Custom Chromebook Image

The following document describes our own method of creating a custom Kali Linux Samsung Chromebook ARM image and is targeted at developers. If you would like to install a pre-made Kali image, check out our Install Kali on Samsung Chromebook article.

In this guide, we create an image with two boot partitions - one containing a kernel hard-coded to boot from the SD card and the other containing a kernel hard-coded to boot from USB. Depending on your USB storage media type, make sure to mark the relevant boot partition with higher priority after you dd the image to your USB device as instructed in the last stages of this guide.

01. Create a Kali rootfs

Start by building a Kali rootfs as described in our Kali documentation, using an armhf architecture. By the end of this process, you should have a populated rootfs directory in ~/arm-stuff/rootfs/kali-armhf.

02. Create the Image File

Next, we create the physical image file that will hold our Chromebook rootfs and boot images.

apt install -y kpartx xz-utils gdisk uboot-mkimage u-boot-tools vboot-kernel-utils vboot-utils cgpt
cd ~
mkdir -p arm-stuff
cd arm-stuff/
mkdir -p images
cd images
dd if=/dev/zero of=kali-custom-chrome.img bs=1MB count=7000

03. Partition and Mount the Image File

parted kali-custom-chrome.img --script -- mklabel msdos
parted kali-custom-chrome.img --script -- mktable gpt
gdisk kali-custom-chrome.img << EOF
x
l
8192
m
n
1

+16M
7f00
n
2

+16M
7f00
n
3

w
y
EOF
loopdevice=`losetup -f --show kali-custom-chrome.img`
device=`kpartx -va $loopdevice| sed -E 's/.*(loop[0-9])p.*/\1/g' | head -1`
device="/dev/mapper/${device}"
bootp1=${device}p1
bootp2=${device}p2
rootp=${device}p3

mkfs.ext4 $rootp
mkdir -p root
mount $rootp root

04. Copy and Modify the Kali rootfs

Copy over the Kali rootfs you bootstrapped earlier using rsync to the mounted image.

cd ~/arm-stuff/images/
rsync -HPavz ~/arm-stuff/rootfs/kali-armhf/ root

echo nameserver 8.8.8.8 > root/etc/resolv.conf

mkdir -p root/etc/X11/xorg.conf.d/
cat << EOF > root/etc/X11/xorg.conf.d/50-touchpad.conf
Section "InputClass"
Identifier "touchpad"
MatchIsTouchpad "on"
Driver "synaptics"
Option "TapButton1" "1"
Option "TapButton2" "3"
Option "TapButton3" "2"
Option "FingerLow" "15"
Option "FingerHigh" "20"
Option "FingerPress" "256"
EndSection
EOF

05. Compile the Samsung Chromium Kernel and Modules

If you’re not using ARM hardware as the development environment, you will need to set up an ARM cross-compilation environment to build an ARM kernel and modules. Once that’s done, proceed with the following instructions.

Fetch the Chromium kernel sources and place them in our development tree structure:

cd ~/arm-stuff
mkdir -p kernel
cd kernel
git clone http://git.chromium.org/chromiumos/third_party/kernel.git -b chromeos-3.4 chromeos
cd chromeos
cat << EOF > kernel.its
/dts-v1/;

/ {
description = "Chrome OS kernel image with one or more FDT blobs";
#address-cells = ;
images {
kernel@1{
description = "kernel";
data = /incbin/("arch/arm/boot/zImage");
type = "kernel_noload";
arch = "arm";
os = "linux";
compression = "none";
load = ;
entry = ;
};
fdt@1{
description = "exynos5250-snow.dtb";
data = /incbin/("arch/arm/boot/exynos5250-snow.dtb");
type = "flat_dt";
arch = "arm";
compression = "none";
hash@1{
algo = "sha1";
};
};
};
configurations {
default = "conf@1";
conf@1{
kernel = "kernel@1";
fdt = "fdt@1";
};
};
};
EOF

Patch the kernel, in our case, with wireless injection patches.

mkdir -p ../patches
wget http://patches.aircrack-ng.org/mac80211.compat08082009.wl_frag+ack_v1.patch -O ../patches/mac80211.patch
wget http://patches.aircrack-ng.org/channel-negative-one-maxim.patch -O ../patches/negative.patch
patch -p1 < ../patches/negative.patch
patch -p1 < ../patches/mac80211.patch

Configure, then cross-compile the Chromium kernel as shown below.

export ARCH=arm
export CROSS_COMPILE=~/arm-stuff/kernel/toolchains/arm-eabi-linaro-4.6.2/bin/arm-eabi-

./chromeos/scripts/prepareconfig chromeos-exynos5
# Disable LSM
sed -i 's/CONFIG_SECURITY_CHROMIUMOS=y/# CONFIG_SECURITY_CHROMIUMOS is not set/g' .config
# If cross compiling, do this once:
sed -i 's/if defined(__linux__)/if defined(__linux__) ||defined(__KERNEL__) /g' include/drm/drm.h

make menuconfig
make -j$(cat /proc/cpuinfo|grep processor|wc -l)
make dtbs
cp ./scripts/dtc/dtc /usr/bin/
mkimage -f kernel.its kernel.itb
make modules_install INSTALL_MOD_PATH=~/arm-stuff/images/root/

# copy over firmware. Ideally use the original firmware (/lib/firmware) from the Chromebook.
git clone git://git.kernel.org/pub/scm/linux/kernel/git/dwmw2/linux-firmware.git
cp -rf linux-firmware/* ~/arm-stuff/images/root/lib/firmware/
rm -rf linux-firmware
echo "console=tty1 debug verbose root=/dev/mmcblk1p3 rootwait rw rootfstype=ext4" > /tmp/config-sd
echo "console=tty1 debug verbose root=/dev/sda3 rootwait rw rootfstype=ext4" > /tmp/config-usb

vbutil_kernel --pack /tmp/newkern-sd --keyblock /usr/share/vboot/devkeys/kernel.keyblock --version 1 --signprivate /usr/share/vboot/devkeys/kernel_data_key.vbprivk --config=/tmp/config-sd --vmlinuz kernel.itb --arch arm
vbutil_kernel --pack /tmp/newkern-usb --keyblock /usr/share/vboot/devkeys/kernel.keyblock --version 1 --signprivate /usr/share/vboot/devkeys/kernel_data_key.vbprivk --config=/tmp/config-usb --vmlinuz kernel.itb --arch arm

06. Prepare the Boot Partition

dd if=/tmp/newkern-sd of=$bootp1 # first boot partition for SD
dd if=/tmp/newkern-usb of=$bootp2 # second boot partition for USB

umount $rootp

kpartx -dv $loopdevice
losetup -d $loopdevice

07. dd the Image and Mark the USB Drive Bootable

dd if=kali-custom-chrome.img of=/dev/sdb bs=512k
cgpt repair /dev/sdb

This is the point where you need to mark either boot partition 1 or 2 to have higher priority. The number with the higher priority will boot first. The example below will give priority 10 to the first partition (-i) and will thus boot successfully from a SD card.

cgpt add -i 1 -S 1 -T 5 -P 10 -l KERN-A /dev/sdb
cgpt add -i 2 -S 1 -T 5 -P 5 -l KERN-B /dev/sdb

To see your partition list and order, use the command cgpt show.

root@kali:~# cgpt show /dev/sdb
start size part contents
0 1 PMBR
1 1 Pri GPT header
2 32 Pri GPT table
8192 32768 1 Label: "KERN-A"
Type: ChromeOS kernel
UUID: 63AD6EC9-AD94-4B42-80E4-798BBE6BE46C
Attr: priority=10 tries=5 successful=1
40960 32768 2 Label: "KERN-B"
Type: ChromeOS kernel
UUID: 37CE46C9-0A7A-4994-80FC-9C0FFCB4FDC1
Attr: priority=5 tries=5 successful=1
73728 3832490 3 Label: "Linux filesystem"
Type: 0FC63DAF-8483-4772-8E79-3D69D8477DE4
UUID: E9E67EE1-C02E-481C-BA3F-18E721515DBB
125045391 32 Sec GPT table
125045423 1 Sec GPT header
root@kali:~#

Once this operation is complete, boot up your Samsung Chromebook with the SD/USB device plugged in. At the developer mode boot screen, hit CTRL+u to boot from from your USB storage device. Log in to Kali (root / toor) and startx.