This page contains instructions to set up syzkaller to run on a FreeBSD or Linux host and fuzz an amd64 FreeBSD kernel running in a virtual machine.
Currently, syzkaller can fuzz FreeBSD running under bhyve, QEMU or GCE (Google Compute Engine). Regardless of the mode of operation, some common steps must be followed.
syz-manager is the component of syzkaller that manages target VMs. It runs on a host system and automatically creates, runs and destroys VMs which share a user-specified image file.
To build syzkaller out of the box, a recent version of FreeBSD 13.0-CURRENT must be used for the host. Older versions of FreeBSD can be used but will require manual tweaking.
The required dependencies can be installed by running:
# pkg install bash gcc git gmake go golangci-lint llvm
When using bhyve as the VM backend, a DHCP server must also be installed:
# pkg install dnsmasq
To checkout the syzkaller sources, run:
$ git clone https://github.com/google/syzkaller
and the binaries can be built by running:
$ cd syzkaller
$ gmake
Once this completes, a syz-manager executable should be available under bin/.
To build Go binaries do:
make manager fuzzer execprog TARGETOS=freebsd
To build C syz-executor binary, copy executor/* files to a FreeBSD machine and build there with:
c++ executor/executor.cc -o syz-executor -O1 -lpthread -DGOOS_freebsd=1 -DGOARCH_amd64=1 -DGIT_REVISION=\"CURRENT_GIT_REVISION\"
Then, copy out the binary back to host into bin/freebsd_amd64 dir.
It is easiest to start with a snapshot image of FreeBSD. Fetch a QCOW2 disk image for QEMU or a raw image for GCE or bhyve. Fetch a copy of the FreeBSD kernel sources and place them in /usr/src. You will likely need to expand the disk of the VM. Before booting the VM, run:
# truncate -s 15G $IMAGEFILE
To enable KCOV on FreeBSD, a custom kernel must be compiled. It is easiest to do this on the host.
Before booting the VM, compile a custom kernel on the host and install it on the VM. On the host:
# mdconfig -a -f $IMAGEFILE # mount /dev/md0p4 /mnt
This will create a memory device for the VM file and allow host to install the custom kernel source on the VM.
To get a copy of the current development sources:
# pkg install git # git clone --depth=1 --branch=main https://github.com/freebsd/freebsd-src /usr/src
To create a custom kernel configuration file for syzkaller and build a new kernel, run:
# cd /usr/src/sys/amd64/conf # cat <<__EOF__ > SYZKALLER include "./GENERIC" ident SYZKALLER options COVERAGE options KCOV __EOF__ # cd /usr/src # make -j $(sysctl -n hw.ncpu) KERNCONF=SYZKALLER buildkernel # make KERNCONF=SYZKALLER installkernel DESTDIR=/mnt
Before booting the VM, make sure to run:
# umount /mnt
The md device will linger and you can use it again later if you want. Otherwise destroy it:
# mdconfig -d -u 0
Use QEMU to start a VM using the downloaded image:
$ qemu-system-x86_64 -hda $IMAGEFILE -nographic -net user,host=10.0.2.10,hostfwd=tcp::10022-:22 -net nic,model=e1000
When the boot loader menu is printed, escape to the loader prompt and enter the commands set console="comconsole" and boot. Once you reach a login prompt, log in as root and add a couple of configuration parameters to /boot/loader.conf:
# cat <<__EOF__ >>/boot/loader.conf autoboot_delay="-1" console="comconsole" __EOF__
After VM is booted, /etc/rc.d/growfs should haven grown its file system automatically. Otherwise run:
# /etc/rc.d/growfs onestart
Verify that uname -i prints SYZKALLER to confirm that your newly built kernel is running.
Then, to permit remote access to the VM, you must configure DHCP and enable sshd:
# sysrc sshd_enable=YES # sysrc ifconfig_DEFAULT=DHCP
If you plan to run the syscall executor as root, ensure that root SSH logins are permitted by adding PermitRootLogin without-password to /etc/ssh/sshd_config. Otherwise, create a new user with adduser. Install an ssh key for the user and verify that you can SSH into the VM from the host. Note that bhyve requires the use of the root user for the time being.
Some additional steps are required on the host in order to use bhyve. First, ensure that the host system is at r346550 or later. Second, since bhyve currently does not support disk image snapshots, ZFS must be used to provide equivalent functionality. Create a ZFS data set and copy the VM image there. The data set can also be used to store the syzkaller workdir. For example, with a zpool named data mounted at /data, write:
# zfs create data/syzkaller # cp FreeBSD-13.0-CURRENT-amd64.raw /data/syzkaller
Third, configure networking and DHCP for the VM instances:
# ifconfig bridge create bridge0 # ifconfig bridge0 inet 169.254.0.1 # echo 'dhcp-range=169.254.0.2,169.254.0.254,255.255.255.0' > /usr/local/etc/dnsmasq.conf # echo 'interface=bridge0' >> /usr/local/etc/dnsmasq.conf # sysrc dnsmasq_enable=YES # service dnsmasq start # echo 'net.link.tap.up_on_open=1' >> /etc/sysctl.conf # sysctl net.link.tap.up_on_open=1
To enable automatic configuration of bridged network every time the system boots, add the following to /etc/rc.conf:
# cloned_interfaces="bridge0 tap0" # ifconfig_bridge0="inet 169.254.0.1 addm tap0 up" # ifconfig_tap0="up"
Finally, ensure that the bhyve kernel module is loaded:
# kldload vmm
If all of the above worked, create a freebsd.cfg configuration file with the following contents (alter paths as necessary):
{
"name": "freebsd",
"target": "freebsd/amd64",
"http": ":10000",
"workdir": "/workdir",
"syzkaller": "/gopath/src/github.com/google/syzkaller",
"sshkey": "/freebsd_id_rsa",
"sandbox": "none",
"procs": 8,
}
If running the fuzzer under QEMU, add:
"image": "/FreeBSD-13.0-CURRENT-amd64.qcow2",
"type": "qemu",
"vm": {
"count": 10,
"cpu": 4,
"mem": 2048
}
For GCE, add the following instead (alter the storage bucket path as necessary):
"image": "/FreeBSD-13.0-CURRENT-amd64.raw",
"type": "gce",
"vm": {
"count": 10,
"instance_type": "n1-standard-4",
"gcs_path": "syzkaller"
}
For bhyve, we need to specify the VM image snapshot name and networking info (alter the dataset name and paths as necessary):
"image": "/data/syzkaller/FreeBSD-13.0-CURRENT-amd64.raw",
"type": "bhyve",
"vm": {
"count": 10,
"bridge": "bridge0",
"hostip": "169.254.0.1",
"dataset": "data/syzkaller"
}
Then, start syz-manager with:
$ bin/syz-manager -config freebsd.cfg
It should start printing output along the lines of:
booting test machines... wait for the connection from test machine... machine check: 253 calls enabled, kcov=true, kleakcheck=false, faultinjection=false, comps=false executed 3622, cover 1219, crashes 0, repro 0 executed 7921, cover 1239, crashes 0, repro 0 executed 32807, cover 1244, crashes 0, repro 0 executed 35803, cover 1248, crashes 0, repro 0
If something does not work, try adding the -debug flag to syz-manager.
pkg/host needs to be taught how to detect supported syscalls/devices.