Lab Assignment 2: Defusing a Binary Bomb

DUE DATES:

For the “trivia” assignment: 11:59pm, Friday, September 18. 

For the main assignment: 11:59pm, Monday, September 28. 

If you want to use slip days you have to tell us before the deadline by emailing Sudhanshu Gupta. 

Introduction

The goal of this assignment is to learn both how program memory is laid out and details of a sample assembly language (x86; and its corresponding machine code) by learning how to read assembly code. As a nice side effect, you will considerably improve your debugging skills.

The nefarious Dr. Evil has planted a slew of “binary bombs” on our machines.  A binary bomb is a program that consists of a sequence of phases.  Each phase expects you to type a particular string on standard input.  If you type the correct string, then the phase is defused and the bomb proceeds to the next phase.  Otherwise, the bomb explodes by printing “BOOM!!!” and then terminating.  The bomb is defused when every phase has been defused. 

There are too many bombs for us to deal with, so we are giving each group a bomb to defuse.  Your mission, which you have no choice but to accept, is to defuse your bomb before the due date.  Good luck, and welcome to the bomb squad! 

Step 1: Get Your Bomb

Each group of students will attempt to defuse their own personalized bomb.  Each bomb is a Linux binary executable file that has been compiled from a C program.  To obtain your group’s bomb, one (and only one) of the group members should point your Web browser to the bomb request daemon at

http://cycle1.csug.rochester.edu:15213,

which is only accessible on the csug network.

Since you are accessing the csug machines remotely, one way to access the webpage is to ssh into any of the CSUG cycle machines and use a text-based browser such as lynx.

On the command line, run "lynx <url>" to open the specified URL in the web browser. For instance, to request the bomb, you will type: "lynx http://cycle1.csug.rochester.edu:15213". Hit enter and the corresponding page will be loaded. Fill in your netID and email address in the corresponding fields. You can use the arrow keys to move your cursor around. Move your cursor to "submit". Press "enter/return" and the browser will prompt you to download bombk.tar. Accept the download by pressing "D". In the download options, select "Save to disk" and then press enter again when prompted to enter the filename. Press 'Q' twice to exit lynx.

Fill out the HTML form with the email address and NetID of just one of the team members, and then submit the form by clicking the “Submit” button.  The request daemon will build your bomb and return it immediately to your browser in a tar file called bombk.tar, where k is the unique number of your bomb. 

Save the bombk.tar file to a (protected) directory in which you plan to do your work.  Then give the command: tar -xvf bombk.tar.  This will create a directory called bombk with the following files: 

Also, if you make any kind of mistake requesting a bomb (such as neglecting to save it or typing the wrong group members), simply request another bomb.  If you change groups but have not had any explosions from your current bomb, request another bomb; we'll sort out the duplicate assignments when we do the grading. You can't request a new bomb just because you want to get away with the explosion penalty from your current bomb.  

Step 2: Defuse Your Bomb

Once you have received your bomb from the bomb daemon, save it in a secure directory.  Your job is to defuse the bomb. 

The bombs are limited to run on the CSUG cycle machines only. You need to ssh to one of the cycle servers (cycle1, cycle2, cycle3.csug.rochester.edu) and run it there. Contact the TA with any problems related to this issue.

You can use many tools to help you with this; please look at the hints section for some tips and ideas.  The best way is to use your favorite debugger to step through the disassembled binary. 

Each time your bomb explodes it notifies the TAs, and you lose 1/4 point (up to a max of 10 points) in the final score for the lab.  So there are consequences to exploding the bomb.  You must be careful! 

Each phase is worth 10 points, for a total of 60 points. 

The phases get progressively harder to defuse, but the expertise you gain as you move from phase to phase should offset this difficulty.  However, the last phase will challenge even the best students, so please don’t wait until the last minute to start. 

The bomb ignores blank input lines.  If you run the bomb with a command line argument such as bomb_codes, it will read the input lines from bomb_codes until it reaches EOF, and then switch over to standard input.  In a moment of weakness, Dr. Evil added this feature so you don’t have to keep retyping the solutions to phases you have already defused. 

To avoid accidentally detonating the bomb, you will need to learn how to single-step through the assembly code and how to set breakpoints.  You will also need to learn how to inspect both the registers and the memory states.  One of the nice side-effects of doing the lab is that you will get very good at using a debugger.  This is a crucial skill that will pay big dividends the rest of your career. 

Logistics

As noted above, you may work in a group of up to 2 people. 

Any clarifications and revisions to the assignment will be posted to the course blackboard.

“Trivia” or Pre-Assignment

Before noon, Thurday, September 18, submit answers to the following questions using blackboard (each student, regardless of whether in a team, is required to submit answers individually): 

  1. Are you working alone or in a team of two?  If the latter, who is your partner? 
  2. Give the names of all the functions in the bomb executable. 
  3. What does bomb print when you run it without any arguments?  (Be careful not to accidentally set if off while doing this! You can press ^C (Ctrl-C) to exit the bomb at any time.) 
  4. Set a breakpoint at line 73 in main. What is the PC value of the line there? (Again, make sure you "kill" the program and "quit" GDB to prevent setting off the bomb. Note that if you are executing the bomb inside GDB but not stopped at a breakpoint, you will still need to use ^C (Ctrl-C) to stop its execution.)

Turn-In

There is no explicit turn-in.  The bomb will notify us automatically after you have successfully defused it.  You can keep track of how you (and the other groups) are doing by looking at the scoreboard.  This web page is updated every minute to show the progress of each group. Note that the scoreboard is also accessible on the CSUG machines only, i.e. you will need to use lynx to access it. 

Hints (Please read this!)

There are many ways to go about defusing your bomb.  You can examine it in great detail without ever running the program, and figure out exactly what it does.  This is a useful technique, but it is not always easy to do.  You can also run it under a debugger, watch what it does step by step, and use this information to defuse it.  This is probably the fastest way of defusing it. 

We do make one request, please do not use brute force!  You could write a program that will try every possible key to find the right one.  But this is no good for several reasons:

There are many tools which are designed to help you figure out both how programs work, and what is wrong when they don’t work.  Here is a list of some of the tools you may find useful in analyzing your bomb, and hints on how to use them. 

Looking for a particular tool?  Don’t forget, the commands apropos and man are your friends.  In particular, man ascii might come in useful.  Also, the web may also be a treasure trove of information.  If you get stumped, feel free to ask the TAs for help.