Bomb Lab

Due: 11:00pm, Tuesday October 6, 2020

Max grace days: 2

Introduction

A binary bomb is a program that consists of a sequence of phases. Each phase expects you to type a particular string on stdin. 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.

Step 1: Your Bomb

You will receive your unique bomb via email. Save the tar file to a (protected) directory on the Linux server. Then execute the command:

    tar -xvf bomb-USER.tar

where USER is your Kutztown University username. This will create a directory called ./bomb-USER with the following files:

• bomb: The executable binary bomb.

• bomb.c: Source file with the bomb’s main routine.

• sol.txt: An empty text file where you will put your solution.

• Makefile: Makefile with a target to submit your sol.txt file.

Step 2: Defuse Your Bomb

Your job for this lab is to defuse your bomb.

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 your favorite debugger to step through the disassembled binary.

Although phases get progressively harder to defuse, the expertise you gain as you move from phase to phase should offset this difficulty.

The bomb ignores blank input lines. If you run your bomb with a command line argument, for example,

    linux> ./bomb sol.txt

then it will read the input lines from sol.txt until it reaches EOF (end of file), and then switch over to stdin. This feature makes it 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.

Handin Instructions

The provided makefile has a target named submit. You need to put your solution in the sol.txt file. Then, to submit your assignment execute the command

    linux> make submit

This will copy the appropriate files to a protected directory owned by your instructor.

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. However, you do not know how long the strings are, nor do you know what characters are in them. Even if you made the (incorrect) assumptions that they all are less than 80 characters long and only contain letters, then you will have 26⁸⁰ guesses for each phase. This will take a very long time to run, and you will not get the answer before the assignment is due.

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.

• gdb

  The GNU debugger, this is a command line debugger tool available on virtually every platform. You can trace through a program line by line, examine memory and registers, look at both the source code and assembly code (we are not giving you the source code for most of your bomb), set breakpoints, set memory watch points, and write scripts.

  The CS:APP web site

  http://csapp.cs.cmu.edu/public/students.html

  has a very handy single-page gdb summary that you can print out and use as a reference. Here are some other tips for using gdb.

  • To keep the bomb from blowing up every time you type in a wrong input, you’ll want to learn how to set breakpoints.

  • For online documentation, type “help” at the gdb command prompt, or type “man gdb”, or “info gdb” at a Unix prompt. Some people also like to run gdb under gdb-mode in emacs.

• objdump -t

  This will print out the bomb’s symbol table. The symbol table includes the names of all functions and global variables in the bomb, the names of all the functions the bomb calls, and their addresses. You may learn something by looking at the function names!

• objdump -d

  Use this to disassemble all of the code in the bomb. You can also just look at individual functions. Reading the assembler code can tell you how the bomb works.

  Although objdump -d gives you a lot of information, it doesn’t tell you the whole story. Calls to system-level functions are displayed in a cryptic form. For example, a call to sscanf might appear as:

  8048c36:  e8 99 fc ff ff  call   80488d4 <_init+0x1a0> 

  To determine that the call was to sscanf, you would need to disassemble within gdb.

• strings

  This utility will display the printable strings in your bomb.

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. Also, the web may also be a treasure trove of information. If you get stumped, feel free to ask your instructor for help.

Grading Criteria

Your grade will be based on how many phases you successfully defuse.

• 1 phase: (60/100) D
• 2 phases: (76/100) C
• 3 phases: (86/100) B
• 4 phases: (90/100) A-
• 5 phases: (100/100) A