Signed-off-by: Alexander Bulekov <alx...@bu.edu> Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com> --- docs/devel/fuzzing.txt | 116 +++++++++++++++++++++++++++++++++++++++++ 1 file changed, 116 insertions(+) create mode 100644 docs/devel/fuzzing.txt
diff --git a/docs/devel/fuzzing.txt b/docs/devel/fuzzing.txt new file mode 100644 index 0000000000..324d2cd92b --- /dev/null +++ b/docs/devel/fuzzing.txt @@ -0,0 +1,116 @@ += Fuzzing = + +== Introduction == + +This document describes the virtual-device fuzzing infrastructure in QEMU and +how to use it to implement additional fuzzers. + +== Basics == + +Fuzzing operates by passing inputs to an entry point/target function. The +fuzzer tracks the code coverage triggered by the input. Based on these +findings, the fuzzer mutates the input and repeats the fuzzing. + +To fuzz QEMU, we rely on libfuzzer. Unlike other fuzzers such as AFL, libfuzzer +is an _in-process_ fuzzer. For the developer, this means that it is their +responsibility to ensure that state is reset between fuzzing-runs. + +== Building the fuzzers == + +NOTE: If possible, build a 32-bit binary. When forking, the 32-bit fuzzer is +much faster, since the page-map has a smaller size. This is due to the fact that +AddressSanitizer mmaps ~20TB of memory, as part of its detection. This results +in a large page-map, and a much slower fork(). + +To build the fuzzers, install a recent version of clang: +Configure with (substitute the clang binaries with the version you installed): + + CC=clang-8 CXX=clang++-8 /path/to/configure --enable-fuzzing + +Fuzz targets are built similarly to system/softmmu: + + make i386-softmmu/fuzz + +This builds ./i386-softmmu/qemu-fuzz-i386 + +The first option to this command is: --fuzz_taget=FUZZ_NAME +To list all of the available fuzzers run qemu-fuzz-i386 with no arguments. + +eg: + ./i386-softmmu/qemu-fuzz-i386 --fuzz-target=virtio-net-fork-fuzz + +Internally, libfuzzer parses all arguments that do not begin with "--". +Information about these is available by passing -help=1 + +Now the only thing left to do is wait for the fuzzer to trigger potential +crashes. + +== Adding a new fuzzer == +Coverage over virtual devices can be improved by adding additional fuzzers. +Fuzzers are kept in tests/qtest/fuzz/ and should be added to +tests/qtest/fuzz/Makefile.include + +Fuzzers can rely on both qtest and libqos to communicate with virtual devices. + +1. Create a new source file. For example ``tests/qtest/fuzz/foo-device-fuzz.c``. + +2. Write the fuzzing code using the libqtest/libqos API. See existing fuzzers +for reference. + +3. Register the fuzzer in ``tests/fuzz/Makefile.include`` by appending the +corresponding object to fuzz-obj-y + +Fuzzers can be more-or-less thought of as special qtest programs which can +modify the qtest commands and/or qtest command arguments based on inputs +provided by libfuzzer. Libfuzzer passes a byte array and length. Commonly the +fuzzer loops over the byte-array interpreting it as a list of qtest commands, +addresses, or values. + += Implementation Details = + +== The Fuzzer's Lifecycle == + +The fuzzer has two entrypoints that libfuzzer calls. libfuzzer provides it's +own main(), which performs some setup, and calls the entrypoints: + +LLVMFuzzerInitialize: called prior to fuzzing. Used to initialize all of the +necessary state + +LLVMFuzzerTestOneInput: called for each fuzzing run. Processes the input and +resets the state at the end of each run. + +In more detail: + +LLVMFuzzerInitialize parses the arguments to the fuzzer (must start with two +dashes, so they are ignored by libfuzzer main()). Currently, the arguments +select the fuzz target. Then, the qtest client is initialized. If the target +requires qos, qgraph is set up and the QOM/LIBQOS modules are initialized. +Then the QGraph is walked and the QEMU cmd_line is determined and saved. + +After this, the vl.c:qemu__main is called to set up the guest. There are +target-specific hooks that can be called before and after qemu_main, for +additional setup(e.g. PCI setup, or VM snapshotting). + +LLVMFuzzerTestOneInput: Uses qtest/qos functions to act based on the fuzz +input. It is also responsible for manually calling the main loop/main_loop_wait +to ensure that bottom halves are executed and any cleanup required before the +next input. + +Since the same process is reused for many fuzzing runs, QEMU state needs to +be reset at the end of each run. There are currently two implemented +options for resetting state: +1. Reboot the guest between runs. + Pros: Straightforward and fast for simple fuzz targets. + Cons: Depending on the device, does not reset all device state. If the + device requires some initialization prior to being ready for fuzzing + (common for QOS-based targets), this initialization needs to be done after + each reboot. + Example target: i440fx-qtest-reboot-fuzz +2. Run each test case in a separate forked process and copy the coverage + information back to the parent. This is fairly similar to AFL's "deferred" + fork-server mode [3] + Pros: Relatively fast. Devices only need to be initialized once. No need + to do slow reboots or vmloads. + Cons: Not officially supported by libfuzzer. Does not work well for devices + that rely on dedicated threads. + Example target: virtio-net-fork-fuzz -- 2.23.0
