Fuzzing libxml2 with AFL++ in Nyx-mode
This tutorial is based on the Fuzzing libxml2 with AFL++ tutorial.
Initial Setup
Setting up our fork of AFL++ and its Nyx backend is rather simple. But there are some requirements: first, Nyx expects either a recent linux kernel (>= 5.11) with KVM or KVM-Nyx to be installed. In both cases, access to the /dev/kvm device is required. So, make sure that the permissions are fine.
To get started, check out our repository and run our setup script to install libnyx, Nyx's packer utilities and QEMU-Nyx:
git clone https://github.com/nyx-fuzz/AFLplusplus-Nyx.git
cd AFLplusplus-Nyx
compile_nyx_mode.sh
Preparing libxml2
This part is basically the same as described in the original tutorial. First, get the latest libxml2 source files by using git:
git clone https://gitlab.gnome.org/GNOME/libxml2.git
cd libxml2
From there on, you have to use -- at least for now -- our AFL-compiler instead of the AFL++ provided compiler. The reason for this is that we currently don't support all of AFL++'s features. Basically, we don't have full support for collision-free bitmaps yet. So, to continue run the following commands and adjust the path to our compiler (our compiler is located in the packer repository):
./autogen.sh
./configure --enable-shared=no
make CC=~/AFLplusplus-Nyx/packer/packer/compiler/afl-clang-fast CXX=~/AFLplusplus-Nyx/packer/packer/compiler/afl-clang-fast++ LD=~/AFLplusplus-Nyx/packer/packer/compiler/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 wells 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 hosthget- this program requests a file from the host's share directoryhget_bulk- an improved version ofhget. It is quite useful if you want to transfer huge files. But please keep in mind that this version ofhgethas a much larger startup overhead and won't improve your transfer rates on small files (typically files smaller than 100MB).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 transfered file will be put in thedump/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 ~/AFLplusplus-Nyx/packer/packer
And run the tool with the following options to pack libxml2:
./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 specmode 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, othwise 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 seperate 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 -argsparameter.
In case you want to fuzz the target only with fast snapshots enabled, you can also set the --fast_reload_modeoption to improve performance.
Finally, we need to generate a Nyx configuration file. Simply run the following command and you're good to proceed:
./nyx_config_gen.py /tmp/nyx_libxml2/ Kernel
Run AFL++Nyx
From here on, we are almost done. Move to the AFL++Nyx folder and start the fuzzer with the following arguments:
mkdir /tmp/in/ # to create an input folder
echo "AA" >> /tmp/in/A # create an input file to make the fuzzer happy for now
./afl-fuzz -i /tmp/in/ -o /tmp/out -d -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 NYX_DISABLE_SNAPSHOT_MODE environment variable.
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 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 here
This custom harness can be statically compiled with by gcc or clang. There is no need to use an AFL compiler, becaues 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 AFLplusplus/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 executable into the sharedir:
cd ~/AFLplusplus-Nyx/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-Nyx/packer/packer
./nyx_config_gen.py /tmp/nyx_custom_agent/ Kernel