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diff --git a/tools/fuzzing/docs/fuzzing_interface.rst b/tools/fuzzing/docs/fuzzing_interface.rst new file mode 100644 index 0000000000..2b45abf35b --- /dev/null +++ b/tools/fuzzing/docs/fuzzing_interface.rst @@ -0,0 +1,499 @@ +Fuzzing Interface +================= + +The fuzzing interface is glue code living in mozilla-central in order to +make it easier for developers and security researchers to test C/C++ +code with either `libFuzzer <https://llvm.org/docs/LibFuzzer.html>`__ or +`afl-fuzz <http://lcamtuf.coredump.cx/afl/>`__. + +These fuzzing tools, are based on *compile-time instrumentation* to measure +things like branch coverage and more advanced heuristics per fuzzing test. +Doing so allows these tools to progress through code with little to no custom +logic/knowledge implemented in the fuzzer itself. Usually, the only thing +these tools need is a code "shim" that provides the entry point for the fuzzer +to the code to be tested. We call this additional code a *fuzzing target* and +the rest of this manual describes how to implement and work with these targets. + +As for the tools used with these targets, we currently recommend the use of +libFuzzer over afl-fuzz, as the latter is no longer maintained while libFuzzer +is being actively developed. Furthermore, libFuzzer has some advanced +instrumentation features (e.g. value profiling to deal with complicated +comparisons in code), making it overall more effective. + +What can be tested? +~~~~~~~~~~~~~~~~~~~ + +The interface can be used to test all C/C++ code that either ends up in +``libxul`` (more precisely, the gtest version of ``libxul``) **or** is +part of the JS engine. + +Note that this is not the right testing approach for testing the full +browser as a whole. It is rather meant for component-based testing +(especially as some components cannot be easily separated out of the +full build). + +.. note:: + + **Note:** If you are working on the JS engine (trying to reproduce a + bug or seeking to develop a new fuzzing target), then please also read + the :ref:`JS Engine Specifics Section <JS Engine Specifics>` at the end + of this documentation, as the JS engine offers additional options for + implementing and running fuzzing targets. + + +Reproducing bugs for existing fuzzing targets +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If you are working on a bug that involves an existing fuzzing interface target, +you have two options for reproducing the issue: + + +Using existing builds +^^^^^^^^^^^^^^^^^^^^^ + +We have several fuzzing builds in CI that you can simply download. We recommend +using ``fuzzfetch`` for this purpose, as it makes downloading and unpacking +these builds much easier. + +You can install ``fuzzfetch`` from +`Github <https://github.com/MozillaSecurity/fuzzfetch>`__ or +`via pip <https://pypi.org/project/fuzzfetch/>`__. + +Afterwards, you can run + +:: + + $ python -m fuzzfetch -a --fuzzing --gtest -n firefox-fuzzing + +to fetch the latest optimized build. Alternatively, we offer non-ASan debug builds +which you can download using + +:: + + $ python -m fuzzfetch -d --fuzzing --gtest -n firefox-fuzzing + +In both commands, ``firefox-fuzzing`` indicates the name of the directory that +will be created for the download. + +Afterwards, you can reproduce the bug using + +:: + + $ FUZZER=TargetName firefox-fuzzing/firefox test.bin + +assuming that ``TargetName`` is the name of the fuzzing target specified in the +bug you are working on and ``test.bin`` is the attached testcase. + +.. note:: + + **Note:** You should not export the ``FUZZER`` variable permanently + in your shell, especially if you plan to do local builds. If the ``FUZZER`` + variable is exported, it will affect the build process. + +If the CI builds don't meet your requirements and you need a local build instead, +you can follow the steps below to create one: + +Local build requirements and flags +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +You will need a Linux environment with a recent Clang. Using the Clang downloaded +by ``./mach bootstrap`` or a newer version is recommended. + +The only build flag required to enable the fuzzing targets is ``--enable-fuzzing``, +so adding + +:: + + ac_add_options --enable-fuzzing + +to your ``.mozconfig`` is already sufficient for producing a fuzzing build. +However, for improved crash handling capabilities and to detect additional errors, +it is strongly recommended to combine libFuzzer with :ref:`AddressSanitizer <Address Sanitizer>` +by adding + +:: + + ac_add_options --enable-address-sanitizer + +at least for optimized builds and bugs requiring ASan to reproduce at all +(e.g. you are working on a bug where ASan reports a memory safety violation +of some sort). + +Once your build is complete, you **must** additionally run + +:: + + $ ./mach gtest dontruntests + +to force the gtest libxul to be built. + +.. note:: + + **Note:** If you modify any code, please ensure that you run **both** build + commands to ensure that the gtest libxul is also rebuilt. It is a common mistake + to only run ``./mach build`` and miss the second command. + +Once these steps are complete, you can reproduce the bug locally using the same +steps as described above for the downloaded builds. + + +Developing new fuzzing targets +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Developing a new fuzzing target using the fuzzing interface only requires a few steps. + + +Determine if the fuzzing interface is the right tool +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +The fuzzing interface is not suitable for every kind of testing. In particular +if your testing requires the full browser to be running, then you might want to +look into other testing methods. + +The interface uses the ``ScopedXPCOM`` implementation to provide an environment +in which XPCOM is available and initialized. You can initialize further subsystems +that you might require, but you are responsible yourself for any kind of +initialization steps. + +There is (in theory) no limit as to how far you can take browser initialization. +However, the more subsystems are involved, the more problems might occur due to +non-determinism and loss of performance. + +If you are unsure if the fuzzing interface is the right approach for you or you +require help in evaluating what could be done for your particular task, please +don't hestitate to :ref:`contact us <Fuzzing#contact-us>`. + + +Develop the fuzzing code +^^^^^^^^^^^^^^^^^^^^^^^^ + +Where to put your fuzzing code +'''''''''''''''''''''''''''''' + +The code using the fuzzing interface usually lives in a separate directory +called ``fuzztest`` that is on the same level as gtests. If your component +has no gtests, then a subdirectory either in tests or in your main directory +will work. If such a directory does not exist yet in your component, then you +need to create one with a suitable ``moz.build``. See `the transport target +for an example <https://searchfox.org/mozilla-central/source/dom/media/webrtc/transport/fuzztest/moz.build>`__ + +In order to include the new subdirectory into the build process, you will +also have to modify the toplevel ``moz.build`` file accordingly. For this +purpose, you should add your directory to ``TEST_DIRS`` only if ``FUZZING_INTERFACES`` +is set. See again `the transport target for an example +<https://searchfox.org/mozilla-central/rev/de7676288a78b70d2b9927c79493adbf294faad5/media/mtransport/moz.build#18-24>`__. + +How your code should look like +'''''''''''''''''''''''''''''' + +In order to define your fuzzing target ``MyTarget``, you only need to implement 2 functions: + +1. A one-time initialization function. + + At startup, the fuzzing interface calls this function **once**, so this can + be used to perform one-time operations like initializing subsystems or parsing + extra fuzzing options. + + This function is the equivalent of the `LLVMFuzzerInitialize <https://llvm.org/docs/LibFuzzer.html#startup-initialization>`__ + function and has the same signature. However, with our fuzzing interface, + it won't be resolved by its name, so it can be defined ``static`` and called + whatever you prefer. Note that the function should always ``return 0`` and + can (except for the return), remain empty. + + For the sake of this documentation, we assume that you have ``static int FuzzingInitMyTarget(int* argc, char*** argv);`` + +2. The fuzzing iteration function. + + This is where the actual fuzzing happens, and this function is the equivalent + of `LLVMFuzzerTestOneInput <https://llvm.org/docs/LibFuzzer.html#fuzz-target>`__. + Again, the difference to the fuzzing interface is that the function won't be + resolved by its name. In addition, we offer two different possible signatures + for this function, either + + ``static int FuzzingRunMyTarget(const uint8_t* data, size_t size);`` + + or + + ``static int FuzzingRunMyTarget(nsCOMPtr<nsIInputStream> inputStream);`` + + The latter is just a wrapper around the first one for implementations that + usually work with streams. No matter which of the two signatures you choose + to work with, the only thing you need to implement inside the function + is the use of the provided data with your target implementation. This can + mean to simply feed the data to your target, using the data to drive operations + on the target API, or a mix of both. + + While doing so, you should avoid altering global state in a permanent way, + using additional sources of data/randomness or having code run beyond the + lifetime of the iteration function (e.g. on another thread), for one simple + reason: Coverage-guided fuzzing tools depend on the **deterministic** nature + of the iteration function. If the same input to this function does not lead + to the same execution when run twice (e.g. because the resulting state depends + on multiple successive calls or because of additional external influences), + then the tool will not be able to reproduce its fuzzing progress and perform + badly. Dealing with this restriction can be challenging e.g. when dealing + with asynchronous targets that run multi-threaded, but can usually be managed + by synchronizing execution on all threads at the end of the iteration function. + For implementations accumulating global state, it might be necessary to + (re)initialize this global state in each iteration, rather than doing it once + in the initialization function, even if this costs additional performance. + + Note that unlike the vanilla libFuzzer approach, you are allowed to ``return 1`` + in this function to indicate that an input is "bad". Doing so will cause + libFuzzer to discard the input, no matter if it generated new coverage or not. + This is particularly useful if you have means to internally detect and catch + bad testcase behavior such as timeouts/excessive resource usage etc. to avoid + these tests to end up in your corpus. + + +Once you have implemented the two functions, the only thing remaining is to +register them with the fuzzing interface. For this purpose, we offer two +macros, depending on which iteration function signature you used. If you +sticked to the classic signature using buffer and size, you can simply use + +:: + + #include "FuzzingInterface.h" + + // Your includes and code + + MOZ_FUZZING_INTERFACE_RAW(FuzzingInitMyTarget, FuzzingRunMyTarget, MyTarget); + +where ``MyTarget`` is the name of the target and will be used later to decide +at runtime which target should be used. + +If instead you went for the streaming interface, you need a different include, +but the macro invocation is quite similar: + +:: + + #include "FuzzingInterfaceStream.h" + + // Your includes and code + + MOZ_FUZZING_INTERFACE_STREAM(FuzzingInitMyTarget, FuzzingRunMyTarget, MyTarget); + +For a live example, see also the `implementation of the STUN fuzzing target +<https://searchfox.org/mozilla-central/source/dom/media/webrtc/transport/fuzztest/stun_parser_libfuzz.cpp>`__. + +Add instrumentation to the code being tested +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +libFuzzer requires that the code you are trying to test is instrumented +with special compiler flags. Fortunately, adding these on a per-directory basis +can be done just by including the following directive in each ``moz.build`` +file that builds code under test: + +:: + + # Add libFuzzer configuration directives + include('/tools/fuzzing/libfuzzer-config.mozbuild') + + +The include already does the appropriate configuration checks to be only +active in fuzzing builds, so you don't have to guard this in any way. + +.. note:: + + **Note:** This include modifies `CFLAGS` and `CXXFLAGS` accordingly + but this only works for source files defined in this particular + directory. The flags are **not** propagated to subdirectories automatically + and you have to ensure that each directory that builds source files + for your target has the include added to its ``moz.build`` file. + +By keeping the instrumentation limited to the parts that are actually being +tested using this tool, you not only increase the performance but also potentially +reduce the amount of noise that libFuzzer sees. + + +Build your code +^^^^^^^^^^^^^^^ + +See the :ref:`Build instructions above <Local build requirements and flags>` for instructions +how to modify your ``.mozconfig`` to create the appropriate build. + + +Running your code and building a corpus +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +You need to set the following environment variable to enable running the +fuzzing code inside Firefox instead of the regular browser. + +- ``FUZZER=name`` + +Where ``name`` is the name of your fuzzing module that you specified +when calling the ``MOZ_FUZZING_INTERFACE_RAW`` macro. For the example +above, this would be ``MyTarget`` or ``StunParser`` for the live example. + +Now when you invoke the firefox binary in your build directory with the +``-help=1`` parameter, you should see the regular libFuzzer help. On +Linux for example: + +:: + + $ FUZZER=StunParser obj-asan/dist/bin/firefox -help=1 + +You should see an output similar to this: + +:: + + Running Fuzzer tests... + Usage: + + To run fuzzing pass 0 or more directories. + obj-asan/dist/bin/firefox [-flag1=val1 [-flag2=val2 ...] ] [dir1 [dir2 ...] ] + + To run individual tests without fuzzing pass 1 or more files: + obj-asan/dist/bin/firefox [-flag1=val1 [-flag2=val2 ...] ] file1 [file2 ...] + + Flags: (strictly in form -flag=value) + verbosity 1 Verbosity level. + seed 0 Random seed. If 0, seed is generated. + runs -1 Number of individual test runs (-1 for infinite runs). + max_len 0 Maximum length of the test input. If 0, libFuzzer tries to guess a good value based on the corpus and reports it. + ... + + +Reproducing a Crash +''''''''''''''''''' + +In order to reproduce a crash from a given test file, simply put the +file as the only argument on the command line, e.g. + +:: + + $ FUZZER=StunParser obj-asan/dist/bin/firefox test.bin + +This should reproduce the given problem. + + +FuzzManager and libFuzzer +''''''''''''''''''''''''' + +Our FuzzManager project comes with a harness for running libFuzzer with +an optional connection to a FuzzManager server instance. Note that this +connection is not mandatory, even without a server you can make use of +the local harness. + +You can find the harness +`here <https://github.com/MozillaSecurity/FuzzManager/tree/master/misc/afl-libfuzzer>`__. + +An example invocation for the harness to use with StunParser could look +like this: + +:: + + FUZZER=StunParser python /path/to/afl-libfuzzer-daemon.py --fuzzmanager \ + --stats libfuzzer-stunparser.stats --libfuzzer-auto-reduce-min 500 --libfuzzer-auto-reduce 30 \ + --tool libfuzzer-stunparser --libfuzzer --libfuzzer-instances 6 obj-asan/dist/bin/firefox \ + -max_len=256 -use_value_profile=1 -rss_limit_mb=3000 corpus-stunparser + +What this does is + +- run libFuzzer on the ``StunParser`` target with 6 parallel instances + using the corpus in the ``corpus-stunparser`` directory (with the + specified libFuzzer options such as ``-max_len`` and + ``-use_value_profile``) +- automatically reduce the corpus and restart if it grew by 30% (and + has at least 500 files) +- use FuzzManager (need a local ``.fuzzmanagerconf`` and a + ``firefox.fuzzmanagerconf`` binary configuration as described in the + FuzzManager manual) and submit crashes as ``libfuzzer-stunparser`` + tool +- write statistics to the ``libfuzzer-stunparser.stats`` file + + +JS Engine Specifics +~~~~~~~~~~~~~~~~~~~ + +The fuzzing interface can also be used for testing the JS engine, in fact there +are two separate options to implement and run fuzzing targets: + +Implementing in C++ +^^^^^^^^^^^^^^^^^^^ + +Similar to the fuzzing interface in Firefox, you can implement your target in +entirely C++ with very similar interfaces compared to what was described before. + +There are a few minor differences though: + +1. All of the fuzzing targets live in `js/src/fuzz-tests`. + +2. All of the code is linked into a separate binary called `fuzz-tests`, + similar to how all JSAPI tests end up in `jsapi-tests`. In order for this + binary to be built, you must build a JS shell with ``--enable-fuzzing`` + **and** ``--enable-tests``. Again, this can and should be combined with + AddressSanitizer for maximum effectiveness. This also means that there is no + need to (re)build gtests when dealing with a JS fuzzing target and using + a shell as part of a full browser build. + +3. The harness around the JS implementation already provides you with an + initialized ``JSContext`` and global object. You can access these in + your target by declaring + + ``extern JS::PersistentRootedObject gGlobal;`` + + and + + ``extern JSContext* gCx;`` + + but there is no obligation for you to use these. + +For a live example, see also the `implementation of the StructuredCloneReader target +<https://searchfox.org/mozilla-central/source/js/src/fuzz-tests/testStructuredCloneReader.cpp>`__. + + +Implementing in JS +^^^^^^^^^^^^^^^^^^ + +In addition to the C++ targets, you can also implement targets in JavaScript +using the JavaScript Runtime (JSRT) fuzzing approach. Using this approach is +not only much simpler (since you don't need to know anything about the +JSAPI or engine internals), but it also gives you full access to everything +defined in the JS shell, including handy functions such as ``timeout()``. + +Of course, this approach also comes with disadvantages: Calling into JS and +performing the fuzzing operations there costs performance. Also, there is more +chance for causing global side-effects or non-determinism compared to a +fairly isolated C++ target. + +As a rule of thumb, you should implement the target in JS if + +* you don't know C++ and/or how to use the JSAPI (after all, a JS fuzzing target is better than none), +* your target is expected to have lots of hangs/timeouts (you can catch these internally), +* or your target is not isolated enough for a C++ target and/or you need specific JS shell functions. + + +There is an `example target <https://searchfox.org/mozilla-central/source/js/src/shell/jsrtfuzzing/jsrtfuzzing-example.js>`__ +in-tree that shows roughly how to implement such a fuzzing target. + +To run such a target, you must run the ``js`` (shell) binary instead of the +``fuzz-tests`` binary and point the ``FUZZER`` variable to the file containing +your fuzzing target, e.g. + +:: + + $ FUZZER=/path/to/jsrtfuzzing-example.js obj-asan/dist/bin/js --fuzzing-safe --no-threads -- <libFuzzer options here> + +More elaborate targets can be found in `js/src/fuzz-tests/ <https://searchfox.org/mozilla-central/source/js/src/fuzz-tests/>`__. + +Troubleshooting +~~~~~~~~~~~~~~~ + + +Fuzzing Interface: Error: No testing callback found +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +This error means that the fuzzing callback with the name you specified +using the ``FUZZER`` environment variable could not be found. Reasons +for are typically either a misspelled name or that your code wasn't +built (check your ``moz.build`` file and build log). + + +``mach build`` doesn't seem to update my fuzzing code +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Keep in mind you always need to run both the ``mach build`` and +``mach gtest dontruntests`` commands in order to update your fuzzing +code. The latter rebuilds the gtest version of ``libxul``, containing +your code. |