175 lines
7.8 KiB
Text
175 lines
7.8 KiB
Text
= 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).
|
|
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.
|
|
|
|
= 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
|