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.

You’ll need to have root privileges to do this procedure, or the ability to escalate your privileges with the command “sudo su”.

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.

kali@kali:~$ sudo apt install -y kpartx xz-utils gdisk uboot-mkimage u-boot-tools vboot-kernel-utils vboot-utils cgpt
kali@kali:~$ mkdir -p ~/arm-stuff/images/
kali@kali:~$ cd ~/arm-stuff/images/
kali@kali:~$ dd if=/dev/zero of=kali-custom-chrome.img bs=4M count=7000

03. Partition and Mount the Image File

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

+16M
7f00
n
2

+16M
7f00
n
3

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

kali@kali:~$ mkfs.ext4 $rootp
kali@kali:~$ mkdir -p root
kali@kali:~$ mount $rootp root

04. Copy and Modify the Kali rootfs

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

kali@kali:~$ cd ~/arm-stuff/images/
kali@kali:~$ rsync -HPavz ~/arm-stuff/rootfs/kali-armhf/ root
kali@kali:~$
kali@kali:~$ echo nameserver 8.8.8.8 > root/etc/resolv.conf
kali@kali:~$
kali@kali:~$ mkdir -p root/etc/X11/xorg.conf.d/
kali@kali:~$ 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:

kali@kali:~$ mkdir -p ~/arm-stuff/kernel/
kali@kali:~$ cd ~/arm-stuff/kernel/
kali@kali:~$ git clone http://git.chromium.org/chromiumos/third_party/kernel.git -b chromeos-3.4 chromeos
kali@kali:~$ cd chromeos/
kali@kali:~$ 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.

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

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

kali@kali:~$ export ARCH=arm
kali@kali:~$ export CROSS_COMPILE=~/arm-stuff/kernel/toolchains/arm-eabi-linaro-4.6.2/bin/arm-eabi-
kali@kali:~$
kali@kali:~$ ./chromeos/scripts/prepareconfig chromeos-exynos5

# Disable LSM
kali@kali:~$ sed -i 's/CONFIG_SECURITY_CHROMIUMOS=y/# CONFIG_SECURITY_CHROMIUMOS is not set/g' .config

# If cross compiling, do this once:
kali@kali:~$ sed -i 's/if defined(__linux__)/if defined(__linux__) ||defined(__KERNEL__) /g' include/drm/drm.h

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

# Copy over firmware. Ideally use the original firmware (/lib/firmware) from the Chromebook.
kali@kali:~$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/dwmw2/linux-firmware.git
kali@kali:~$ cp -rf linux-firmware/* ~/arm-stuff/images/root/lib/firmware/
kali@kali:~$ rm -rf linux-firmware
kali@kali:~$ echo "console=tty1 debug verbose root=/dev/mmcblk1p3 rootwait rw rootfstype=ext4" > /tmp/config-sd
kali@kali:~$ echo "console=tty1 debug verbose root=/dev/sda3 rootwait rw rootfstype=ext4" > /tmp/config-usb
kali@kali:~$
kali@kali:~$ vbutil_kernel --pack /tmp/newkern-sd --keyblock /usr/share/vboot/devkeys/kernel.keyblock --version 1 --signprivate /usr/share/vboot/devkeys/kali@kali:~$ kernel_data_key.vbprivk --config=/tmp/config-sd --vmlinuz kernel.itb --arch arm
kali@kali:~$ vbutil_kernel --pack /tmp/newkern-usb --keyblock /usr/share/vboot/devkeys/kernel.keyblock --version 1 --signprivate /usr/share/vboot/devkeys/kali@kali:~$ kernel_data_key.vbprivk --config=/tmp/config-usb --vmlinuz kernel.itb --arch arm

06. Prepare the Boot Partition

kali@kali:~$ dd if=/tmp/newkern-sd of=$bootp1 # first boot partition for SD
kali@kali:~$ dd if=/tmp/newkern-usb of=$bootp2 # second boot partition for USB
kali@kali:~$
kali@kali:~$ umount $rootp
kali@kali:~$
kali@kali:~$ kpartx -dv $loopdevice
kali@kali:~$ losetup -d $loopdevice

07. dd the Image and Mark the USB Drive Bootable

kali@kali:~$ dd if=kali-linux-chrome.img of=/dev/sdb bs=4M
kali@kali:~$ 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.

kali@kali:~$ cgpt add -i 1 -S 1 -T 5 -P 10 -l KERN-A /dev/sdb
kali@kali:~$ 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.

kali@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
kali@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.


Updated on: 2021-Jun-22
Author: steev