Home

Awesome

About JTAGenum

JTAGenum is an open source Arduino JTAGenum.ino or RaspbberyPi JTAGenum.sh (experimental) scanner. This code was built with three primary goals:

  1. Given a large set of pins on a device determine which are JTAG lines
  2. Enumerate the Instruction Register to find undocumented functionality
  3. be easy to build and apply

JTAGenum is a more Arduino'y fork of Arduinull by Sébastien Bourdeauducq (lekernel), which is inspired by Benedikt Heinz's JTAG scanner. JTAGenum also includes instruction scanning functionality best described by Felix Domke (tmbinc) in his 26c3 paper. The initial version of this branch was built for personal research and while working on various projects at Recurity Labs.

Please feel free to contact me with any questions, problems, targets or updates. I would be more than happy if you fork and take the code in whatever direction you choose.

Links

Authors and code branches

Similar tools or branches:

Hardware

JTAGenum has been tested on the following hardware:

When picking your micro-controller platform consider two issues:

  1. How many pins do you want to check on your target.
  2. what voltage level does your target device require.

Concerning voltage RaspberryPi's I/O operate at 3.3v, many Arduinos work at 5 volts. Some are switchable but even those that are not could be modified. Alternatively voltage shifting Arduino shields or voltage shifting gadgets can be used. See the Voltage Shifting Appendix discussion on the Embedded Analysis wiki for more details. https://github.com/cyphunk/JTAGenum/wiki/Embedded-Analysis#Voltage_Shifting

When connecting the micro-controller to the pins of your target one thing to be aware of is possible cross-talk between wires. The loopback check function in JTAGenum cab help you determine which wires may produce cross talk.

Usage

For use on Raspberry Pi use and consult the JTAGenum.sh. The Raspberry Pi pins being used for scanning should be specified inside the script file. This script is experimental and only provides the functions for finding JTAG. To use the script should be sourc'ed on the console the user should execute the desired scan. See the comments in the header of the script for further details.

For use on a Arduino the JTAGenum.ino sketch is loaded. The Arduino pins being used for scanning should first be specified at the top of the sketch. This is all that is required for basic JTAG scanning functionality. Once the correct JTAG pins on the target have been determined they can be specified in the script and along with the defining the proper IR_LENGTH the user can then execute the search for hidden instructions or print the boundary scan register.

Before loading the sketch first define the pins[] and pinnames[] arrays. After loadin the sketch open a serial console at baud of 115200 to access the user interface. Sending a h to the console will print usage information that describes each function. Each function is enacted by sending the defined one character code:

v > verbose

Toggles verbose output. At times verbose might present too much information or without it too little.

l > loopback check

Find loopback pairs that will generate false-positives for other tests. After running you should remove any loopback pairs from your pins[]/pinnames[]. Looback pairs are found by sending a predetermined pattern[] to all possible pins while checking all pins for matching output. Because the JTAG clock (TCK) and state (TMS) pins are NOT being stimulated the input/output pairs where the pattern is found represent loopbacks. NOTE: you should probably run this once with and without internal pull-up resistors set (r) to avoid problems of cross-talk which is discussed in detail later.

s > scan

This routine is used to check all possible pins and find JTAG clock, state, input and output pins lines (TCK,TMS,TDI,TDO). This is done by setting the JTAG state (TMS) into Shift_IR mode and then sending pattern[] to TDI and checking for it on TDO while clocking TCK. This check is run for every possible pin combination and it is important that you remove loopback pins before running. While this scan is meant to determine all of the JTAG pins required it is possible that the TMS pin found is incorrect. This depends on if the target uses the bypass register by default (described later). If an IDCODE register is present then bypass mode is not the default and you can assume that the pin this scan defines as TMS is correct. Otherwise, only the TCK, TDI and TDO pins can be determined. NOTE: run with pull-ups on (r) as any cross-talk might result in false-positives.

y > brute force IR search

This will set the instruction register (IR) to all possible values and check the output. This can be used to find undocumented instructions and examine their results via the data register (DR). To run this scan you should have already determined the 4 JTAG pins and define pins[] as such: [0]=TCK [1]=TMS [2]=TDO [3]=TDI. NOTE: run with pull-ups on (r) as any cross-talk might result in false-positives.

x > boundary scan

This will return the state of all the pins on the target. Actually it is not just the pins but the contents of the scan/sample register. This should be a rather large register and is defined in the code by SCAN_LEN+100. You can check your targets documentation and specify this or just leave it as a large number (currently 1800). To run this scan you should have already determined the 4 JTAG pins and define pins[] as such: [0]=TCK [1]=TMS [2]=TDO [3]=TDI. NOTE: run with pull-ups on (r) as any cross-talk might result in false-positives.

i > idcode scan

The JTAG standards specify that if an idcode register is present it should be set as the default data register (DR) and attached to output (TDO) by default. Meaning, regardless of the state of the JTAG chip (set with TMS line) and regardless of input being sent to the chip (TDI) by clocking the chip (TCK) it should return the contents of the idcode to the output (TDO). Hence, this routine iterates through all possible TCK,TDO pairs of pins and prints the output when it changes (we assume an idcode will not be all 0s or 1s). You should examine the documentation of your target(s) to see if the idcode matches. NOTE: run with pull-ups on (r) as any cross-talk might result in false-positives.

b > shift_bypass

Broken atm (need to add TCK enumeration). The JTAG standards specify that if and idcode register is NOT present on the chip then the bypass register (length of 1) should be the default DR. Essentially this means what is sent to the input (TDI) should come out on the output (TDI) with a one clock delay (TCK). It is important that you remove loopbacks before running this test otherwise the loopback pins will look like valid JTAG lines. NOTE: run with pull-ups on (r) as any cross-talk might result in false-positives.

r > set pull-up resistors & cross-talk

If like me the cables you use to connect between JTAGenum to your targets are flimsy or uninsulated you might run into issues of cross-talk whereby when one pin is transmitting a nearby pin picks up the transmission even though they are not connected. To avoid this you can turn on the internal pull-up resistors which will force the pin to a default state. If for some reason you continue to have sporadic issues run the following in sequence to check if the problem is the cable, target or other:

  1. Disconnect the cables between your target and JTAGenum. Disconnected them entirely from JTAGenum as well.

  2. Run a loopback check (l) with pull-ups off. In this state the pins are in open mode and might fluctuate. Youll notice that as you move the microcontroller around, turn lights on and off or move other devices close to or away from it that the results change.

  3. Turn on pull-ups (r) and run the test again. The results should now be consistent. If they arent, then let me know.

  4. Now attach your cables to JTAGenum but not the target. Run steps 2 and 3 again. Step 2 will give you a feel for how much inconsistency the cable may add. If the loopback check results in actual pattern matches then your cable has cross-talk. Step 3 should still result in a consistent state of either all high (1s) or all low (0s) and if it doesnt then your cross-talk issues are such that all JTAGenum tests are going to be buggy at best. Feel free to give me an email and I will happily try to help solve the problem.

A bit about JTAG

Basic understanding of how JTAG works will be helpful when using JTAGenum. There are 4 lines/pins: TDO=output, TDI=input, TCK=clock, TMS=state machine control. Say you want to read the ID of the chip. First you would send the IDCODE instruction to the instruction register (IR). The JTAG controller then places the actual id code value of the chip in a data register which you could then read out. You would think that it would be enough to have one input line going to the IR and one output coming from the DR but JTAG also supports writing to the DR. As apposed to adding another input line specific to the DR instead JTAG works by moving the input and output lines between IR and DR. The TMS line is used to switch TDI/TDO to IR when you want to place an instruction and back to DR when you want to read or write data. With all operations, be it state change (TMS) reading (TDI) or writing (TDO), the clock line must be cycled once (TCK) for every bit or change. This was a brutal and drastic simplification but with that understood reading the Usage section should be comprehensible.

For a more detailed discussion of JTAG see https://github.com/cyphunk/JTAGenum/wiki

TODO

  1. upload pictures of the hardware setups
  2. add ESP32 support
  3. BusPirate bitbang support