Setup: Ubuntu host, QEMU vm, x86-64 kernel

These are the instructions on how to fuzz the x86-64 kernel in a QEMU with Ubuntu on the host machine and Debian Bullseye in the QEMU instances.

In the instructions below, the $VAR notation (e.g. $GCC, $KERNEL, etc.) is used to denote paths to directories that are either created when executing the instructions (e.g. when unpacking GCC archive, a directory will be created), or that you have to create yourself before running the instructions. Substitute the values for those variables manually.

Install Prerequisites

Command:

sudo apt update
sudo apt install make gcc flex bison libncurses-dev libelf-dev libssl-dev

GCC

If your distro’s GCC is older, it’s preferable to get the latest GCC from this list. Download and unpack into $GCC, and you should have GCC binaries in $GCC/bin/

Ubuntu 20.04 LTS: You can ignore this section. GCC is up-to-date.

Command:

ls $GCC/bin/
# Sample output:
# cpp     gcc-ranlib  x86_64-pc-linux-gnu-gcc        x86_64-pc-linux-gnu-gcc-ranlib
# gcc     gcov        x86_64-pc-linux-gnu-gcc-9.0.0
# gcc-ar  gcov-dump   x86_64-pc-linux-gnu-gcc-ar
# gcc-nm  gcov-tool   x86_64-pc-linux-gnu-gcc-nm

Kernel

Checkout Linux Kernel source

Command:

git clone --branch v6.2 git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git $KERNEL

We recommend to start with the latest stable version. v6.2 is an example here.

Generate default configs

Command:

cd $KERNEL
make defconfig
make kvm_guest.config

Or if you want to specify a compiler.

Command:

cd $KERNEL
make CC="$GCC/bin/gcc" defconfig
make CC="$GCC/bin/gcc" kvm_guest.config

Enable required config options

Enable kernel config options required for syzkaller as described here. It’s not required to enable all of them, but at the very least you need:

# Coverage collection.
CONFIG_KCOV=y

# Debug info for symbolization.
CONFIG_DEBUG_INFO_DWARF4=y

# Memory bug detector
CONFIG_KASAN=y
CONFIG_KASAN_INLINE=y

# Required for Debian Stretch and later
CONFIG_CONFIGFS_FS=y
CONFIG_SECURITYFS=y

Edit .config file manually and enable them (or do that through make menuconfig if you prefer).

Since enabling these options results in more sub options being available, we need to regenerate config:

Command:

make olddefconfig

Or if you want to specify a compiler.

Command:

make CC="$GCC/bin/gcc" olddefconfig

You might also be interested in disabling the Predictable Network Interface Names mechanism. This can be disabled either in the syzkaller configuration (see details here) or by updating these kernel configuration parameters:

CONFIG_CMDLINE_BOOL=y
CONFIG_CMDLINE="net.ifnames=0"

Build the Kernel

Command:

make -j`nproc`

Or if you want to specify a compiler.

Command:

make CC="$GCC/bin/gcc" -j`nproc`

Now you should have vmlinux (kernel binary) and bzImage (packed kernel image):

Command:

ls $KERNEL/vmlinux
# sample output - $KERNEL/vmlinux
ls $KERNEL/arch/x86/boot/bzImage
# sample output - $KERNEL/arch/x86/boot/bzImage

Image

Install debootstrap

Command:

sudo apt install debootstrap

Create Debian Bullseye Linux image

Create a Debian Bullseye Linux image with the minimal set of required packages.

Command:

mkdir $IMAGE
cd $IMAGE/
wget https://raw.githubusercontent.com/google/syzkaller/master/tools/create-image.sh -O create-image.sh
chmod +x create-image.sh
./create-image.sh

The result should be $IMAGE/bullseye.img disk image.

OR Create Debian Linux image with a different version

To create a Debian image with a different version (e.g. buster, stretch, sid), specify the --distribution option.

Command:

./create-image.sh --distribution buster

Image extra tools

Sometimes it’s useful to have some additional packages and tools available in the VM even though they are not required to run syzkaller. To install a set of tools we find useful do (feel free to edit the list of tools in the script):

Command:

./create-image.sh --feature full

To install perf (not required to run syzkaller; requires $KERNEL to point to the kernel sources):

Command:

./create-image.sh --add-perf

For additional options of create-image.sh, please refer to ./create-image.sh -h

QEMU

Install QEMU

Command:

sudo apt install qemu-system-x86

Verify

Make sure the kernel boots and sshd starts.

Command:

qemu-system-x86_64 \
	-m 2G \
	-smp 2 \
	-kernel $KERNEL/arch/x86/boot/bzImage \
	-append "console=ttyS0 root=/dev/sda earlyprintk=serial net.ifnames=0" \
	-drive file=$IMAGE/bullseye.img,format=raw \
	-net user,host=10.0.2.10,hostfwd=tcp:127.0.0.1:10021-:22 \
	-net nic,model=e1000 \
	-enable-kvm \
	-nographic \
	-pidfile vm.pid \
	2>&1 | tee vm.log

early console in setup code
early console in extract_kernel
input_data: 0x0000000005d9e276
input_len: 0x0000000001da5af3
output: 0x0000000001000000
output_len: 0x00000000058799f8
kernel_total_size: 0x0000000006b63000

Decompressing Linux... Parsing ELF... done.
Booting the kernel.
[    0.000000] Linux version 4.12.0-rc3+ ...
[    0.000000] Command line: console=ttyS0 root=/dev/sda debug earlyprintk=serial
...
[ ok ] Starting enhanced syslogd: rsyslogd.
[ ok ] Starting periodic command scheduler: cron.
[ ok ] Starting OpenBSD Secure Shell server: sshd.

After that you should be able to ssh to QEMU instance in another terminal.

Command:

ssh -i $IMAGE/bullseye.id_rsa -p 10021 -o "StrictHostKeyChecking no" root@localhost

Troubleshooting

If this fails with “too many tries”, ssh may be passing default keys before the one explicitly passed with -i. Append option -o "IdentitiesOnly yes".

To kill the running QEMU instance press Ctrl+A and then X or run:

Command:

kill $(cat vm.pid)

If QEMU works, the kernel boots and ssh succeeds, you can shutdown QEMU and try to run syzkaller.

syzkaller

Build syzkaller as described here. Then create a manager config like the following, replacing the environment variables $GOPATH, $KERNEL and $IMAGE with their actual values.

{
	"target": "linux/amd64",
	"http": "127.0.0.1:56741",
	"workdir": "$GOPATH/src/github.com/google/syzkaller/workdir",
	"kernel_obj": "$KERNEL",
	"image": "$IMAGE/bullseye.img",
	"sshkey": "$IMAGE/bullseye.id_rsa",
	"syzkaller": "$GOPATH/src/github.com/google/syzkaller",
	"procs": 8,
	"type": "qemu",
	"vm": {
		"count": 4,
		"kernel": "$KERNEL/arch/x86/boot/bzImage",
		"cpu": 2,
		"mem": 2048
	}
}

Run syzkaller manager:

mkdir workdir
./bin/syz-manager -config=my.cfg

Now syzkaller should be running, you can check manager status with your web browser at 127.0.0.1:56741.

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