Fuzzing

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 maps ~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 targets:

make qemu-fuzz-i386

This builds ./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:

./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 one of

  QEMU_FUZZ_OBJECTS='virtio-net'
  QEMU_FUZZ_OBJECTS='virtio*'
  QEMU_FUZZ_OBJECTS='virtio* pcspk' # Fuzz the virtio devices and the speaker
  QEMU_FUZZ_OBJECTS='*' # 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 / Fuzzer 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 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:

• 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

• 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