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docs/fuzz: rST-ify the fuzzing documentation
Signed-off-by: Alexander Bulekov <alxndr@bu.edu> Message-Id: <20201106180600.360110-2-alxndr@bu.edu> Signed-off-by: Thomas Huth <thuth@redhat.com>
This commit is contained in:
Alexander Bulekov
Thomas Huth
parent
3758e88bb8
commit
f3a0208f24
@ -2525,7 +2525,7 @@ R: Thomas Huth <thuth@redhat.com>
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S: Maintained
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F: tests/qtest/fuzz/
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F: scripts/oss-fuzz/
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F: docs/devel/fuzzing.txt
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F: docs/devel/fuzzing.rst
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Register API
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M: Alistair Francis <alistair@alistair23.me>
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236
docs/devel/fuzzing.rst
Normal file
236
docs/devel/fuzzing.rst
Normal file
@ -0,0 +1,236 @@
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========
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Fuzzing
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========
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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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------
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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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|
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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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|
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Building the fuzzers
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--------------------
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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 maps ~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 targets::
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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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----------------------
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As mentioned above, libFuzzer accepts some arguments. Passing ``-help=1`` will
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list 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.
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libFuzzer stores each "interesting" input in this corpus directory. The next
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time you run libFuzzer, it will read all of the inputs from the corpus, and
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continue fuzzing from there. You can also specify multiple directories.
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libFuzzer loads existing inputs from all specified directories, but will only
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write new ones to the first one specified.
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* ``-max_len=4096`` : specify the maximum byte-length of the inputs libFuzzer
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will 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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|
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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
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coverage table. Useful for targets with "magic" constants. If Arg1 came from
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the fuzzer's input and Arg2 is a magic constant, then each time the Hamming
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distance between Arg1 and Arg2 decreases, libFuzzer adds the input to the
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corpus.
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|
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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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|
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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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---------------------------
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|
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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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|
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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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|
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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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-------------------
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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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|
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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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------------------
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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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|
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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
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the 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 one of ::
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QEMU_FUZZ_OBJECTS='virtio-net'
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QEMU_FUZZ_OBJECTS='virtio*'
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QEMU_FUZZ_OBJECTS='virtio* pcspk' # Fuzz the virtio devices and the speaker
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QEMU_FUZZ_OBJECTS='*' # Fuzz the whole machine``
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The ``"info mtree"`` and ``"info qom-tree"`` monitor commands can be especially
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useful for identifying the ``MemoryRegion`` and ``Object`` names used for
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matching.
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As a generic rule-of-thumb, the more ``MemoryRegions``/Devices we match, the
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greater the input-space, and the smaller the probability of finding crashing
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inputs for individual devices. As such, it is usually a good idea to limit the
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fuzzer to 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 / Fuzzer Lifecycle
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-----------------------------------------
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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 ``main_loop_wait`` to ensure
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that bottom halves are executed and any cleanup required before the 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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- 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 (common
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for QOS-based targets), this initialization needs to be done after each
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reboot.
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- *Example target*: ``i440fx-qtest-reboot-fuzz``
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- 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 to
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do slow reboots or vmloads.
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- *Cons*: Not officially supported by libfuzzer. Does not work well for
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devices that rely on dedicated threads.
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- *Example target*: ``virtio-net-fork-fuzz``
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@ -1,214 +0,0 @@
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= Fuzzing =
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== Introduction ==
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|
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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.
|
||||
|
||||
== 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).
|
||||
Here, enable-sanitizers, is optional but it allows us to reliably detect bugs
|
||||
such as out-of-bounds accesses, use-after-frees, double-frees etc.
|
||||
|
||||
CC=clang-8 CXX=clang++-8 /path/to/configure --enable-fuzzing \
|
||||
--enable-sanitizers
|
||||
|
||||
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-target=FUZZ_NAME
|
||||
To list all of the available fuzzers run qemu-fuzz-i386 with no arguments.
|
||||
|
||||
For example:
|
||||
./i386-softmmu/qemu-fuzz-i386 --fuzz-target=virtio-scsi-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.
|
||||
|
||||
== Useful libFuzzer flags ==
|
||||
|
||||
As mentioned above, libFuzzer accepts some arguments. Passing -help=1 will list
|
||||
the available arguments. In particular, these arguments might be helpful:
|
||||
|
||||
$CORPUS_DIR/ : Specify a directory as the last argument to libFuzzer. libFuzzer
|
||||
stores each "interesting" input in this corpus directory. The next time you run
|
||||
libFuzzer, it will read all of the inputs from the corpus, and continue fuzzing
|
||||
from there. You can also specify multiple directories. libFuzzer loads existing
|
||||
inputs from all specified directories, but will only write new ones to the
|
||||
first one specified.
|
||||
|
||||
-max_len=4096 : specify the maximum byte-length of the inputs libFuzzer will
|
||||
generate.
|
||||
|
||||
-close_fd_mask={1,2,3} : close, stderr, or both. Useful for targets that
|
||||
trigger many debug/error messages, or create output on the serial console.
|
||||
|
||||
-jobs=4 -workers=4 : These arguments configure libFuzzer to run 4 fuzzers in
|
||||
parallel (4 fuzzing jobs in 4 worker processes). Alternatively, with only
|
||||
-jobs=N, libFuzzer automatically spawns a number of workers less than or equal
|
||||
to half the available CPU cores. Replace 4 with a number appropriate for your
|
||||
machine. Make sure to specify a $CORPUS_DIR, which will allow the parallel
|
||||
fuzzers to share information about the interesting inputs they find.
|
||||
|
||||
-use_value_profile=1 : For each comparison operation, libFuzzer computes
|
||||
(caller_pc&4095) | (popcnt(Arg1 ^ Arg2) << 12) and places this in the coverage
|
||||
table. Useful for targets with "magic" constants. If Arg1 came from the fuzzer's
|
||||
input and Arg2 is a magic constant, then each time the Hamming distance
|
||||
between Arg1 and Arg2 decreases, libFuzzer adds the input to the corpus.
|
||||
|
||||
-shrink=1 : Tries to make elements of the corpus "smaller". Might lead to
|
||||
better coverage performance, depending on the target.
|
||||
|
||||
Note that libFuzzer's exact behavior will depend on the version of
|
||||
clang and libFuzzer used to build the device fuzzers.
|
||||
|
||||
== Generating Coverage Reports ==
|
||||
Code coverage is a crucial metric for evaluating a fuzzer's performance.
|
||||
libFuzzer's output provides a "cov: " column that provides a total number of
|
||||
unique blocks/edges covered. To examine coverage on a line-by-line basis we
|
||||
can use Clang coverage:
|
||||
|
||||
1. Configure libFuzzer to store a corpus of all interesting inputs (see
|
||||
CORPUS_DIR above)
|
||||
2. ./configure the QEMU build with:
|
||||
--enable-fuzzing \
|
||||
--extra-cflags="-fprofile-instr-generate -fcoverage-mapping"
|
||||
3. Re-run the fuzzer. Specify $CORPUS_DIR/* as an argument, telling libfuzzer
|
||||
to execute all of the inputs in $CORPUS_DIR and exit. Once the process
|
||||
exits, you should find a file, "default.profraw" in the working directory.
|
||||
4. Execute these commands to generate a detailed HTML coverage-report:
|
||||
llvm-profdata merge -output=default.profdata default.profraw
|
||||
llvm-cov show ./path/to/qemu-fuzz-i386 -instr-profile=default.profdata \
|
||||
--format html -output-dir=/path/to/output/report
|
||||
|
||||
== 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.
|
||||
|
||||
== The Generic Fuzzer ==
|
||||
Writing a fuzz target can be a lot of effort (especially if a device driver has
|
||||
not be built-out within libqos). Many devices can be fuzzed to some degree,
|
||||
without any device-specific code, using the generic-fuzz target.
|
||||
|
||||
The generic-fuzz target is capable of fuzzing devices over their PIO, MMIO,
|
||||
and DMA input-spaces. To apply the generic-fuzz to a device, we need to define
|
||||
two env-variables, at minimum:
|
||||
|
||||
QEMU_FUZZ_ARGS= is the set of QEMU arguments used to configure a machine, with
|
||||
the device attached. For example, if we want to fuzz the virtio-net device
|
||||
attached to a pc-i440fx machine, we can specify:
|
||||
QEMU_FUZZ_ARGS="-M pc -nodefaults -netdev user,id=user0 \
|
||||
-device virtio-net,netdev=user0"
|
||||
|
||||
QEMU_FUZZ_OBJECTS= is a set of space-delimited strings used to identify the
|
||||
MemoryRegions that will be fuzzed. These strings are compared against
|
||||
MemoryRegion names and MemoryRegion owner names, to decide whether each
|
||||
MemoryRegion should be fuzzed. These strings support globbing. For the
|
||||
virtio-net example, we could use QEMU_FUZZ_OBJECTS=
|
||||
* 'virtio-net'
|
||||
* 'virtio*'
|
||||
* 'virtio* pcspk' (Fuzz the virtio devices and the PC speaker...)
|
||||
* '*' (Fuzz the whole machine)
|
||||
|
||||
The "info mtree" and "info qom-tree" monitor commands can be especially useful
|
||||
for identifying the MemoryRegion and Object names used for matching.
|
||||
|
||||
As a generic rule-of-thumb, the more MemoryRegions/Devices we match, the greater
|
||||
the input-space, and the smaller the probability of finding crashing inputs for
|
||||
individual devices. As such, it is usually a good idea to limit the fuzzer to
|
||||
only a few MemoryRegions.
|
||||
|
||||
To ensure that these env variables have been configured correctly, we can use:
|
||||
|
||||
./qemu-fuzz-i386 --fuzz-target=generic-fuzz -runs=0
|
||||
|
||||
The output should contain a complete list of matched MemoryRegions.
|
||||
|
||||
= 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
|
||||
@ -22,6 +22,7 @@ Contents:
|
||||
stable-process
|
||||
testing
|
||||
qtest
|
||||
fuzzing
|
||||
decodetree
|
||||
secure-coding-practices
|
||||
tcg
|
||||
|
||||
Loading…
Reference in New Issue
Block a user