Machine problem 2: Binary bomb

Objectives

  1. Learn to use GDB and other command line debugging utilities
  2. Practice debugging and tracing binary executables
  3. Learn to read and understand x86 assembly, including:
    • addressing modes
    • procedure call conventions
    • control structure implementation

Introduction

The nefarious Dr. Evil has planted a slew of "binary bombs" on our class 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 student 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

As with the previous lab, start by claiming your repository on GitHub via this invitation link. After accepting it, clone your repository on Fourier with the command (replacing USER with your own username):

git clone https://github.com/cs351/mp-bomb-USER.git

You should now have a directory named "mp-bomb-USER". cd into it and obtain your bomb by running the command ./getbomb.sh.

This command will download a bomb from a remote server and create a directory named "bombN", where N is the unique number of your bomb. In that directory you'll find the following files:

If for some reason you request multiple bombs, this is not a problem. Choose one bomb to work on and delete the rest.

Step 2: Defuse Your Bomb

Your job for this lab is to defuse your bomb.

For this assignment you must do the assignment on fourier.cs.iit.edu. In fact, there is a rumor that Dr. Evil really is evil, and the bomb will always blow up if run elsewhere. There are several other tamper-proofing devices built into the bomb as well, or so we hear.

You can use many tools to help you defuse your bomb. Please look at the hints section for some tips and ideas. The best way is to use gdb to step through the disassembled binary.

Each time your bomb explodes it notifies the bomblab server, 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!

The first four phases are worth 6 points each. Phases 5 and 6 are a little more difficult, so they are worth 8 points each. So the maximum score you can get is 40 points.

Although phases get progressively harder to defuse, 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 your bomb with a command line argument, for example,

> ./bomb psol.txt

then it will read the input lines from psol.txt until it reaches EOF (end of file), and then switch over to stdin. 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 (and this class).

Submission

There is no explicit handin for this lab. You do not need to commit or push any files to GitHub. The bomb will notify your instructor automatically about your progress as you work on it. You can keep track of how you are doing by looking at the class scoreboard at https://moss.cs.iit.edu/cs351/bomblab-scoreboard.html. This web page is updated continuously to show the progress for each bomb.

Hints (Please read this!)

There are many ways of 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 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? How about documentation? Don't forget, the commands apropos, man, and info are your friends. In particular, man ascii might come in useful. info gas will give you more than you ever wanted to know about the GNU Assembler. If you get stumped, feel free to ask for help.