CS-211 Summer 2020
Lab Assignment L2: Defusing a Binary Bomb Assigned: July 14, Due: July 28
Shaleen Garg (shaleen.garg@rutgers.edu) is the lead person for this lab.
1 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 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. 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
You can obtain your bomb by pointing your Web browser at:
http://kill.cs.rutgers.edu:17200/
This will display a binary bomb request form for you to fill in. Enter your user name and email address and hit the Submit button. The server will build your bomb and return it to your browser in a tar file called bomb
The other method is to use the terminal command curl to download the file. This method is quite volatile and sending a wrong request may shut- down the server, so take caution. You want to use this command from the terminal:
curl -JLO
“http://kill.cs.rutgers.edu:17200/?username=
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with everything in a single line. < N > is your NetID and < E > should be your email address. Make sure that you ahve typed in all the punctuations correctly including quotation marks around the url. This command will download bomb < id > .tar to the P W D of your terminal. Refrain from downloading multiple versions of the bomb for yourself or your class mates. You need an iLab machine to do this assignment.
Save the bomb
• 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 from Dr. Evil.
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.
You must do the assignment on one ilab machines like {kill,ls,cd}.cs.rutgers.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 your favorite debugger to step through the disassembled binary.
Each time your bomb explodes it notifies the bomblab server, and you lose 1/2 point (up to a max of 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 15 points each. Phases 5 and 6 are a little more difficult, so they are worth 20 points each. So the maximum score you can get is 100 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,
linux> ./bomb solution.txt
then it will read the input lines from solution.txt until it reaches EOF (end of file), and then switch over to stdin. Each line in the file constitutes an input string for each phase. 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.
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Each time your bomb explodes it notifies the bomblab server , and you lose 1/2 point in the final score for the assignemnt. So there are consequences to exploding the bomb. You must be careful!
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.
Although phases get progressively difficult to defuse, the expertise you gain as you move from phase to phase should offset the difficulty. However, the last phase will challenge even the best of students, so please don’t wait until the last minute to start.
As always, if you have trouble, go to the office hours. If something doesn’t look good, contact us.
Grading
You can keep track of how you are doing by looking at the scoreboard at:
http://kill.cs.rutgers.edu:17200/scoreboard
Submission
You have to submit the assignment using Sakai. Your submission should be a tar file named bomb < id > .tar that can be extracted using the command:
tar -xvf bomb
Extracting your tar file should give a directory called bomb < id >. This directory should contain the same files that you downloaded, along with the file solution.txt to defuse the bomb. In other words, your directory should look like this:
.
– bomb
– bomb.c
– solution.txt
– README
To create the tar file that you will submit after finishing your assignment , you should use the following command in the parent directory of bomb < id >:
tar -cvf bomb
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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:
• Youlose1/2point(uptoamaxof20points)everytimeyouguessincorrectlyandthebombexplodes.
• 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 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!
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• 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.
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