Healer

Healer is a kernel fuzzer inspired by Syzkaller.

Similar to Syzkaller, Healer uses the syscall information provided by the Syzlang description to generate sequences of system calls that confirm to the parameter structure constraints and partial semantic constraints, and finds kernel bugs by continuously executing the generated call sequences to cause kernel crashes.

Unlike Syzkaller, Healer does not use an empirical choice-table, but detects the influence relationships between syscalls by dynamically removing calls in the minimized call sequences and observing coverage changes, and uses the influence relationships to guide the generation and mutation of call sequences. In addition, Healer also uses a different architectural design than Syzkaller.

NOTE: This is just a prototype and many important features haven’t been published.

Build Healer

Healer is written in pure rust, except for some patching code. Therefore, rust toolchain should be installed first.

> curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
> rustc --version # check install

In order to use the Syzlang descriptions, Healer’s build script will automatically download Syzkaller and add patches to the source code and build Syzkaller, which may increase the build time. Therefore, the build tool required by Syzkaller needs to be installed, e.g., golang compiler with GO111MODULE on, GCC 6.1.0 or later. You can also skip the automated building process via setting related env variables. However, you need provide syscall description in json format manually in this case.

Once all the required tools have been installed, Healer can be easily built using following command:

> cargo build --release

Finally, Healer itself and the patched Syzkaller binary (syz-bin) can be found in the target/release directory.

Fuzz Linux Kernel

Overall, fuzzing Linux kernel with Healer requires three steps: (1) prepare the disk image, (2) compile the kernel, and (3) start Healer.

Healer uses qumu to boot the kernel, so the disk image and kernel image need to be prepared. The booted qemu needs to be able to login via the ssh key, and the kernel needs to have at least the kcov feature. This document from Syzkaller describes in detail about how to build stretch.img and compile the Linux kernel with specific configuration, so follow the instructions there to complete the first two steps.

Once the stretch.img, ssh-stretch.id_rsa, bzImage are ready, my recommendation is to create a working directory. Then, create a bin directory inside the workdir and copy the patched Syzkaller binary and healer binary to that directory, taking care not to change the syz-bin directory structure. The final working directory needs to have the following files.

> cd path/to/workdir && ls 
bin  bzImage  stretch.id_rsa  stretch.img
> ls ./bin
healer linux_amd64  syz-repro  syz-symbolize  syz-sysgen

Finally, executing following command to start the fuzzing, where -d specifies the path to disk image, -k specifies the path to kernel image and --ssh-key specifies the path to ssh key.

> # `sudo` maybe needed for `kvm` accessing. 
> healer -d stretch.img --ssh-key stretch.id_rsa -k bzImage

One can also specify the parallel fuzzing instance (thread) via -j, the path to kernel object file (vmlinux) and srouce code via -b and -r so that Healer can symbolize the kernel crash log. See more options via healer --help. If everything works ok, you’ll see following log:

 ___   ___   ______   ________   __       ______   ______
/__/\ /__/\ /_____/\ /_______/\ /_/\     /_____/\ /_____/\
\::\ \  \ \::::_\/_\::: _  \ \:\ \    \::::_\/_\:::_ \ \
 \::\/_\ .\ \:\/___/\::(_)  \ \:\ \    \:\/___/\:(_) ) )_
  \:: ___::\ \::___\/_\:: __  \ \:\ \____\::___\/_\: __ `\ \
   \: \ \::\ \:\____/\:.\ \  \ \:\/___/\:\____/\ \ `\ \ \
    \__\/ \::\/ \_____\/ \__\/\__\/ \_____\/ \_____\/ \_\/ \_\/

[2021-08-30T03:05:28Z INFO] loading target linux/amd64...
[2021-08-30T03:05:30Z INFO] loading input progs
[2021-08-30T03:05:30Z INFO] progs loaded: 1765/1765
[2021-08-30T03:05:30Z INFO] pre-booting one vm...
[2021-08-30T03:05:58Z INFO] boot cost around 28s
[2021-08-30T03:05:59Z INFO] detecting features
[2021-08-30T03:05:59Z INFO] code coverage               : enabled
[2021-08-30T03:05:59Z INFO] setuid sandbox              : enabled
[2021-08-30T03:05:59Z INFO] namespace sandbox           : enabled
[2021-08-30T03:05:59Z INFO] fault injection             : enabled
[2021-08-30T03:05:59Z INFO] net packet injection        : enabled
[2021-08-30T03:05:59Z INFO] net device setup            : enabled
[2021-08-30T03:05:59Z INFO] USB emulation               : enabled
[2021-08-30T03:05:59Z INFO] hci packet injection        : enabled
[2021-08-30T03:05:59Z INFO] wifi device emulation       : enabled
[2021-08-30T03:05:59Z INFO] pre-setup one executor...
[2021-08-30T03:06:02Z INFO] ok, fuzzer-0 should be ready
...

Contributing

All contributions are welcome, if you have a feature request don’t hesitate to open an issue!