Lab 2: Disassembling and Defusing a Binary Bomb

Learning Objectives

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 stdin (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 everyone 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!

Everyone gets a different bomb to defuse! Enter your UW NetID to download your personal bomb:

You will be prompted to sign in with your UW NetID.

Alternately, you may open the following link in your browser (replacing <netid> with your UW NetID):

https://courses.cs.washington.edu/courses/cse351/23sp/files/labs/lab2/<netid>/lab2-bomb.tar

Unzip

Running tar xvf lab2-bomb.tar will extract the lab files to a directory called bomb<num> (where <num> is the ID of your bomb) with the following files:

You should do this assignment on a 64-bit CSE Linux VM or a CSE lab Linux machine or on attu. Be sure to test your solution on one of those platforms before submitting it, to make sure it works when we grade it!In fact, there is a rumor that Dr. Evil has ensured 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.

Your job is to find to correct strings to defuse the bomb. Look at the Tools section for ideas and tools to use. Two of the best ways are to (a) use a debugger to step through the disassembled binary and (b) print out the disassembled code and step through it by hand.

The bomb has 5 regular phases. The 6th phase is extra credit, and rumor has it that a secret 7th phase exists. If it does and you can find and defuse it, you will receive additional extra credit points. The phases get progressively harder to defuse, but the expertise you gain as you move from phase to phase should offset this difficulty. Nonetheless, the latter phases are not easy, so please don't wait until the last minute to start! (If you're stumped, check the Hints section at the end of this document.)

Bomb Input

From the keyboard

Running the bomb executable without arguments will take user input typed into the terminal (i.e., the program will pause, you will type a string, then press [Enter]):

$ ./bomb

If you are running the bomb within gdb, then you use the standard run command and type input into the terminal when the program is paused and you don't see the usual gdb prompt:

(gdb) run

Blank input lines are ignored. The bomb will print some text in-between each phase which will indicate success or failure. On success, it will pause again for the next user input; on failure, it will exit back to the shell prompt.

From a file

Running the bomb executable with the name of a file will read the phase input strings from the lines of the file. For this lab, we are dictating that you use defuser.txt:

$ ./bomb defuser.txt

If you are running the bomb within gdb, then you use append this argument to the run command:

(gdb) run defuser.txt

It will then read the input lines from defuser.txtuntil it reaches EOF (end of file), and then switch over to stdin (standard input from the terminal). 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, instead you can put them in defuser.txt.

Note: you will turn in your defuser.txt file as part of this lab. It is extremely important that your defuser.txt has a newline character at the end of the file. You can check this by using the Linux cat command from the terminal. If you type:

$ hexdump -C defuser.txt

and your defuser.txt contains the sentence This is not the real answer., the output will look something like:

00000000  54 68 69 73 20 69 73 20  6e 6f 74 20 61 20 72 65  |This is not a re|
00000010  61 6c 20 61 6e 73 77 65  72 2e 0a                 |al answer..|
0000001b

This shows the contents of the file, printed out as bytes in hex. (Remember, each byte is two hex characters.) Notice that the last hex character in the file is 0x0a, which is the newline character in ASCII. So thisdefuser.txt does end with a newline. However, if you type hexdump -C defuser.txt and your file does not end with a newline, you'll instead get something like this:

00000000  54 68 69 73 20 69 73 20  6e 6f 74 20 61 20 72 65  |This is not a re|
00000010  61 6c 20 61 6e 73 77 65  72 2e                    |al answer.|
0000001a

Notice that the last character is now 0x2e, which corresponds to the . character in ASCII (the period in the sentence). To add a newline to the end of your file, you can use the command:

$ echo "" >> defuser.txt

The echo command prints out its argument, and the >> redirects that argument to the file defuser.txt. By default, echo appends a newline character to its output. So if you supply an empty string to echo, it will just add the newline character to the end of the file.

To avoid accidentally detonating the bomb, you will need to learn how to single-step through the assembly code in gdb 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.

During this lab, we strongly recommend that you keep notes of the steps you took in solving each stage. This will be immensly helpful in helping you to keep track of what's stored at important addresses in memory and in registers at different points in the program's execution. (A good strategy for this might be to keep a notetaking app open on your computer so you can copy and paste values between it and gdb.)

Helpful Information

There are many online resources that will help you understand any assembly instructions you may encounter. In particular, the instruction references for x86-64 processors distributed by Intel and AMD are exceptionally valuable. They both describe the same ISA, but sometimes one may be easier to understand than the other.

Important Note: The instruction format used in these manuals is known as “Intel format”. This format is very different than the format used in our text, in lecture slides, and in what is produced by gcc, objdump and other tools (which is known as “AT&T format”. You can read more about these differences in our textbook (p.177) or on Wikipedia. The biggest difference is that the order of operands is SWITCHED. This also serves as a warning that you may see both formats come up in web searches.

x86-64 Calling Conventions

The x86-64 ISA passes the first six arguments to a function in the following registers (in order):%rdi, %rsi, %rdx, %rcx, %r8, %r9.
The return value of a function is passed in %rax.

Using sscanf

First let's look at scanf ("scan format"), which reads in data from stdin (the keyboard) and stores it according to the format specifier into the locations pointed to by the additional arguments:

int i;
printf("Enter a number: ");
scanf("%d", &i);

The printf prints a prompt, once the user enters in a number and hits enterscanf will store the input from stdin into i with the format of an integer. Notice how scanf uses the address of i as the argument.

Lab 2 uses sscanf ("string scan format"), which is similiar to scanf but reads in data from a string instead of stdin:

char* mystring = "123, 456";
int a, b;
sscanf(mystring, "%d, %d", &a, &b);

Reference information can be found online for sscanf, scanf, and printf.

Tools (Read This!!)

There are many ways of defusing your bomb. You can print out the assembly and 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. Both are useful skills to develop.

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 the number of possibilities is so large that you won't be able to try them all in time.

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.

// DO NOT RUN THIS unless you are going to CSE 002 to pick up the printout (save the trees)!!!
$ enscript -h -2r -Pps002 -DDuplex:true bomb_disas

Looking for a particular tool? How about documentation? Don't forget, the commands apropos and man are your friends. In particular, man ascii is more useful than you'd think. If you get stumped, use the course's discussion board.

Hints

The correct string has a particular format for each phase: It could be a phrase, sequence of numbers, characters, or both. We suggest figuring out what format is required before trying to understand what is happening at each phase! If you're still having trouble figuring out what your bomb is doing, here are some hints for what to think about at each stage:

REMINDER: You will need to use the CSE Linux environment in order to get addresses that are consistent with our solutions.

Start with a fresh copy of lab0.c and examine part_2() using the following commands:

$ wget https://courses.cs.washington.edu/courses/cse351/23wi/files/labs/lab0.c
$ gcc -g -std=c18 -o lab0 lab0.c
$ gdb lab0
(gdb) layout split
(gdb) break fill_array
(gdb) break part_2
(gdb) run 2

Now answer the following questions. You will find the following GDB commands useful: nexti, finish, print, and refresh.

It is important to make sure that defuser.txt obeys the following formatting rules, otherwise our grading script is likely to conclude you defused zero bombs:

There is no compilation for this lab, so it is your responsibility to make sure that your defuser.txt file works on your assigned bomb.

Submit your completed defuser.txt and lab2synthesis.txt files to the "Lab 2" assignment on Gradescope.

If you completed the extra credit, also submit the same defuser.txt file to the "Lab 2 Extra Credit" assignment on Gradescope.

After submitting, please wait until the autograder is done running and double-check that you passed the "File Check" and "Compilation and Execution Issues" tests. If either test returns a score of -1, be sure to read the output and fix any problems before resubmitting. Failure to do so will result in a programming score of 0 for the lab.

It is fine to submit multiple times up until the deadline, we will only grade your last submission. NOTE that if you do re-submit, you MUST RE-submit BOTH files again.