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Nyx mode

Nyx is a full system emulation fuzzing mode that supports snapshotting and can be used for both source code based instrumentation and binary-only targets.

It is recommended to be used if the target cannot be fuzzed in persistent mode (so default fork mode fuzzing is used).

It is only available on Linux and is currently restricted to x86_x64 however aarch64 support is in the works (but the host must then run on aarch64 too).

Underneath it is built upon KVM and QEMU and requires a modern Linux kernel (5.11+) for fuzzing source code based instrumented targets (e.g., afl-clang-fast). To fuzz binary-only targets, this is done via Intel PT and requires an Intel processor (6th generation onwards) and a special 5.10 kernel (see KVM-Nyx).

Building Nyx mode

  1. Install all the packages from docs/INSTALL.md.

  2. Additionally, install the following packages:

    apt-get install -y libgtk-3-dev pax-utils python3-msgpack python3-jinja2

  3. As Nyx is written in Rust, install the newest rust compiler (rust packages in the Linux distribution are usually too old to be able to build Nyx):

    curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

  4. Finally build Nyx mode:

    ./build_nyx_support.sh

  5. Optionally, for binary-only fuzzing: set up the required 5.10 kernel, see KVM-Nyx.

Preparing to fuzz a target with Nyx mode

For source instrumented fuzzing you can use any afl-cc mode, with LTO even auto-dictionary is supported. Note the CMPLOG is currently not supported (yet).

Nyx uses full system emulation hence your fuzzing targets have to be especially packaged.

With your target ready at hand execute the following command (note that for binary-only fuzzing with the special 5.10 kernel switch the option instrumentation below with processor_trace):

python3 nyx_mode/packer/packer/nyx_packer.py \
    /PATH/TO/TARGET \
    PACKAGE-DIRECTORY \
    afl \
    instrumentation \
    --fast_reload_mode \
    --purge

This will create a directory with all necessary files and the Nyx configuration. The name of the directory will be whatever you choose for PACKAGE-DIRECTORY above.

Note that if the target reads from a file then use the -file /path/to/file parameter to the above command.

Note that Nyx does not support the afl @@ argument. Instead pass something like -file /foo.file -args "--file /foo.file --other-args" to the above command.

Then the final step: we generate the Nyx package configuration:

python3 nyx_mode/packer/packer/nyx_config_gen.py PACKAGE-DIRECTORY Kernel

Fuzzing with Nyx mode

Note that you need to load the kvm kernel modules for Nyx:

sudo modprobe -r kvm-intel
sudo modprobe -r kvm
sudo modprobe  kvm enable_vmware_backdoor=y
sudo modprobe  kvm-intel
cat /sys/module/kvm/parameters/enable_vmware_backdoor | grep -q Y && echi OK || echo KVM module problem

All the hard parts are done, fuzzing with Nyx mode is easy - just supply the PACKAGE-DIRECTORY as fuzzing target and specify the -X option to afl-fuzz:

afl-fuzz -i in -o out -X -- ./PACKAGE-DIRECTORY

If you get a forkserver error upon starting then you did not load the Linux kvm kernel modules, see above.

If you want to fuzz in parallel (and you should!), then this has to be done in a special way:

  • Instead of -X (standalone mode), you specify -Y (multiprocessor mode).
  • First, a Main afl-fuzz instance has to be started with -M 0.
  • Only afterwards you can start Secondary afl-fuzz instances, which must have an increasing number value, starting at 1, e.g., -S 1.
afl-fuzz -i in -o out -Y -M 0 -- ./PACKAGE-DIRECTORY
afl-fuzz -i in -o out -Y -S 1 -- ./PACKAGE-DIRECTORY
afl-fuzz -i in -o out -Y -S 2 -- ./PACKAGE-DIRECTORY

AFL++ companion tools (afl-showmap etc.)

AFL++ companion tools support Nyx mode and can be used to analyze or minimize one specific input or an entire output corpus. These tools work similarly to afl-fuzz.

To run a target with one of these tools, add the -X parameter to the command line to enable Nyx mode, and pass the path to a Nyx package directory:

afl-tmin -i in_file -o out_file -X  -- ./PACKAGE-DIRECTORY
afl-analyze -i in_file -X  -- ./PACKAGE-DIRECTORY
afl-showmap -i in_dir -o out_file -X -- ./PACKAGE-DIRECTORY
afl-cmin -i in_dir -o out_dir -X -- ./PACKAGE-DIRECTORY

On each program startup of one the AFL++ tools in Nyx mode, a Nyx VM is spawned, and a bootstrapping procedure is performed inside the VM to prepare the target environment. As a consequence, due to the bootstrapping procedure, the launch performance is much slower compared to other modes. However, this can be optimized by reusing an existing fuzzing snapshot to avoid the slow re-execution of the bootstrap procedure.

A fuzzing snapshot is automatically created and stored in the output directory at out_dir/workdir/snapshot/ by the first parent process of afl-fuzz if parallel mode is used. To enable this feature, set the path to an existing snapshot directory in the AFL_NYX_REUSE_SNAPSHOT environment variable and use the tools as usual:

afl-fuzz -i ./in_dir -o ./out_dir -Y -M 0 ./PACKAGE-DIRECTORY

AFL_NYX_REUSE_SNAPSHOT=./out_dir/workdir/snapshot/ afl-analyze -i in_file -X  -- ./PACKAGE-DIRECTORY

Real-world examples

Fuzzing libxml2 with AFL++ in Nyx-mode

This tutorial is based on the Fuzzing libxml2 with AFL++ tutorial.

Preparing libxml2

First, get the latest libxml2 source files by using git:

git clone https://gitlab.gnome.org/GNOME/libxml2
cd libxml2

Next, compile libxml2:

./autogen.sh
./configure --enable-shared=no
make CC=afl-clang-fast CXX=afl-clang-fast++ LD=afl-clang-fast

Nyx share directories

Nyx expects that the target is provided in a certain format. More specifically, the target is passed as a so-called „share directory“ to a Nyx-frontend implementation. The share directory contains the target as well as a folder containing all dependencies and other files that are copied over to the guest. But more importantly, this share directory also contains a bootstrap script (fuzz.shif you are using KVM-Nyxotherwise fuzz_no_pt.sh) that is also executed right after launching the fuzzer. Both bootstrap scripts use several tools to communicate with the "outer world":

  • hcat - this tool copies a given string to the host
  • hget - this program requests a file from the host's share directory
  • hget_bulk - an improved version of hget. It is quite useful if you want to transfer huge files. But please keep in mind that this version of hget has a much larger startup overhead and won't improve your transfer rates on small files (typically files smaller than 100 MB).
  • habort - this tool basically sends an abort signal to the host (useful if something went wrong during bootstrap)
  • hpush - a tool to transfer a given file to the host (the transferred file will be put in the dump/ folder of your Nyx workdir)

Those tools are all using hypercalls which are defined in packer/nyx.h. We will give some more examples later on how to use these hypercalls directly to implement custom fuzzing harnesses.

Pack libxml2 into Nyx sharedir format

To turn a given linux target into the Nyx format, you can simply use nyx_packer.py. To do so, move to the following directory:

cd nyx_mode/packer/packer

And run the tool with the following options to pack libxml2:

python3 ./nyx_packer.py \
    ~/libxml2/xmllint \
    /tmp/nyx_libxml2 \
    afl \
    instrumentation \
    -args "/tmp/input" \
    -file "/tmp/input" \
    --fast_reload_mode \
    --purge

In this example, the packer will take xmllint, recursively get all dependencies and put both into the specified share directory (/tmp/nyx_libxml2 in this case). Because we have selected the afl option, an ld_preload-based agent is also automatically built and put into the sharedir. Another option would be spec. Without going into too much detail here, the spec mode is only used by Nyx's spec-fuzzer implementation. Next, since our target is built with compile-time instrumentations, we must select the instrumentation option, otherwise we could also use processor-trace option to enable Intel-PT fuzzing on targets without instrumentation.

To specify that the input generated by the fuzzer is passed as a separate file to the target, we need to set the -file option. Otherwise, the input will be passed over to the target via stdin. To specify any required argv options, you can use the -args parameter.

In case you want to fuzz the target only with fast snapshots enabled, you can also set the --fast_reload_mode option to improve performance.

Finally, we need to generate a Nyx configuration file. Simply run the following command and you're good to proceed:

python3 ./nyx_config_gen.py /tmp/nyx_libxml2/ Kernel

Run Nyx mode

From here on, we are almost done. Move to the AFL++ top directory and start the fuzzer with the following arguments:

mkdir /tmp/in/          # create an input folder
echo "AAAA" >> /tmp/in/A    # create a dummy input file
 ./afl-fuzz -i /tmp/in/ -o /tmp/out -X /tmp/nyx_libxml2/

If everything has been successfully set up to this point, you will now be welcomed by the following AFL++ screen:

        american fuzzy lop ++3.15a {default} (/tmp/nyx_libxml2/) [fast] - NYX
┌─ process timing ────────────────────────────────────┬─ overall results ────┐
│        run time : 0 days, 0 hrs, 0 min, 14 sec      │  cycles done : 0     │
│   last new find : 0 days, 0 hrs, 0 min, 0 sec       │ corpus count : 96    │
│last saved crash : none seen yet                     │saved crashes : 0     │
│ last saved hang : none seen yet                     │  saved hangs : 0     │
├─ cycle progress ─────────────────────┬─ map coverage┴──────────────────────┤
│  now processing : 28.0 (29.2%)       │    map density : 2.17% / 3.61%      │
│  runs timed out : 0 (0.00%)          │ count coverage : 1.67 bits/tuple    │
├─ stage progress ─────────────────────┼─ findings in depth ─────────────────┤
│  now trying : havoc                  │ favored items : 27 (28.12%)         │
│ stage execs : 22.3k/32.8k (68.19%)   │  new edges on : 58 (60.42%)         │
│ total execs : 55.9k                  │ total crashes : 0 (0 saved)         │
│  exec speed : 3810/sec               │  total tmouts : 0 (0 saved)         │
├─ fuzzing strategy yields ────────────┴─────────────┬─ item geometry ───────┤
│   bit flips : disabled (default, enable with -D)   │    levels : 3         │
│  byte flips : disabled (default, enable with -D)   │   pending : 95        │
│ arithmetics : disabled (default, enable with -D)   │  pend fav : 27        │
│  known ints : disabled (default, enable with -D)   │ own finds : 95        │
│  dictionary : n/a                                  │  imported : 0         │
│havoc/splice : 57/32.8k, 0/0                        │ stability : 100.00%   │
│py/custom/rq : unused, unused, unused, unused       ├───────────────────────┘
│    trim/eff : n/a, disabled                        │          [cpu000: 25%]
└────────────────────────────────────────────────────┘

If you want to run the fuzzer in distributed mode, which might be especially useful if you want to keep your memory footprint low, we got you covered. To start an initiating parent process, which will also create the snapshot which is later shared across all other childs, simply run AFL++Nyx with the following arguments:

./afl-fuzz -i /tmp/in/ -o /tmp/out -d -Y -M 0 /tmp/nyx_libxml2/

To attach other child processes adjust the -S <id> and run the following command:

./afl-fuzz -i /tmp/in/ -o /tmp/out -d -Y -S 1 /tmp/nyx_libxml2/

If you want to disable fast snapshots (except for crashes), you can simply set the AFL_NYX_DISABLE_SNAPSHOT_MODE environment variable.

Nyx crash reports

If the Nyx agent detects a crash in the target application, it can pass additional information on that crash to AFL++ (assuming that the agent implements this feature). For each saved crashing input AFL++ will also create an additional file in the crashes directory with a .log file extension. Crash reports generated by the default agent shipped with the Nyx packer will contain information such as the faulting address and signal number. Additionally, if the target is compiled with AddressSanitizer, the crash report will also contain the entire ASan report.

From a technical perspective, the crash report is passed from QEMU-Nyx to AFL++ via a shared memory region called Nyx Auxiliary Buffer which is by default 4096 bytes in size. In this shared memory region a specific amount is reserved for the header (1408 bytes) and the remaining bytes can be used to transfer crash reports (also the hprintf feature utilizes the very same shared memory for transferring data). By default a crash report will be truncated to 2688 bytes. However, if you want to increase the size of the shared memory region, you can set the AFL_NYX_AUX_SIZE environment variable to a higher value (keep in mind that this value must be a multiple of 4096).

Run AFL++Nyx with a custom agent

Most of the common use-cases for linux userland targets are already handled by our general purpose agent implementation. But in case you want to build your own agent, or write a custom harness for a specific target or you just want to implement all the hypercall and shared memory communication on your own, you can use our custom harness example as a starting point for that. You can find the code in custom_harness/.

This custom harness can be statically compiled with by gcc or clang. There is no need to use an AFL compiler, because this agent implements its own very basic coverage tracking by simply setting specific bytes in the "coverage" bitmap after specific branches have been covered.

To prepare this target, we must first create a new folder that will later become the sharedir.

mkdir /tmp/nyx_custom_agent/

To compile this example, run the following command (remove the -DNO_PT_NYX option if you are using KVM-Nyx):

gcc example.c -DNO_PT_NYX -static -I ../packer/ -o /tmp/nyx_custom_agent/target

Copy both bootstrap scripts into the sharedir:

cp fuzz.sh /tmp/nyx_custom_agent
cp fuzz_no_pt.sh /tmp/nyx_custom_agent

Copy all htools executables into the sharedir:

cd ~/AFLplusplus/packer/packer/linux_x86_64-userspace/
sh compile_64.sh
cp bin64/h* /tmp/nyx_custom_agent/

And finally, generate a Nyx configuration:

cd ~/AFLplusplus/packer/packer
python3 ./nyx_config_gen.py /tmp/nyx_custom_agent/ Kernel