591 lines
25 KiB
TeX
591 lines
25 KiB
TeX
Linux is now one of the most widespread operating system for embedded devices due
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to its openess as well as the wide variety of platforms it can run on. Many
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manufacturer actually use it in firmware you can find on many devices: DVB-T
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decoders, routers, print servers, DVD players ... Most of the time the stock
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firmware is not really open to the consumer, even if it uses open source software.
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You might be interested in running a Linux based firmware for your router for
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various reasons: extending the use of a network protocol (such as IPv6), having
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new features, new piece of software inside, or for security reasons. A fully
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open-source firmware is de-facto needed for such applications, since you want to
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be free to use this or that version of a particular reason, be able to correct a
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particular bug. Few manufacturers do ship their routers with a Sample Development Kit,
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that would allow you to create your own and custom firmware and most of the time,
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when they do, you will most likely not be able to complete the firmware creation process.
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This is one of the reasons why OpenWrt and other firmware exists: providing a
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version independent, and tools independent firmware, that can be run on various
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platforms, known to be running Linux originally.
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\subsection{Which Operating System does this device run?}
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There is a lot of methods to ensure your device is running Linux. Some of them do
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need your router to be unscrewed and open, some can be done by probing the device
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using its external network interfaces.
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\subsubsection{Operating System fingerprinting and port scanning}
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A large bunch of tools over the Internet exists in order to let you do OS
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fingerprinting, we will show here an example using \textbf{nmap}:
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\begin{Verbatim}
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nmap -P0 -O <IP address>
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Starting Nmap 4.20 ( http://insecure.org ) at 2007-01-08 11:05 CET
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Interesting ports on 192.168.2.1:
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Not shown: 1693 closed ports
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PORT STATE SERVICE
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22/tcp open ssh
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23/tcp open telnet
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53/tcp open domain
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80/tcp open http
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MAC Address: 00:13:xx:xx:xx:xx (Cisco-Linksys)
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Device type: broadband router
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Running: Linksys embedded
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OS details: Linksys WRT54GS v4 running OpenWrt w/Linux kernel 2.4.30
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Network Distance: 1 hop
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\end{Verbatim}
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nmap is able to report whether your device uses a Linux TCP/IP stack, and if so,
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will show you which Linux kernel version is probably runs. This report is quite
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reliable and it can make the distinction between BSD and Linux TCP/IP stacks and others.
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Using the same tool, you can also do port scanning and service version discovery.
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For instance, the following command will report which IP-based services are running
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on the device, and which version of the service is being used:
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\begin{verbatim}
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nmap -P0 -sV <IP address>
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Starting Nmap 4.20 ( http://insecure.org ) at 2007-01-08 11:06 CET
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Interesting ports on 192.168.2.1:
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Not shown: 1693 closed ports
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PORT STATE SERVICE VERSION
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22/tcp open ssh Dropbear sshd 0.48 (protocol 2.0)
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23/tcp open telnet Busybox telnetd
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53/tcp open domain ISC Bind dnsmasq-2.35
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80/tcp open http OpenWrt BusyBox httpd
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MAC Address: 00:13:xx:xx:xx:xx (Cisco-Linksys)
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Service Info: Device: WAP
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\end{verbatim}
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The web server version, if identified, can be determining in knowing the Operating
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System. For instance, the \textbf{BOA} web server is typical from devices running
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an open-source Unix or Unix-like.
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\subsubsection{Wireless Communications Fingerprinting}
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Although this method is not really known and widespread, using a wireless scanner
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to discover which OS your router or Access Point run can be used. We do not have
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a clear example of how this could be achieved, but you will have to monitor raw
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802.11 frames and compare them to a very similar device running a Linux based firmware.
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\subsubsection{Web server security exploits}
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The Linksys WRT54G was originally hacked by using a "ping bug" discovered in the
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web interface. This tip has not been fixed for months by Linksys, allowing people
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to enable the "boot\_wait" helper process via the web interface. Many web servers
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used in firmwares are open source web server, thus allowing the code to be audited
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to find an exploit. Once you know the web server version that runs on your device,
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by using \textbf{nmap -sV} or so, you might be interested in using exploits to reach
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shell access on your device.
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\subsubsection{Native Telnet/SSH access}
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Some firmwares might have restricted or unrestricted Telnet/SSH access, if so,
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try to log in with the web interface login/password and see if you can type in
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some commands. This is actually the case for some Broadcom BCM963xx based firmwares
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such as the one in Neuf/Cegetel ISP routers, Club-Internet ISP CI-Box and many
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others. Some commands, like \textbf{cat} might be left here and be used to
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determine the Linux kernel version.
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\subsubsection{Analysing a binary firmware image}
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You are very likely to find a firmware binary image on the manufacturer website,
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even if your device runs a proprietary operating system. If so, you can download
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it and use an hexadecimal editor to find printable words such as \textbf{vmlinux},
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\textbf{linux}, \textbf{ramdisk}, \textbf{mtd} and others.
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Some Unix tools like \textbf{hexdump} or \textbf{strings} can be used to analyse
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the firmware. Below there is an example with a binary firmware found other the Internet:
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\begin{verbatim}
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hexdump -C <binary image.extension> | less (more)
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00000000 46 49 52 45 32 2e 35 2e 30 00 00 00 00 00 00 00 |FIRE2.5.0.......|
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00000010 00 00 00 00 31 2e 30 2e 30 00 00 00 00 00 00 00 |....1.0.0.......|
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00000020 00 00 00 00 00 00 00 38 00 43 36 29 00 0a e6 dc |.......8.C6)..??|
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00000030 54 49 44 45 92 89 54 66 1f 8b 08 08 f8 10 68 42 |TIDE..Tf....?.hB|
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00000040 02 03 72 61 6d 64 69 73 6b 00 ec 7d 09 bc d5 d3 |..ramdisk.?}.???|
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00000050 da ff f3 9b f7 39 7b ef 73 f6 19 3b 53 67 ea 44 |???.?9{?s?.;Sg?D|
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\end{verbatim}
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Scroll over the firmware to find printable words that can be significant.
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\subsubsection{Amount of flash memory}
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Linux can hardly fit in a 2MB flash device, once you have opened the device and
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located the flash chip, try to find its characteristics on the Internet. If
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your flash chip is a 2MB or less device, your device is most likely to run a
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proprietary OS such as WindRiver VxWorks, or a custom manufacturer OS like Zyxel ZynOS.
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OpenWrt does not currently run on devices which have 2MB or less of flash memory.
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This limitation will probably not be worked around since those devices are most
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of the time micro-routers, or Wireless Access Points, which are not the main
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OpenWrt target.
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\subsubsection{Pluging a serial port}
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By using a serial port and a level shifter, you may reach the console that is being shown by the device
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for debugging or flashing purposes. By analysing the output of this device, you can
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easily notice if the device uses a Linux kernel or something different.
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\subsection{Finding and using the manufacturer SDK}
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Once you are sure your device run a Linux based firmware, you will be able to start
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hacking on it. If the manufacturer respected the GPL, it will have released a Sample
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Development Kit with the device.
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\subsubsection{GPL violations}
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Some manufacturers do release a Linux based binary firmware, with no sources at all.
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The first step before doing anything is to read the license coming with your device,
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then write them about this lack of Open Source code. If the manufacturer answers
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you they do not have to release a SDK containing Open Source software, then we
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recommend you get in touch with the gpl-violations.org community.
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You will find below a sample letter that can be sent to the manufacturer:
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\begin{verse}
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Miss, Mister,
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I am using a <device name>, and I cannot find neither on your website nor on the
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CD-ROM the open source software used to build or modify the firmware.
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In conformance to the GPL license, you have to release the following sources:
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\begin{itemize}
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\item complete toolchain that made the kernel and applications be compiled (gcc, binutils, libc)
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\item tools to build a custom firmware (mksquashfs, mkcramfs ...)
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\item kernel sources with patches to make it run on this specific hardware, this does not include binary drivers
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\end{itemize}
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Thank you very much in advance for your answer.
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Best regards, <your name>
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\end{verse}
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\subsubsection{Using the SDK}
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Once the SDK is available, you are most likely not to be able to build a complete
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or functional firmware using it, but parts of it, like only the kernel, or only
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the root filesystem. Most manufacturers do not really care releasing a tool that
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do work every time you uncompress and use it.
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You should anyway be able to use the following components:
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\begin{itemize}
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\item kernel sources with more or less functional patches for your hardware
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\item binary drivers linked or to be linked with the shipped kernel version
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\item packages of the toolchain used to compile the whole firmware: gcc, binutils, libc or uClibc
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\item binary tools to create a valid firmware image
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\end{itemize}
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Your work can be divided into the following tasks:
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\begin{itemize}
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\item create a clean patch of the hardware specific part of the linux kernel
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\item spot potential kernel GPL violations especially on netfilter and USB stack stuff
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\item make the binary drivers work, until there are open source drivers
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\item use standard a GNU toolchain to make working executables
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\item understand and write open source tools to generate a valid firmware image
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\end{itemize}
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\subsubsection{Creating a hardware specific kernel patch}
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Most of the time, the kernel source that comes along with the SDK is not really
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clean, and is not a standard Linux version, it also has architecture specific
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fixes backported from the \textbf{CVS} or the \textbf{git} repository of the
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kernel development trees. Anyway, some parts can be easily isolated and used as
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a good start to make a vanilla kernel work your hardware.
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Some directories are very likely to have local modifications needed to make your
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hardware be recognized and used under Linux. First of all, you need to find out
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the linux kernel version that is used by your hardware, this can be found by
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editing the \textbf{linux/Makefile} file.
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\begin{verbatim}
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head -5 linux-2.x.x/Makefile
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VERSION = 2
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PATCHLEVEL = x
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SUBLEVEL = y
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EXTRAVERSION = z
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NAME=A fancy name
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\end{verbatim}
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So now, you know that you have to download a standard kernel tarball at
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\textbf{kernel.org} that matches the version being used by your hardware.
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Then you can create a \textbf{diff} file between the two trees, especially for the
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following directories:
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\begin{verbatim}
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diff -urN linux-2.x.x/arch/<sub architecture> linux-2.x.x-modified/arch/<sub architecture> > 01-architecture.patch
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diff -urN linux-2.x.x/include/ linux-2.x.x-modified/include > 02-includes.patch
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diff -urN linux-2.x.x/drivers/ linux-2.x.x-modified/drivers > 03-drivers.patch
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\end{verbatim}
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This will constitute a basic set of three patches that are very likely to contain
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any needed modifications that has been made to the stock Linux kernel to run on
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your specific device. Of course, the content produced by the \textbf{diff -urN}
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may not always be relevant, so that you have to clean up those patches to only
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let the "must have" code into them.
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The first patch will contain all the code that is needed by the board to be
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initialized at startup, as well as processor detection and other boot time
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specific fixes.
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The second patch will contain all useful definitions for that board: addresses,
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kernel granularity, redefinitions, processor family and features ...
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The third patch may contain drivers for: serial console, ethernet NIC, wireless
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NIC, USB NIC ... Most of the time this patch contains nothing else than "glue"
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code that has been added to make the binary driver work with the Linux kernel.
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This code might not be useful if you plan on writing drivers from scratch for
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this hardware.
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\subsubsection{Using the device bootloader}
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The bootloader is the first program that is started right after your device has
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been powered on. This program, can be more or less sophisticated, some do let you
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do network booting, USB mass storage booting ... The bootloader is device and
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architecture specific, some bootloaders were designed to be universal such as
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RedBoot or U-Boot so that you can meet those loaders on totally different
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platforms and expect them to behave the same way.
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If your device runs a proprietary operating system, you are very likely to deal
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with a proprietary boot loader as well. This may not always be a limitation,
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some proprietary bootloaders can even have source code available (i.e : Broadcom CFE).
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According to the bootloader features, hacking on the device will be more or less
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easier. It is very probable that the bootloader, even exotic and rare, has a
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documentation somewhere over the Internet. In order to know what will be possible
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with your bootloader and the way you are going to hack the device, look over the
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following features :
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\begin{itemize}
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\item does the bootloader allow net booting via bootp/DHCP/NFS or tftp
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\item does the bootloader accept loading ELF binaries ?
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\item does the bootloader have a kernel/firmware size limitation ?
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\item does the bootloader expect a firmware format to be loaded with ?
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\item are the loaded files executed from RAM or flash ?
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\end{itemize}
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Net booting is something very convenient, because you will only have to set up network
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booting servers on your development station, and keep the original firmware on the device
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till you are sure you can replace it. This also prevents your device from being flashed,
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and potentially bricked every time you want to test a modification on the kernel/filesystem.
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If your device needs to be flashed every time you load a firmware, the bootlader might
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only accept a specific firmware format to be loaded, so that you will have to
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understand the firmware format as well.
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\subsubsection{Making binary drivers work}
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As we have explained before, manufacturers do release binary drivers in their GPL
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tarball. When those drivers are statically linked into the kernel, they become GPL
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as well, fortunately or unfortunately, most of the drivers are not statically linked.
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This anyway lets you a chance to dynamically link the driver with the current kernel
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version, and try to make them work together.
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This is one of the most tricky and grey part of the fully open source projects.
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Some drivers require few modifications to be working with your custom kernel,
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because they worked with an earlier kernel, and few modifications have been made
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to the kernel in-between those versions. This is for instance the case with the
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binary driver of the Broadcom BCM43xx Wireless Chipsets, where only few differences
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were made to the network interface structures.
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Some general principles can be applied no matter which kernel version is used in
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order to make binary drivers work with your custom kernel:
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\begin{itemize}
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\item turn on kernel debugging features such as:
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\begin{itemize}
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\item CONFIG\_DEBUG\_KERNEL
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\item CONFIG\_DETECT\_SOFTLOCKUP
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\item CONFIG\_DEBUG\_KOBJECT
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\item CONFIG\_KALLSYMS
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\item CONFIG\_KALLSYMS\_ALL
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\end{itemize}
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\item link binary drivers when possible to the current kernel version
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\item try to load those binary drivers
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\item catch the lockups and understand them
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\end{itemize}
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Most of the time, loading binary drivers will fail, and generate a kernel oops.
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You can know the last symbol the binary drivers attempted to use, and see in the
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kernel headers file, if you do not have to move some structures field before or
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after that symbol in order to keep compatibily with both the binary driver and
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the stock kernel drivers.
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\subsubsection{Understanding the firmware format}
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You might want to understand the firmware format, even if you are not yet capable
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of running a custom firmware on your device, because this is sometimes a blocking
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part of the flashing process.
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A firmware format is most of the time composed of the following fields:
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\begin{itemize}
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\item header, containing a firmware version and additional fields: Vendor, Hardware version ...
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\item CRC32 checksum on either the whole file or just part of it
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\item Binary and/or compressed kernel image
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\item Binary and/or compressed root filesystem image
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\item potential garbage
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\end{itemize}
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Once you have figured out how the firmware format is partitioned, you will have
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to write your own tool that produces valid firmware binaries. One thing to be very
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careful here is the endianness of either the machine that produces the binary
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firmware and the device that will be flashed using this binary firmware.
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\subsubsection{Writing a flash map driver}
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The flash map driver has an important role in making your custom firmware work
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because it is responsible of mapping the correct flash regions and associated
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rights to specific parts of the system such as: bootloader, kernel, user filesystem.
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Writing your own flash map driver is not really a hard task once you know how your
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firmware image and flash is structured. You will find below a commented example
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that covers the case of the device where the bootloader can pass to the kernel its partition plan.
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First of all, you need to make your flash map driver be visible in the kernel
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configuration options, this can be done by editing the file \
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\textbf{linux/drivers/mtd/maps/Kconfig}:
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\begin{verbatim}
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config MTD_DEVICE_FLASH
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tristate "Device Flash device"
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depends on ARCHITECTURE && DEVICE
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help
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Flash memory access on DEVICE boards. Currently only works with
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Bootloader Foo and Bootloader Bar.
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\end{verbatim}
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Then add your source file to the \textbf{linux/drivers/mtd/maps/Makefile}, so
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that it will be compiled along with the kernel.
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\begin{verbatim}
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obj-\$(CONFIG_MTD_DEVICE_FLASH) += device-flash.o
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\end{verbatim}
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You can then write the kernel driver itself, by creating a
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\textbf{linux/drivers/mtd/maps/device-flash.c} C source file.
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\begin{verbatim}
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// Includes that are required for the flash map driver to know of the prototypes:
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#include <asm/io.h>
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#include <linux/init.h>
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#include <linux/kernel.h>
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#include <linux/mtd/map.h>
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#include <linux/mtd/mtd.h>
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#include <linux/mtd/partitions.h>
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#include <linux/vmalloc.h>
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// Put some flash map definitions here:
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#define WINDOW_ADDR 0x1FC00000 /* Real address of the flash */
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#define WINDOW_SIZE 0x400000 /* Size of flash */
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#define BUSWIDTH 2 /* Buswidth */
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static void __exit device_mtd_cleanup(void);
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static struct mtd_info *device_mtd_info;
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static struct map_info devicd_map = {
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.name = "device",
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.size = WINDOW_SIZE,
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.bankwidth = BUSWIDTH,
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.phys = WINDOW_ADDR,
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};
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static int __init device_mtd_init(void)
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{
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// Display that we found a flash map device
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printk("device: 0x\%08x at 0x\%08x\n", WINDOW_SIZE, WINDOW_ADDR);
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// Remap the device address to a kernel address
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device_map.virt = ioremap(WINDOW_ADDR, WINDOW_SIZE);
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// If impossible to remap, exit with the EIO error
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if (!device_map.virt) {
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printk("device: Failed to ioremap\n");
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return -EIO;
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}
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// Initialize the device map
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simple_map_init(&device_map);
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/* MTD informations are closely linked to the flash map device
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you might also use "jedec_probe" "amd_probe" or "intel_probe" */
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device_mtd_info = do_map_probe("cfi_probe", &device_map);
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if (device_mtd_info) {
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device_mtd_info->owner = THIS_MODULE;
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int parsed_nr_parts = 0;
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// We try here to use the partition schema provided by the bootloader specific code
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if (parsed_nr_parts == 0) {
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int ret = parse_bootloader_partitions(device_mtd_info, &parsed_parts, 0);
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if (ret > 0) {
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part_type = "BootLoader";
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parsed_nr_parts = ret;
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}
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}
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add_mtd_partitions(devicd_mtd_info, parsed_parts, parsed_nr_parts);
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return 0;
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}
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iounmap(device_map.virt);
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return -ENXIO;
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}
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// This function will make the driver clean up the MTD device mapping
|
|
static void __exit device_mtd_cleanup(void)
|
|
{
|
|
// If we found a MTD device before
|
|
if (device_mtd_info) {
|
|
// Delete every partitions
|
|
del_mtd_partitions(device_mtd_info);
|
|
// Delete the associated map
|
|
map_destroy(device_mtd_info);
|
|
}
|
|
|
|
// If the virtual address is already in use
|
|
if (device_map.virt) {
|
|
// Unmap the physical address to a kernel space address
|
|
iounmap(device_map.virt);
|
|
// Reset the structure field
|
|
device_map.virt = 0;
|
|
}
|
|
}
|
|
|
|
|
|
// Macros that indicate which function is called on loading/unloading the module
|
|
module_init(device_mtd_init);
|
|
module_exit(device_mtd_cleanup);
|
|
|
|
|
|
// Macros defining license and author, parameters can be defined here too.
|
|
MODULE_LICENSE("GPL");
|
|
MODULE_AUTHOR("Me, myself and I <memyselfandi@domain.tld");
|
|
\end{verbatim}
|
|
|
|
\subsection{Adding your target in OpenWrt}
|
|
|
|
Once you spotted the key changes that were made to the Linux kernel
|
|
to support your target, you will want to create a target in OpenWrt
|
|
for your hardware. This can be useful to benefit from the toolchain
|
|
that OpenWrt builds as well as the resulting user-space and kernel
|
|
configuration options.
|
|
|
|
Provided that your target is already known to OpenWrt, it will be
|
|
as simple as creating a \texttt{target/linux/board} directory
|
|
where you will be creating the following directories and files.
|
|
|
|
Here for example, is a \texttt{target/linux/board/Makefile}:
|
|
|
|
\begin{Verbatim}[frame=single,numbers=left]
|
|
#
|
|
# Copyright (C) 2009 OpenWrt.org
|
|
#
|
|
# This is free software, licensed under the GNU General Public License v2.
|
|
# See /LICENSE for more information.
|
|
#
|
|
include $(TOPDIR)/rules.mk
|
|
|
|
ARCH:=mips
|
|
BOARD:=board
|
|
BOARDNAME:=Eval board
|
|
FEATURES:=squashfs jffs2 pci usb
|
|
|
|
LINUX_VERSION:=2.6.27.10
|
|
|
|
include $(INCLUDE_DIR)/target.mk
|
|
|
|
DEFAULT_PACKAGES += hostapd-mini
|
|
|
|
define Target/Description
|
|
Build firmware images for Evaluation board
|
|
endef
|
|
|
|
$(eval $(call BuildTarget))
|
|
\end{Verbatim}
|
|
|
|
\begin{itemize}
|
|
\item \texttt{ARCH} \\
|
|
The name of the architecture known by Linux and uClibc
|
|
\item \texttt{BOARD} \\
|
|
The name of your board that will be used as a package and build directory identifier
|
|
\item \texttt{BOARDNAME} \\
|
|
Expanded name that will appear in menuconfig
|
|
\item \texttt{FEATURES} \\
|
|
Set of features to build filesystem images, USB, PCI, VIDEO kernel support
|
|
\item \texttt{LINUX\_VERSION} \\
|
|
Linux kernel version to use for this target
|
|
\item \texttt{DEFAULT\_PACKAGES} \\
|
|
Set of packages to be built by default
|
|
\end{itemize}
|
|
|
|
A partial kernel configuration which is either named \texttt{config-default} or which matches the kernel version \texttt{config-2.6.x} should be present in \texttt{target/linux/board/}.
|
|
This kernel configuration will only contain the relevant symbols to support your target and can be changed using \texttt{make kernel\_menuconfig}.
|
|
|
|
To patch the kernel sources with the patches required to support your hardware, you will have to drop them in \texttt{patches} or in \texttt{patches-2.6.x} if there are specific
|
|
changes between kernel versions. Additionnaly, if you want to avoid creating a patch that will create files, you can put those files into \texttt{files} or \texttt{files-2.6.x}
|
|
with the same directory structure that the kernel uses (e.g: drivers/mtd/maps, arch/mips ..).
|
|
|
|
The build system will require you to create a \texttt{target/linux/board/image/Makefile}:
|
|
|
|
\begin{Verbatim}[frame=single,numbers=left]
|
|
#
|
|
# Copyright (C) 2009 OpenWrt.org
|
|
#
|
|
# This is free software, licensed under the GNU General Public License v2.
|
|
# See /LICENSE for more information.
|
|
#
|
|
include $(TOPDIR)/rules.mk
|
|
include $(INCLUDE_DIR)/image.mk
|
|
|
|
define Image/BuildKernel
|
|
cp $(KDIR)/vmlinux.elf $(BIN_DIR)/openwrt-$(BOARD)-vmlinux.elf
|
|
gzip -9n -c $(KDIR)/vmlinux > $(KDIR)/vmlinux.bin.gz
|
|
$(STAGING_DIR_HOST)/bin/lzma e $(KDIR)/vmlinux $(KDIR)/vmlinux.bin.l7
|
|
dd if=$(KDIR)/vmlinux.bin.l7 of=$(BIN_DIR)/openwrt-$(BOARD)-vmlinux.lzma bs=65536 conv=sync
|
|
dd if=$(KDIR)/vmlinux.bin.gz of=$(BIN_DIR)/openwrt-$(BOARD)-vmlinux.gz bs=65536 conv=sync
|
|
endef
|
|
|
|
define Image/Build/squashfs
|
|
$(call prepare_generic_squashfs,$(KDIR)/root.squashfs)
|
|
endef
|
|
|
|
define Image/Build
|
|
$(call Image/Build/$(1))
|
|
dd if=$(KDIR)/root.$(1) of=$(BIN_DIR)/openwrt-$(BOARD)-root.$(1) bs=128k conv=sync
|
|
|
|
-$(STAGING_DIR_HOST)/bin/mkfwimage \
|
|
-B XS2 -v XS2.ar2316.OpenWrt \
|
|
-k $(BIN_DIR)/openwrt-$(BOARD)-vmlinux.lzma \
|
|
-r $(BIN_DIR)/openwrt-$(BOARD)-root.$(1) \
|
|
-o $(BIN_DIR)/openwrt-$(BOARD)-ubnt2-$(1).bin
|
|
endef
|
|
|
|
$(eval $(call BuildImage))
|
|
|
|
\end{Verbatim}
|
|
|
|
\begin{itemize}
|
|
\item \texttt{Image/BuildKernel} \\
|
|
This template defines changes to be made to the ELF kernel file
|
|
\item \texttt{Image/Build} \\
|
|
This template defines the final changes to apply to the rootfs and kernel, either combined or separated
|
|
firmware creation tools can be called here as well.
|
|
\end{itemize}
|