Reviewed-by: Darren Kenny <darren.kenny@oracle.com> Signed-off-by: Alexander Bulekov <alxndr@bu.edu> Message-Id: <20201023150746.107063-13-alxndr@bu.edu> Signed-off-by: Thomas Huth <thuth@redhat.com>
215 lines
9.5 KiB
Plaintext
215 lines
9.5 KiB
Plaintext
= Fuzzing =
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== Introduction ==
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This document describes the virtual-device fuzzing infrastructure in QEMU and
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how to use it to implement additional fuzzers.
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== Basics ==
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Fuzzing operates by passing inputs to an entry point/target function. The
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fuzzer tracks the code coverage triggered by the input. Based on these
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findings, the fuzzer mutates the input and repeats the fuzzing.
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To fuzz QEMU, we rely on libfuzzer. Unlike other fuzzers such as AFL, libfuzzer
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is an _in-process_ fuzzer. For the developer, this means that it is their
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responsibility to ensure that state is reset between fuzzing-runs.
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== Building the fuzzers ==
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NOTE: If possible, build a 32-bit binary. When forking, the 32-bit fuzzer is
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much faster, since the page-map has a smaller size. This is due to the fact that
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AddressSanitizer mmaps ~20TB of memory, as part of its detection. This results
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in a large page-map, and a much slower fork().
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To build the fuzzers, install a recent version of clang:
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Configure with (substitute the clang binaries with the version you installed).
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Here, enable-sanitizers, is optional but it allows us to reliably detect bugs
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such as out-of-bounds accesses, use-after-frees, double-frees etc.
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CC=clang-8 CXX=clang++-8 /path/to/configure --enable-fuzzing \
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--enable-sanitizers
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Fuzz targets are built similarly to system/softmmu:
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make i386-softmmu/fuzz
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This builds ./i386-softmmu/qemu-fuzz-i386
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The first option to this command is: --fuzz-target=FUZZ_NAME
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To list all of the available fuzzers run qemu-fuzz-i386 with no arguments.
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For example:
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./i386-softmmu/qemu-fuzz-i386 --fuzz-target=virtio-scsi-fuzz
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Internally, libfuzzer parses all arguments that do not begin with "--".
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Information about these is available by passing -help=1
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Now the only thing left to do is wait for the fuzzer to trigger potential
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crashes.
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== Useful libFuzzer flags ==
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As mentioned above, libFuzzer accepts some arguments. Passing -help=1 will list
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the available arguments. In particular, these arguments might be helpful:
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$CORPUS_DIR/ : Specify a directory as the last argument to libFuzzer. libFuzzer
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stores each "interesting" input in this corpus directory. The next time you run
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libFuzzer, it will read all of the inputs from the corpus, and continue fuzzing
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from there. You can also specify multiple directories. libFuzzer loads existing
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inputs from all specified directories, but will only write new ones to the
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first one specified.
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-max_len=4096 : specify the maximum byte-length of the inputs libFuzzer will
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generate.
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-close_fd_mask={1,2,3} : close, stderr, or both. Useful for targets that
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trigger many debug/error messages, or create output on the serial console.
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-jobs=4 -workers=4 : These arguments configure libFuzzer to run 4 fuzzers in
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parallel (4 fuzzing jobs in 4 worker processes). Alternatively, with only
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-jobs=N, libFuzzer automatically spawns a number of workers less than or equal
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to half the available CPU cores. Replace 4 with a number appropriate for your
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machine. Make sure to specify a $CORPUS_DIR, which will allow the parallel
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fuzzers to share information about the interesting inputs they find.
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-use_value_profile=1 : For each comparison operation, libFuzzer computes
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(caller_pc&4095) | (popcnt(Arg1 ^ Arg2) << 12) and places this in the coverage
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table. Useful for targets with "magic" constants. If Arg1 came from the fuzzer's
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input and Arg2 is a magic constant, then each time the Hamming distance
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between Arg1 and Arg2 decreases, libFuzzer adds the input to the corpus.
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-shrink=1 : Tries to make elements of the corpus "smaller". Might lead to
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better coverage performance, depending on the target.
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Note that libFuzzer's exact behavior will depend on the version of
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clang and libFuzzer used to build the device fuzzers.
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== Generating Coverage Reports ==
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Code coverage is a crucial metric for evaluating a fuzzer's performance.
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libFuzzer's output provides a "cov: " column that provides a total number of
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unique blocks/edges covered. To examine coverage on a line-by-line basis we
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can use Clang coverage:
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1. Configure libFuzzer to store a corpus of all interesting inputs (see
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CORPUS_DIR above)
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2. ./configure the QEMU build with:
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--enable-fuzzing \
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--extra-cflags="-fprofile-instr-generate -fcoverage-mapping"
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3. Re-run the fuzzer. Specify $CORPUS_DIR/* as an argument, telling libfuzzer
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to execute all of the inputs in $CORPUS_DIR and exit. Once the process
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exits, you should find a file, "default.profraw" in the working directory.
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4. Execute these commands to generate a detailed HTML coverage-report:
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llvm-profdata merge -output=default.profdata default.profraw
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llvm-cov show ./path/to/qemu-fuzz-i386 -instr-profile=default.profdata \
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--format html -output-dir=/path/to/output/report
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== Adding a new fuzzer ==
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Coverage over virtual devices can be improved by adding additional fuzzers.
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Fuzzers are kept in tests/qtest/fuzz/ and should be added to
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tests/qtest/fuzz/Makefile.include
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Fuzzers can rely on both qtest and libqos to communicate with virtual devices.
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1. Create a new source file. For example ``tests/qtest/fuzz/foo-device-fuzz.c``.
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2. Write the fuzzing code using the libqtest/libqos API. See existing fuzzers
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for reference.
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3. Register the fuzzer in ``tests/fuzz/Makefile.include`` by appending the
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corresponding object to fuzz-obj-y
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Fuzzers can be more-or-less thought of as special qtest programs which can
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modify the qtest commands and/or qtest command arguments based on inputs
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provided by libfuzzer. Libfuzzer passes a byte array and length. Commonly the
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fuzzer loops over the byte-array interpreting it as a list of qtest commands,
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addresses, or values.
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== The Generic Fuzzer ==
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Writing a fuzz target can be a lot of effort (especially if a device driver has
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not be built-out within libqos). Many devices can be fuzzed to some degree,
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without any device-specific code, using the generic-fuzz target.
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The generic-fuzz target is capable of fuzzing devices over their PIO, MMIO,
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and DMA input-spaces. To apply the generic-fuzz to a device, we need to define
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two env-variables, at minimum:
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QEMU_FUZZ_ARGS= is the set of QEMU arguments used to configure a machine, with
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the device attached. For example, if we want to fuzz the virtio-net device
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attached to a pc-i440fx machine, we can specify:
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QEMU_FUZZ_ARGS="-M pc -nodefaults -netdev user,id=user0 \
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-device virtio-net,netdev=user0"
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QEMU_FUZZ_OBJECTS= is a set of space-delimited strings used to identify the
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MemoryRegions that will be fuzzed. These strings are compared against
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MemoryRegion names and MemoryRegion owner names, to decide whether each
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MemoryRegion should be fuzzed. These strings support globbing. For the
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virtio-net example, we could use QEMU_FUZZ_OBJECTS=
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* 'virtio-net'
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* 'virtio*'
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* 'virtio* pcspk' (Fuzz the virtio devices and the PC speaker...)
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* '*' (Fuzz the whole machine)
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The "info mtree" and "info qom-tree" monitor commands can be especially useful
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for identifying the MemoryRegion and Object names used for matching.
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As a generic rule-of-thumb, the more MemoryRegions/Devices we match, the greater
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the input-space, and the smaller the probability of finding crashing inputs for
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individual devices. As such, it is usually a good idea to limit the fuzzer to
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only a few MemoryRegions.
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To ensure that these env variables have been configured correctly, we can use:
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./qemu-fuzz-i386 --fuzz-target=generic-fuzz -runs=0
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The output should contain a complete list of matched MemoryRegions.
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= Implementation Details =
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== The Fuzzer's Lifecycle ==
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The fuzzer has two entrypoints that libfuzzer calls. libfuzzer provides it's
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own main(), which performs some setup, and calls the entrypoints:
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LLVMFuzzerInitialize: called prior to fuzzing. Used to initialize all of the
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necessary state
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LLVMFuzzerTestOneInput: called for each fuzzing run. Processes the input and
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resets the state at the end of each run.
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In more detail:
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LLVMFuzzerInitialize parses the arguments to the fuzzer (must start with two
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dashes, so they are ignored by libfuzzer main()). Currently, the arguments
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select the fuzz target. Then, the qtest client is initialized. If the target
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requires qos, qgraph is set up and the QOM/LIBQOS modules are initialized.
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Then the QGraph is walked and the QEMU cmd_line is determined and saved.
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After this, the vl.c:qemu__main is called to set up the guest. There are
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target-specific hooks that can be called before and after qemu_main, for
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additional setup(e.g. PCI setup, or VM snapshotting).
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LLVMFuzzerTestOneInput: Uses qtest/qos functions to act based on the fuzz
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input. It is also responsible for manually calling the main loop/main_loop_wait
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to ensure that bottom halves are executed and any cleanup required before the
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next input.
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Since the same process is reused for many fuzzing runs, QEMU state needs to
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be reset at the end of each run. There are currently two implemented
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options for resetting state:
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1. Reboot the guest between runs.
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Pros: Straightforward and fast for simple fuzz targets.
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Cons: Depending on the device, does not reset all device state. If the
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device requires some initialization prior to being ready for fuzzing
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(common for QOS-based targets), this initialization needs to be done after
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each reboot.
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Example target: i440fx-qtest-reboot-fuzz
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2. Run each test case in a separate forked process and copy the coverage
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information back to the parent. This is fairly similar to AFL's "deferred"
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fork-server mode [3]
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Pros: Relatively fast. Devices only need to be initialized once. No need
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to do slow reboots or vmloads.
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Cons: Not officially supported by libfuzzer. Does not work well for devices
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that rely on dedicated threads.
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Example target: virtio-net-fork-fuzz
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