Wink Hub – Rooting via JTAG – Part II

In my last post I detailed the method I used to find the JTAG pinout.

This time, I’ll show how I configured openocd, loaded a custom u-boot via JTAG, and rooted the hub.

Step 1 – Setting Up OpenODC

The version of openocd in the Ubuntu Trusty repo is 0.7 and I wanted to use the latest (currently 0.10), so I compiled it from source. The process is fairly painless if you have all the prerequisites installed:

$ git clone git://git.code.sf.net/p/openocd/code openocd-code
$ ./bootstrap
$ ./configure --enable-buspirate --enable-ftdi
$ make -j4
$ sudo make install

On my first attempt to connect to the hub I got an “memory write caused data abort”, but a quick search led me to a helpful post with a solution. Click here to download my config file for the Wink hub that incorporates the fixes to the startup script.

You can also download my config the Olimex ARM-USB-OCD.

once openocd is built and your config files are in place you can start it with:

openocd -f olimex-arm-usb-ocd.cfg -f imx28evk.cfg

Step 2 – Loading A Custom U-Boot

The goal here is to build a custom version of u-boot, load it to RAM and execute it in place of the factory bootloader. This makes it easy to root the hub without risking damage to the hardware or filesystem corruption like some folks over at the XDA Developers forum have reported.

If you connect a 3.3V serial adapter to the UART on the hub you will see :

U-Boot 2014.01-14400-gda781c6-dirty (Apr 30 2014 - 22:35:38)

And this gives you an idea of where to start. To build the custom u-boot:

$ git clone git://git.denx.de/u-boot.git
$ cd u-boot
$ git checkout v2014.01 -b tmp
$ export ARCH=arm
$ export CROSS_COMPILE=/usr/bin/arm-linux-gnueabi-
$ make mx28evk_nand_config
$ wget http://jalderman.org/wink-files/wink-hub.patch
$ patch -p1 < wink-hub.patch
$ make -j4

My patch sets up the environment to (mostly) match the Wink u-boot, but sets init=/bin/sh and enables the “Hit any key to stop autoboot” function. As an aside, I searched for the Wink u-boot sources and wasn’t able to find them. This seems to be a GPL license violation.

Assuming u-boot compiles successfully, you need to know where to load it into memory using openocd:

$ grep __image_copy_start u-boot.map
 *(.__image_copy_start)
 .__image_copy_start
                0x0000000040000100                __image_copy_start

Now start openocd in one terminal:

$ sudo openocd -f olimex-arm-usb-ocd.cfg -f imx28evk.cfg

And connect via telnet in another:

$ telnet localhost 4444
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
Open On-Chip Debugger

Now reset and halt the CPU and load our custom bootloader into memory. NOTE: I’ve observed that the halt command doesn’t always work. Sometimes it’ll take multiple reset/halt commands to get the CPU actually halted and in a state where we can write to RAM. Watch the serial terminal output and it’ll be obvious when the CPU is properly halted.

> reset
> halt
> cd u-boot
> load_image u-boot.bin 0x40000100
downloaded 455212 bytes in 12.666913s (35.095 KiB/s)
> resume 0x40000100

If the above is successful, you’ll see a new u-boot prompt in your serial terminal window:

U-Boot 2014.01-dirty (Aug 24 2015 - 18:24:43)

CPU:   Freescale i.MX28 rev1.2 at 454 MHz
BOOT:  NAND, 3V3
DRAM:  64 MiB
NAND:  128 MiB
*** Warning - bad CRC, using default environment

Video: MXSFB: 'videomode' variable not set!
In:    serial
Out:   serial
Err:   serial
Net:   FEC0 [PRIME], FEC1
Warning: FEC1 using MAC address from net device

Hit any key to stop autoboot:  0 

You can interrupt u-boot now, but if you simply let it run it’ll boot with init=/bin/sh which will allow you to remove the root password.

Step 4 – Getting Root

Delete the root password, add a getty to the serial port, and enable ssh. For detailed instructions, go here.

In summary:

/ # passwd -d root
/ # echo 'ttyAM0::respawn:/sbin/getty -L  ttyAM0 115200 vt100' >> /etc/inittab
/ # mount -a
/ # ubiattach -p /dev/mtd3
/ # mount -t ubifs ubi1:database /database
/ # echo '1' > /database/ENABLE_SSH

You’ll also want to add your public key to /root/.ssh/authorized_keys so that you can log in via ssh.

Next up: dumping a NAND image via JTAG.

One thought on “Wink Hub – Rooting via JTAG – Part II

  1. Don Smyth

    Hi Jason, I was able to get JTAG working with my flyswatter2, but when I load u-boot, it doesn’t run. If I verify_image, the loaded data doesn’t match. Any ideas on what might be going wrong? I’m using your version of imx28evk.cfg. I use this as a startup script:

    adapter_khz 4000
    source [find interface/ftdi/flyswatter2.cfg]
    source [find imx28evk.cfg]

    Here’s what I get on startup:
    openocd -f wink.cfg
    Open On-Chip Debugger 0.10.0-dev-00002-g79fdeb3-dirty (2015-07-28-11:32)
    Licensed under GNU GPL v2
    For bug reports, read
    http://openocd.org/doc/doxygen/bugs.html
    adapter speed: 4000 kHz
    trst_and_srst separate srst_gates_jtag trst_push_pull srst_open_drain connect_deassert_srst
    adapter_nsrst_delay: 100
    Info : auto-selecting first available session transport “jtag”. To override use ‘transport select ‘.
    jtag_ntrst_delay: 100
    dcc downloads are enabled
    imx28evk_init
    Info : clock speed 4000 kHz
    Info : JTAG tap: imx28.cpu tap/device found: 0x079264f3 (mfg: 0x279, part: 0x7926, ver: 0x0)
    Info : Embedded ICE version 6
    Info : imx28.cpu: hardware has 2 breakpoint/watchpoint units

    Then I do the following from telnet:

    telnet localhost 4444
    Trying 127.0.0.1…
    Connected to localhost.
    Escape character is ‘^]’.
    Open On-Chip Debugger
    > soft_reset_halt
    requesting target halt and executing a soft reset
    target state: halted
    target halted in ARM state due to debug-request, current mode: Supervisor
    cpsr: 0x000000d3 pc: 0x00000000
    MMU: disabled, D-Cache: disabled, I-Cache: disabled
    > reset init
    JTAG tap: imx28.cpu tap/device found: 0x079264f3 (mfg: 0x279, part: 0x7926, ver: 0x0)
    target state: halted
    target halted in ARM state due to debug-request, current mode: Abort
    cpsr: 0x200000d7 pc: 0x00000010
    MMU: disabled, D-Cache: disabled, I-Cache: disabled
    NOTE! Severe performance degradation without working memory enabled.
    NOTE! Severe performance degradation without fast memory access enabled. Type ‘help fast’.
    > cd u-boot
    > load_image u-boot.bin 0x40000100
    No working memory available. Specify -work-area-phys to target.
    not enough working area available(requested 24)
    no working area available, falling back to memory writes
    451108 bytes written at address 0x40000100
    downloaded 451108 bytes in 5.772132s (76.321 KiB/s)
    > resume 0x40000100
    >

    Any thoughts?

    – Don

Leave a Reply

Your email address will not be published. Required fields are marked *