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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 14.04 on the host machine and Debian Stretch in the QEMU instances.

GCC

Get the latest compiler from this list and unpack into $GCC.

Now you should have GCC binaries in $GCC/bin/:

$ ls $GCC/bin/
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:

git clone https://github.com/torvalds/linux.git $KERNEL

Generate default configs:

cd $KERNEL
make defconfig
make kvmconfig

Now we need to enable some config options required for syzkaller. Edit .config file manually and enable:

CONFIG_KCOV=y
CONFIG_DEBUG_INFO=y
CONFIG_KASAN=y
CONFIG_KASAN_INLINE=y

You may also need the following for a recent linux image:

CONFIG_CONFIGFS_FS=y
CONFIG_SECURITYFS=y

You might also want to enable some other kernel configs as described here.

Since enabling these options results in more sub options being available, we need to regenerate config. Run this and press enter each time when prompted for some config value to leave it as default:

make oldconfig

Build the kernel with previously built GCC:

make CC="$GCC/bin/gcc" -j64

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

$ ls $KERNEL/vmlinux
$KERNEL/vmlinux
$ ls $KERNEL/arch/x86/boot/bzImage 
$KERNEL/arch/x86/boot/bzImage

Image

Install debootstrap:

sudo apt-get install debootstrap

Create a Debian-stretch Linux 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

By default, this script will create a minimal Debian-stretch Linux image. The result should be $IMAGE/stretch.img disk image.

If you would like to generate wheezy debian image, instead of stretch, just add one option of the script

./create-image.sh --distribution wheezy

Sometimes it's useful to have some additional packages and tools available in the VM even though they are not required to run syzkaller. The instructions to install some useful tools are below.

To install other packages, like make sysbench git vim tmux usbutils (not required to run syzkaller):

./create-image.sh --feature full

To install perf (not required to run syzkaller):

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

Note: remember to set $KERNEL before installing perf

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

QEMU

Install QEMU:

sudo apt-get install qemu-system-x86

Make sure the kernel boots and sshd starts:

qemu-system-x86_64 \
  -kernel $KERNEL/arch/x86/boot/bzImage \
  -append "console=ttyS0 root=/dev/sda debug earlyprintk=serial slub_debug=QUZ"\
  -hda $IMAGE/stretch.img \
  -net user,hostfwd=tcp::10021-:22 -net nic \
  -enable-kvm \
  -nographic \
  -m 2G \
  -smp 2 \
  -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:

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

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:

kill $(cat vm.pid)

syzkaller

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": "$IMAGE/stretch.img",
	"sshkey": "$IMAGE/stretch.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.

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