By Gavin L. Rebeiro

Contents

Cover

Copyright

1 Acknowledgements

2 Introduction

2.1 Prerequisite Knowledge 2.2 Apparatus

3 Fundamentals

3.1 Communication 3.2 Kernel Ring Buffer 3.3 Drivers 3.4 Operating Systems 3.5 Special Files

4 YOU ARE HERE ☞ Doing The Task

4.1 Preparing The Boot Media 4.2 Connecting Physical Components 4.3 Using Picocom 4.4 OS Installation

5 Thanks

6 OpenPGP Key

A Malicious Hardware

B Linux Kernel Source Tree Analysis

C Digital Multimeter Tests

Summary: We now spell out the steps taken to actually replace the Raspberry Pi OS with something more trustworthy (for background see Part I, Part II, and Part III)

We’ve now covered enough ground to make the installation of NetBSD on our Raspberry Spy (over our UTUB) a relatively painless matter.

Let’s go through the process in little steps.

4.1 Preparing The Boot Media

I’m going to grab the appropriate NetBSD image by taking hints from the following:

● NetBSD/evbarm on Raspberry Pi tells us everything we need to know to pick the right image. All the sections here related to booting are worth reading at least once. Also read sections about consoles and serial consoles at least once.

● Raspberry Pi boot modes is useful if you want to dig deeper into the booting mechanisms of the Raspberry Spy. USB mass storage boot is particularly useful for booting off USB. Trust me, you don’t want to muck around with SD cards; they’re a nightmare.

● NetBSD/evbarm can be referenced for general information about NetBSD on ARM boards.

The above links should give you a good idea of what’s going on and what needs to be done with regards to putting a NetBSD on a boot media that goes into a Raspberry Spy.

Let’s go through a concrete example.

My Raspberry Spy is of the model “3 B+” variety so I’m dealing with an ARM64 CPU architecture. We’ll follow along the instructions outlined in Installation procedure for NetBSD/evbarm; pay close attention to the section “NetBSD/evbarm subdirectory structure”; I follow these instructions as I explore Index of pub/NetBSD/NetBSD-9.1/evbarm-aarch64/.

I grab the appropriate image like so:

$ mkdir ~/Downloads/netbsd
$ cd ~/Downloads/minted
$ wget https://cdn.netbsd.org/pub/NetBSD/NetBSD-9.1/evb c
 → arm-aarch64/binary/gzimg/arm64.img.gz

Now that we’ve got the image, we can write it to our boot media. I’m going to assume you have an appropriate reader already plugged into your GNU/Linux box. I’ve got my USB thumb drive as “/dev/sdg” on my system. Use the right block device file on your system¹. We base our procedure along the lines of “Installation for ARMv7 and AArch64 devices with U-Boot” section from Installation procedure for NetBSD/evbarm:

$ gzip --decompress --keep arm64.img.gz
# dd if=arm64.img of=/dev/sdg bs=1M conv=sync
 → status=progress
$ lsblk -f | grep sdg

We’re going to ignore the minutiae of writing to block devices, bootloaders, and other adjacent topics related to the utilities we just used; that’s left for another time. We care about learning how to use a serial console in this project so we must stay focused on our primary target.

We’re going to have a look at how to make a serial install possible via some editing of the “cmdline.txt” file that now resides in the boot media (on the boot partition which is of type “vfat”):

# mkdir /media/netbsd_image
# mount /dev/sdg1 /media/netbsd_image
# grep "console" < cmdline.txt
# root=ld0a console=fb
# grep "enable_uart" < config.txt
# enable_uart=1

The “console=fb” part is to get out OS image to use the HDMI output. We will get rid of that string from the file “cmdline.txt”. Who needs that anyway? One way to do it²:

# ed cmdline.txt
21
,p
root=ld0a console=fb
1
root=ld0a console=fb
s/console=fb//
,p
root=ld0a
wq
11
# echo ",p" | ed cmdline.txt
11
root=ld0a

Remember to check your edits!

We also ensure that “enable_uart=1” is set in the file “config.txt":

# echo ",p" | ed config.txt
82
arm_64bit=1

kernel=netbsd.img
kernel_address=0x200000
enable_uart=1
force_turbo=0

Everything looks good! Additional useful information on the Raspberry Spy UART can be found in UART configuration. Pretty self-explanatory. That wasn’t so hard. Was it? Note that the following links document the files we’ve been messing around with:

● The Kernel Command Line ● config.txt

It’s a good idea to back up the state of your image, at this point³. We can now safely unmount our boot media and get on with the project:

# cd ~
# umount /media/netbsd_image

We change directory, before we unmount, so that we don’t get any “device busy” errors.

We’ve now got our boot media ready. Onwards!

4.2 Connecting Physical Components

Before you power up your UTUB, you should really check that the pins are working properly. The very basic test you should do is to check that the right voltage is being supplied. Check out Appendix C.

The pins on our UTUB and Raspberry Spy that we’re interested are the following:

● Raspberry Spy: Pin6 (Ground), Pin8 (GPIO14, TXD), Pin10 (GPIO15, RXD). You can find the layout in the official GPIO page.

● UTUB: I’ve got a CP2104 UTUB so I’ve got to only worry about the pins marked TX, RX, and GND. I have other pins on the module but they’re not relevant for this task.

We won’t be using any of the voltage pins on the boards because it’s more prone to errors. Just use the USB power supply that comes with your Raspberry Spy.

Don’t plug anything into power for the following sequence. Connect the jump-wires like so:

● Ground on UTUB to Ground (Pin6) on Raspberry Spy.

● TX on UTUB to RX (Pin10) on Raspbery Spy.

● RX on UTUB to TX on (Pin8) Raspberry Spy.

"We won’t be using any of the voltage pins on the boards because it’s more prone to errors."Don’t make the rookie mistake of matching TX with TX and RX with RX; TX always goes to RX and RX always goes to TX. Keep this in mind, always, when working with UARTs. Colour-coding your jump-wires helps.

We’ll just go over the order of attaching the stuff to do with power on our devices:

● Attach the USB power adapter to the Raspberry Pi without plugging the adapter into the power outlet.

● Attach the UTUB to your GNU/Linux box.

● Attach your USB power adapter to your power outlet.

The logic for the above procedure is that you can ensure that your serial interface is up and running before you start getting input from your Raspberry Spy.

4.3 Using Picocom

Using picocom(1) is simple. All we need to do is select the correct baud rate and give the right device file as a parameter to picocom(1).

I’ll give you an extract from the manual page to enlighten you:

In effect, picocom is not an "emulator" per-se. It is a
simple program that opens, configures, manages a serial
port (tty device) and its settings, and connects to it
the terminal emulator you are, most likely, already
→ using
(the terminal window application, xterm, rxvt, system
console, etc).
When picocom starts it opens the tty (serial port)
given as its non-option argument. Unless the
--noinit option is given, it configures the port to
the settings specified by the option-arguments (or
to some default settings), and sets it to "raw"
mode. If --noinit is given, the initialization and
configuration is skipped; the port is just opened.
Following this, if standard input is a tty, picocom
sets the tty to raw mode. Then it goes in a loop
where it listens for input from stdin, or from the
serial port. Input from the serial port is copied
to the standard output while input from the standard
input is copied to the serial port. Picocom also
scans its input stream for a user-specified control
character, called the escape character (being by
default C-a). If the escape character is seen, then
instead of sending it to the serial-device, the
program enters "command mode" and waits for the next
character (which is called the "function
character"). Depending on the value of the function
character, picocom performs one of the operations
described in the COMMANDS section below.

We use “C-a C-x” (Ctrl+a followed by Ctrl+x)⁴ to tell picocom(1) to exit; for more, RTFM; in particular, pay close attention to the “COMMANDS” section.

Make sure you’ve set up all the physical connections, as advised. It’s time to attach our UTUB to our GNU/Linux box and then make sure we invoke picocom(1) correctly:

# picocom --baud 115200 /dev/ttyUSB0
picocom v3.1





port is         : /dev/ttyUSB0
flowcontrol     : none
baudrate is     : 115200
parity is       : none
databits are    : 8
stopbits are    : 1
escape is       : C-a
local echo is   : no
noinit is       : no
noreset is      : no
hangup is       : no
nolock is       : no
send_cmd is     : sz -vv
receive_cmd is  : rz -vv -E
imap is         : 
omap is         :
emap is         : crcrlf,delbs
logfile is      : none
initstring      : none
exit_after is   : not set
exit is         : no





Type [C-a] [C-h] to see available commands
Terminal ready

It really is that simple. You’ve now got a serial terminal ready and listening.

4.4 OS Installation

Now that you’ve got a serial terminal operational, all we have to do to install NetBSD on the Raspberry Spy is to plug the USB power adapter into the power outlet. Keep a close eye on what goes on in the output of your serial terminal:

...
[   7.4246937] root device:
[  11.6252523] use one of: mue0 sd0[a-p] ddb halt reboot
[  11.6252523] root device: sd0
[  13.9755661] dump device (default sd0b):
[  15.7257992] file system (default generic):
...

You should be promoted to pick a root device. I pick “sd0” as it’s the first ’disk’ offered by NetBSD (which can only be my boot media)⁵. I go for the suggested defaults, for everything else. No need to overcomplicate things, at this point.

You will probably see your Raspberry Spy reboot once or twice during the OS install process. Just pass the same parameters for the boot device, and you should be good to go.

Eventually, you should be met with the following:

...
NetBSD/evbarm (arm64) (constty)
...





login:

If you login as “root”, you should have a nice login shell presented to you.

And we are done! You’ve successfully done some tinkering over a serial terminal. That wasn’t so hard. Was it? You can shutdown your device (halt the OS) like so:

# shutdown -p now
...
[   910.5814809] The operating system has halted.
[   910.5814809] Please press any key to reboot.

You can now disconnect the power supply from your Raspberry Spy. Then just send “C-a C-x” to picocom(1); after which, you should see:

...
Terminating...
Thanks for using picocom
#

Welcome to the world of serial terminals; hack your heart out! ⬆ ____ ¹ The command lsblk -f should help you out here. Don’t wipe the wrong device by accident. ² If you use another text editor, that’s fine. You really should learn ed(1) at some point though, especially if you want to get into embedded systems. ³ At least keep track of the files that you tweaked. If you use some sort of version-control-system, you get bonus points. ⁴ I don’t know why the manual doesn’t bother to explicitly mention that these are GNU-Emacs-style key sequences. ⁵ See the NetBSD sd(4) manpage for details.