Bomblab

Due: Wednesday, October 21, 11:55 PM

Introduction

A mysterious villain 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 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.

There are too many bombs for us to deal with, so we are giving each student a bomb to defuse (don’t question the morality of this decision). Your mission, if you choose to accept it (just kidding you have to accept it), is to defuse your bomb before the due date. Good luck, and welcome to the bomb squad!

0. Get Your Bomb

Connect to UCLA Wi-Fi or the UCLA VPN, then log in to the class machine and create a private working directory:

ssh YOUR_SEASNET_USERNAME@cs33.seas.ucla.edu
mkdir -m 700 ~/bomblab
cd ~/bomblab

You can obtain your bomb by pointing your Web browser at:

http://cs33.seas.ucla.edu:15213/

This will display a binary bomb request form for you to fill in. Enter your 9-digit UID without dashes and your email address and hit the Submit button. The server will build your bomb and return it to your browser in a tar file called bombk.tar, where k is the unique number of your bomb. Use your own UID; requesting again with the same UID returns the same bomb.

Upload the bombk.tar file to your private working directory on the server. For macOS/Linux users, run this command on your own computer, replacing bombk.tar with the downloaded filename:

scp bombk.tar YOUR_SEASNET_USERNAME@cs33.seas.ucla.edu:~/bomblab/

For Windows users, you can use WinSCP. Useful resources about WinSCP here.

On the server, extract the archive:

tar -xvf bombk.tar

This will create a directory called ./bombk with the following files:

• README: Identifies the bomb and its owners.

• bomb: The executable binary bomb.

• bomb.c: Source file with the bomb’s main routine and a friendly greeting.

Change into the extracted directory with cd bombk, replacing k with your bomb number.

Alternative Method: Get Your Bomb Using Command Line

If you’re comfortable with the command line, you can download your bomb directly using curl in your private working directory on cs33.seas.ucla.edu:

curl --fail --show-error --get \
  --data-urlencode "username=YOURUID" \
  --data-urlencode "usermail=YOUR_EMAIL@g.ucla.edu" \
  --data-urlencode "submit=Submit" \
  --output bomb.tar \
  "http://cs33.seas.ucla.edu:15213/"

Replace the placeholders with your UID and email address, then extract the archive:

tar -xvf bomb.tar

This creates a directory such as bomb71. Change into your own bomb’s directory before running GDB or the bomb (for example, cd bomb71).

1. Defuse Your Bomb

Your job for this lab is to defuse your bomb on cs33.seas.ucla.edu.

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 :).

Each time your bomb explodes it notifies the bomblab server. The first 3 explosions are free; afterward, explosions incur a 1/2-point penalty, with deductions rounded down to whole points and capped at 20 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 10 points each. Phases 5 and 6 are a little more difficult, so they are worth 15 points each. So the maximum score you can get is 70 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 kindne…er I mean weakness, this feature was added so you don’t have to keep retyping the solutions to phases you have already defused (you’re welcome).

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 as a final gift, you will be given 3 free explosions, no questions asked. Use them wisely :)

2. Logistics

This is an individual project. All handins are electronic. Clarifications and corrections will be posted on the course message board.

3. Handin

There is no explicit handin. 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:

http://cs33.seas.ucla.edu:15213/scoreboard

This web page is updated every 30 seconds to show the progress for each bomb.

4. 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:

• Incorrect guesses can cost points when the bomb explodes, as described in the grading rules above.

• Every time you guess wrong, a message is sent to the bomblab server. You could very quickly saturate the network with these messages, and cause the system administrators to revoke your computer access.

• We haven’t told you how long the strings are, nor have we told you 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 26^80 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.

Don’t Try to Cheat with GDB: Jumping to success functions with GDB won’t help - the server checks your actual input strings, not whether you bypassed local validation.

Note on phase variants: Each phase has multiple variants (a, b, c). Different students will get different variants, so your bomb’s assembly will look different from your classmates’. Don’t assume their solutions will work for you - understand the logic instead.

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 following web site 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.

• Feel free to trust the names of function calls that parse strings. For instance, if you see a function called “read_two_numbers”, you can trust that it will read two numbers, with a space inbetween the numbers.

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.

Another helpful aside, when going through the assembly code, you can safely step over any call to a function with a name prefixed with “__” (double underscore) or suffixed with “@plt”