The nefarious Dr. Evil has planted a slew of “binary bombs” on our 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 group 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!
Each group of students will attempt 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 group’s bomb, one (and only one) of the group members should point your Web browser to the bomb request daemon at
http://cycle1.csug.rochester.edu:15213
Fill out the HTML form with the email addresses and names of your team
members, and then submit the form by clicking the “Submit”
button. The request daemon will build your bomb and return it
immediately to your browser in a tar file called
bombk.tar, where k is the unique number of your
bomb.
Save the bombk.tar
file to a (protected) directory in which you plan to do your
work. Then give the command: 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.
If you change groups, simply request another bomb and we’ll sort out the duplicate assignments later on when we grade the lab.
Also, if you make any kind of mistake requesting a bomb (such as neglecting to save it or typing the wrong group members), simply request another bomb. If you change groups, simply request another bomb and we’ll sort out the duplicate assignments later on when we grade the lab.
Once you have received your bomb from the bomb daemon, save it in a secure directory. Your job is to defuse the bomb.
The bombs are limited to run on the cs undergraduate network only. We have tried to enable all machine on the undergraduate network, but machines come and go. If your machine doesn't want to run the bomb executable, ssh to one of the cycle servers (cycle1, cycle2, cycle3.csug.rochester.edu) and run it there. Contact the TA with any problems related to this issue.
You can use many tools to help you with this; 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 TAs, 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!
Each phase is worth 10 points, for a total of 60 points.
The phases get progressively harder to defuse, but 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 the bomb with a
command line argument such as bomb_codes, it will read the
input lines from bomb_codes until it reaches EOF, and then
switch over to standard input. 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.
As noted above, you may work in a group of up to 2 people.
Any clarifications and revisions to the assignment will be posted to the course blackboard.
You should do the assignment on 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 they say.
Before noon, Wednesday, Feb. 6, submit answers to the following questions using blackboard (each student, regardless of whether in a team, is required to submit answers individually):
gdb, how do you print a string whose base address is
stored in the register eax? (Hint: Something
similar to C)
There is no explicit turn-in. The bomb will notify us automatically after you have successfully defused it. You can keep track of how you (and the other groups) are doing by looking at spring08.html. This web page is updated continuously to show the progress of each group.
For the “trivia” assignment: noon, Tuesday, February 5.
For the main assignment: 11:59pm, Thursday, February 14.
There are many ways to go about 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 is 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.
gdb
The GNU debugger, 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.
Here are some tips for using gdb.
gdb summary.
gdb command prompt, or type “man gdb”,
“info gdb”, or “xinfo gdb” at a Unix
prompt. Many people prefer to run info mode inside
emacs.
You can even run gdb itself under gdb-mode in
emacs.
objdump -t or nm
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.
strings
This utility will display the printable strings in your bomb.
readelf
Displays all types of information on the object file format.
Looking for a particular tool? Don’t
forget, the commands apropos and man are
your friends. In
particular, man ascii might come in useful. Also,
the web may also be a treasure trove of information. If you get
stumped, feel free to ask the TAs for help.