Project 5: Bomblab
Each pair of students will aempt 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 pair’s bomb, one (and only one) of the pair members should point your Web browser to the bomb
Once you have downloaded a bomb, you must complete the assignment using that bomb; making submissions from different bomb downloads can confuse the record- keeping on the server side and the wrong score is likely to be assigned to your work.
Enter your user name and the email address for one of you, and hit the Submit buon. 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.
Save the file to a (protected) directory in which you plan to do your work. Then give the command: . This will create a directory called ./bombk with the following files:
README : Idenfies the bomb and its owners.
bomb : The executable binary bomb.
bomb.c : Source file with the bomb’s main roune and a friendly greeng from Dr. Evil.
Step 2: Defuse Your Bomb
Your job for this lab is to defuse your bomb.
You must do the assignment on one of the class machines. 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 secon for some ps and ideas. The best way is to use your favorite debugger to step through the disassembled binary.
Each me your bomb explodes it nofies the bomblab server, and you lose 1/4 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!
tar -xvf bombk.tar
The first four phases are worth 10 points each. Phases 5 and 6 are a lile 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 experse 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 unl 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 psol.txt
then it will read the input lines from psol.txt unl 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 soluons to phases you have already defused.
To avoid accidentally detonang 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.
All hand ins are electronic. Clarificaons and correcons will be posted on the course Forum Board.
There is no explicit hand in. The bomb will nofy your instructor automacally about your progress as you work on it. You can keep track of how you are doing by looking at the class scoreboard at:
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 informaon 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:
You lose 1/4 point (up to a max of 20 points) every me you guess incorrectly and the bomb explodes.
Every me 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) assumpons that they all are less than 80 characters long and only contain leers, then you will have 2680 guesses for each phase. This will take a very long me 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.
The GNU debugger, this is a command line debugger tool available on virtually every plaorm. 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
has a very handy single-page gdb summary that you can print out and use as a reference. Here are some other ps for using gdb .
To keep the bomb from blowing up every me you type in a wrong input, you’ll want to learn how to set breakpoints.
For online documentaon, type “ help ” at the gdb command prompt, or type “ ”, or “ info gdb ” at a Unix prompt. Some people also like to run gdb under in .
This will print out the bomb’s symbol table. The symbol table includes the names of all funcons and global variables in the bomb, the names of all the funcons the bomb calls, and their addresses. You may learn something by looking at the funcon names!
Use this to disassemble all of the code in the bomb. You can also just look at individual funcons. Reading the assembler code can tell you how the bomb works.
Although objdump -d gives you a lot of informaon, it doesn’t tell you the whole story. Calls to system-level funcons are displayed in a crypc form. For example, a call to sscanf might appear as:
To determine that the call was to sscanf , you would need to disassemble within gdb . strings
This ulity will display the printable strings in your bomb.
Looking for a parcular tool? How about documentaon? Don’t forget, the commands apropos , man , and info are your friends. In parcular, 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 informaon. If you get stumped, feel free to ask your instructor for help.