLITE_TEMP_STORE=3d

a Be careful with _GLIBCXX_DEBUG when using pre-compiled libraries such as Boost from a distribution. There are ABI incompatibilities, and the result will likely be a crash. You will have to compile Boost with _GLIBCXX_DEBUG or omit _GLIBCXX_DEBUG.

b SQLite secure deletion zeroizes memory on destruction. Define as required, and always define in US Federal since zeroization is required for FIPS 140-2, Level 1.

c N is 0644 by default, which means everyone has some access.

d Force temporary tables into memory (no unencrypted data to disk).