Setup: Linux host, QEMU vm, arm64 kernel

This document will detail the steps involved in setting up a Syzkaller instance fuzzing any ARM64 linux kernel of your choice.

Create a disk image

We will use buildroot to create the disk image. You can obtain buildroot from here. Extract the tarball and perform a make menuconfig inside it. Choose the following options.

Target options
    Target Architecture - Aarch64 (little endian)
Toolchain type
    External toolchain - Linaro AArch64
System Configuration
[*] Enable root login with password
        ( ) Root password ⇐= set your password using this option
[*] Run a getty (login prompt) after boot  --->
    TTY port - ttyAMA0
Target packages
    [*]   Show packages that are also provided by busybox
    Networking applications
        [*] dhcpcd
        [*] iproute2
        [*] openssh
Filesystem images
    [*] ext2/3/4 root filesystem
        ext2/3/4 variant - ext3
        exact size in blocks - 6000000
    [*] tar the root filesystem

Run make. After the build, confirm that output/images/rootfs.ext3 exists.

Get the ARM64 toolchain from Linaro

You will require an ARM64 kernel with gcc plugin support. If not, obtain the ARM64 toolchain from Linaro. Get gcc-linaro-6.1.1-2016.08-x86_64_aarch64-linux-gnu.tar.xz from here. Extract and add its bin/ to your PATH. If you have another ARM64 toolchain on your machine, ensure that this newly downloaded toolchain takes precedence.

Compile the kernel

Once you have obtained the source code for the linux kernel you wish to fuzz, do the following.

$ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make defconfig
$ vim .config

Change the following options :

    CONFIG_KCOV=y
    CONFIG_KASAN=y
    CONFIG_DEBUG_INFO=y
    CONFIG_CMDLINE=”console=ttyAMA0”
    CONFIG_KCOV_INSTRUMENT_ALL=y
    CONFIG_DEBUG_FS=y
    CONFIG_NET_9P=y
    CONFIG_NET_9P_VIRTIO=y
    CONFIG_CROSS_COMPILE="aarch64-linux-gnu-"

    $ ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make -j40

If the build was successful, you should have a arch/arm64/boot/Image file.

Obtain qemu for ARM64

Obtain the QEMU source from git or from the latest source release.

$ ./configure
$ make -j40

If the build was successful, you should have a aarch64-softmmu/qemu-system-aarch64 binary.

Boot up manually

You should be able to start up the kernel as follows.

$ /path/to/aarch64-softmmu/qemu-system-aarch64 \
  -machine virt \
  -cpu cortex-a57 \
  -nographic -smp 1 \
  -hda /path/to/rootfs.ext3 \
  -kernel /path/to/arch/arm64/boot/Image \
  -append "console=ttyAMA0 root=/dev/vda oops=panic panic_on_warn=1 panic=-1 ftrace_dump_on_oops=orig_cpu debug earlyprintk=serial slub_debug=UZ" \
  -m 2048 \
  -net user,hostfwd=tcp::10023-:22 -net nic

At this point, you should be able to see a login prompt.

Set up the QEMU disk

Now that we have a shell, let us add a few lines to existing init scripts so that they are executed each time Syzkaller brings up the VM.

At the top of /etc/init.d/S50sshd add the following lines:

ifconfig eth0 up
dhcpcd
mount -t debugfs none /sys/kernel/debug
chmod 777 /sys/kernel/debug/kcov

Comment out the line

/usr/bin/ssh-keygen -A

Next we set up ssh. Create an ssh keypair locally and copy the public key to /authorized_keys in /. Ensure that you do not set a passphrase when creating this key.

Open /etc/ssh/sshd_config and modify the following lines as shown below.

PermitRootLogin yes
PubkeyAuthentication yes
AuthorizedKeysFile      /authorized_keys
PasswordAuthentication yes

Reboot the machine, and ensure that you can ssh from host to guest as.

$ ssh -i /path/to/id_rsa root@localhost -p 10023

Build syzkaller

make TARGETARCH=arm64 [CC=/gcc-linaro-6.3.1-2017.05-x86_64_aarch64-linux-gnu/bin/aarch64-linux-gnu-g++]

Modify your config file and start off syzkaller

A sample config file that exercises the required options are shown below. Modify according to your needs.

{
    "name": "QEMU-aarch64",
    "target": "linux/arm64",
    "http": ":56700",
    "workdir": "/path/to/a/dir/to/store/syzkaller/corpus”,
    "vmlinux": “/path/to/vmlinux",
    "syzkaller": "/path/to/syzkaller/arm64/",
    "image": "/path/to/rootfs.ext3",
    "sshkey": "/path/to/ida_rsa",
    "procs": 8,
    "type": "qemu",
    "vm": {
        "count": 1,
        "qemu": "/path/to/qemu-system-aarch64",
        "cmdline": "console=ttyAMA0 root=/dev/vda",
        "kernel": “/path/to/Image",
        "cpu": 2,
        "mem": 2048
    }
}

At this point, you should be able to visit localhost:56700 and view the results of the fuzzing.

If you get issues after syz-manager starts, consider running it with the -debug flag. Also see this page for troubleshooting tips.

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