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When net.ifnames is set to 1, syzkaller might not be able to connect
to the virtual machines. Add the corresponding recommendations to the
documentation as well as a note to the troubleshooting guide.
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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 on the host machine and Debian Stretch 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.

GCC

If your distro's GCC is older, it's preferable to get the lastest GCC from this list. Download and unpack into $GCC, and 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 git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git $KERNEL

Generate default configs:

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

Note: If you are using your distro's GCC, you don't need to set CC in the make command.

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=y

# Memory bug detector
CONFIG_KASAN=y
CONFIG_KASAN_INLINE=y

# Required for Debian Stretch
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:

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:

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

To create a Debian Stretch Linux image with the minimal set of required packages do:

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/stretch.img disk image.

If you would like to generate an image with Debian Buster, instead of Stretch, do:

./create-image.sh --distribution buster

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):

./create-image.sh --feature full

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

./create-image.sh --add-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 \
	-m 2G \
	-smp 2 \
	-kernel $KERNEL/arch/x86/boot/bzImage \
	-append "console=ttyS0 root=/dev/sda earlyprintk=serial net.ifnames=0" \
	-drive file=$IMAGE/stretch.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:

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 press Ctrl+A and then X or run:

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