Lab 2: Disassembling and Defusing a Binary Bomb

• Gain basic familiarity with x86-64 assembly instructions and how they are used to implement comparison, loops, switch statements, recursion, pointers, and arrays.
• Gain experience using the gdb debugger to step through assembly code and other tools such as objdump.

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 diffuse! Substitute <username> in the URL below with your UWNetID in order to find yours.

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.)

The bomb ignores blank input lines. If you run your bomb with a command line argument, for example,

$ ./bomb defuser.txt

then it will read the input lines from defuser.txt until 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 this defuser.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.)

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

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

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 part2() using the following commands:

$ wget https://courses.cs.washington.edu/courses/cse351/19sp/labs/lab0.c
$ gcc -g -std=c99 -o lab0 lab0.c
$ gdb lab0
(gdb) layout split
(gdb) break fillArray
(gdb) break part2
(gdb) run 2

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

1. At which memory addresses are the variables value and array from part2() stored?  [2 pt]
2. Which two registers (be specific as to the register width) determine if the assert() call in fillArray() fails?  [2 pt]
3. Give the relative addresses (i.e. of the form <function+#>) of the instructions that perform the initalization and update statements for the for-loop in fillArray.  [2 pt]
4. Consider the lea instruction at the relative address <part2+18>. Give an equivalent/replacement lea instruction that does not use %rbp.  [2 pt]
5. What address is the string "*** LAB 0 PART 2 ***\n" stored at in memory? Which part of the memory layout is this?  [2 pt]

Submit defuser.txt and lab2reflect.txt to the assignments page.

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

• Put your answer for each phase in one line. Your answer for phase 1 should be in the first line, answer for phase 2 on the second line, and so on.
• Do not put your name or other information at the top of the file. Again, you want the first line in the file to be your answer for phase 1.
• Do not add numbering or other “comments” for your answers (e.g. 1. This is my answer for phase 1).
• Make sure all your answers, including the last one, have a newline character afterwards, so even your last-phase answer is a complete line with a newline. This last newline is important for our grading script even though you will not notice the difference in your own testing.