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Add "U-Boot Installation" sec w/ netcat suggestion

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Denver Gingerich 2014-11-08 14:52:52 -05:00 • committed by Bradley M. Kuhn
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154
enforcement-case-studies.tex
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@ -597,59 +597,113 @@ compilation).
%same directory at thinkpenguin_librecmc-built-busybox_output.log - you may want
%to only use part of the BusyBox output (maybe even just the login) for brevity
%% \section{U-Boot Installation}
\section{U-Boot Installation}
%% The U-Boot installation process is substantially more complicated than the
%% firmware update. The investigator purchased the optional a serial cable
%% along with the TPE-NWIFIROUTER, in order to complete the U-Boot installation
%% per the instructions in'' -boot\verb0_0reflash''.
The U-Boot installation process is substantially more complicated than the
firmware update. The investigator purchased the optional serial cable
along with the TPE-NWIFIROUTER, in order to complete the U-Boot installation
per the instructions in ``u-boot\verb0_0reflash''.
%% However, we were
%% only able to read data from the serial port; we were unable to interrupt the
%% boot process or access the U-Boot console to complete the U-Boot re-flash. Here
%% are the steps we tried:
However, the investigator was only able to read data from the serial port; the
investigator was unable to send key events via the serial port so the U-Boot
console could not be accessed in that way. The investigator did find another
way of accessing the U-Boot console, though, which was used to complete the
U-Boot installation and verification. The likely issue with the serial port was
initial mis-wiring of the serial connector, causing the receive pin to be
permanently disabled. Here are the steps the investigator tried, including the
alternate method of installation that did not require the serial console:
%% * We found the serial cable included was a USB serial adapter that had a male
%% USB type A connector on one end and 4 female jumper wires at the other end.
%% These female jumper wires were red, black, white, and green.
%% * The instructions did not specify how to connect these wires, but we were able
%% to determine this in part using the "v8.4" image (close to our "v8.2" router)
%% at \url{http://wiki.openwrt.org/toh/tp-link/tl-wr841nd#serial.console} . Aside from
%% power and ground (red and black), we did have to guess which of the wires was
%% RX and TX. By experimentation we found that green was RX and white was TX.
%% When we tried the other way, we received no data to our serial console at boot
%% time.
%% * We did have to use the included jumper pin gender changer with the USB serial
%% adapter, which we put through the holes on the router's mainboard and then
%% connected to the USB serial adapter. The fit was fairly loose so it would be
%% nice if future router versions included a tighter gender changer or (ideally)
%% had the jumper pins soldered onto the board to begin with (so no gender
%% changer would be required).
%% * We used 115200 8N1 as our serial console settings (with no hardware or
%% software flow control). This was tested with both the minicom and screen
%% commands. We found that if we connected all 4 wires on the USB serial adapter
%% that the router would start without additional power and our console would
%% receive the startup messages. We could replicate the same behavior by
%% omitting the power cable from the USB serial adapter (red wire) and connecting
%% the main power adapter to the router instead.
%% * While we did see the U-Boot and kernel boot logs in our serial console, we
%% were unable to interrupt the boot process as u-boot\verb0_0reflash indicated we
%% should. We suspect this is a misconfiguration of our serial console, but it's
%% unclear exactly how it is misconfigured, as we were able to receive data fine
%% (we just couldn't send data to the router).
%% * As a result, we were unable to complete the U-Boot installation test. We did
%% appreciate that installation instructions were included, though these
%% instructions should be updated to include more specifics about connecting the
%% serial cable. Since ThinkPenguin does have the option to ship a serial
%% adapter with the router, it would be helpful if instructions specific to that
%% adapter were included, as the wiring configuration one should use was unclear.
%% * Additionally, instructions for removing the router's case should be included.
%% We found that the two screws that needed removal to open the case were hidden
%% underneath rubber feet on the case. Indicating which feet need removal to
%% unscrew the case would be helpful. The instructions should also note that the
%% case needs to be carefully separated once the screws are removed; it
%% effectively snaps apart, but care must be taken to avoid breaking the plastic
%% fasteners that keep the case together after the screws are removed.
\begin{itemize}
\item The investigator found the serial cable included was a USB serial adapter
that had a male USB type A connector on one end and 4 female jumper wires at the
other end. These female jumper wires were red, black, white, and green.
\item The instructions did not specify how to connect these wires, but the
investigator was able to determine this in part using the "v8.4" image (close to
the "v8.2" version string the investigator found on the bottom of the router) at
\url{http://wiki.openwrt.org/toh/tp-link/tl-wr841nd#serial.console} . Aside
from power and ground (red and black), the investigator did have to guess which
of the wires was RX and TX. By experimentation the investigator found that
green was RX and white was TX. When the investigator tried the other way, no
data was received to the serial console at boot time. While determining which
wires connected to which pins, the investigator may have connected the power pin
to the RX pin; this could explain why the receive (RX) pin later failed to work.
\item The investigator did have to use the included jumper pin gender changer
with the USB serial adapter, which the investigator put through the holes on the
router's mainboard and then connected to the USB serial adapter. The fit was
fairly loose so it would be nice if future router versions included a tighter
gender changer or (ideally) had the jumper pins soldered onto the board to begin
with (so no gender changer would be required). Since the serial cable is not
strictly required for U-Boot installation (see below), this may not be issue.
\item The investigator used 115200 8N1 as the serial console setting (with no
hardware or software flow control). This was tested with both the
\verb0minicom0 and \verb0screen0 commands. The investigator found that if all 4
wires were connected on the USB serial adapter that the router would start
without additional power and the console would receive the startup messages.
The investigator could replicate the same behavior by omitting the power cable
from the USB serial adapter (red wire) and connecting the main power adapter to
the router instead.
\item While the investigator did see the U-Boot and kernel boot logs in the
serial console, the investigator was unable to interrupt the boot process as
u-boot\verb0_0reflash indicated one should. This is likely related to the
accidental connection of power to the RX pin mentioned earlier, which may have
disabled the pin, preventing the serial port on the router from receiving the
commands sent to it.
\item The investigator then contacted one of the libreCMC developers to
determine what the serial console issue might be and whether there was an
alternate way to install U-Boot that did not rely on the serial console working.
The developer agreed the the receive pin had likely been disabled so a different
installation method would be needed. The developer indicated that the console
could be accessed via \verb0netcat0 when the router was booted into a special
mode by holding the reset button on the router for 7 seconds after turning on
the router.
\item The investigator turned on the router while pressing reset as mentioned
above and then ran \verb0nc -u -p 6666 192.168.1.1 66660 on the desktop that was
connected to the router (after changing its IP address to 192.168.1.2). After
pressing Enter, a \verb0uboot>0 prompt appeared and the investigator was able to
confirm the running version by typing \verb0version0 to which the router
responded with "U-Boot 1.1.4 (Jul 28 2014)".
\item A TFTP server was then setup according to step 1 of the U-Boot
installation steps in u-boot\verb0_0reflash. These instructions did not
explicitly state that the U-Boot image mentioned in step 4 of the build
instructions should be placed in /srv/tftp, but this was evident based on the
instructions that followed. This should be corrected in a future version of
u-boot\verb0_0reflash but, because it was straight-forward based on the context,
did not amount to a compliance issue.
\item The u-boot\verb0_0reflash steps were then followed starting at step 4,
using the \verb0netcat0 console rather than the serial console (described in
steps 2 and 3). The U-Boot image was downloaded onto the device and then copied
over top of the old U-Boot image. The router was then restarted with the
\verb0reset0 command.
\item Since the serial cable was still connected, the investigator noticed at
startup that U-Boot now printed "U-Boot 1.1.4 (Oct 17 2014)" as its version
string. This was also confirmed by using the \verb0netcat0 console and the
\verb0version0 command, as was previously done above. The new version string
showed that the router was now running the version of U-Boot that the
investigator built, rather than the one it was shipped with, thus fulfilling the
GPL's requirements that one must be able to build and install the software and
any modified versions.
\end{itemize}
While the u-boot\verb0_0reflash instructions appear to be functional for those
able to use a serial console, we would prefer if these instructions were updated
to use the \verb0netcat0 console instead. This provides a number of advantages,
such as no requirement for additional hardware to install a new version of
U-Boot, and less chance of mis-configuring one's serial connector (which would
reduce the risk of damage to the router). The existing instructions appear to
be compliant without modification; this suggestion would merely make it easier
for users to take advantage of the freedoms provided to them by U-Boot and the
rest of the system.
\section{Firmware Comparison}