Lab 2: Disassembling and Defusing a Binary Bomb

Lab Format

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 the correct strings to defuse the bomb. To avoid detonating the bomb, you will need to learn how to set breakpoints and single-step through the assembly code in gdb (our debugger) while inspecting the current state of the registers and memory. Look at the Tools section for ideas and tools to use.

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 Tips section and the Hints section.

Bomb Input

There are two different ways to use the bomb, as described below.

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

Blank input lines are ignored. The bomb will stop reading the file if it blows up. If the bomb reaches the EOF (end of file) before blowing up, then it will switch over to stdin (standard input from the terminal via the keyboard), meaning that you can use defuser.txt with partial bomb solutions.

Defusing Tools

There are many ways of defusing your bomb. At the "highest" level, you can read the assembly code and reason through what it is doing without ever running the program. At the "lowest" level, you can step through the program execution in a debugger to determine what each assembly instruction does without looking up its behavior. As this lab is about learning to read assembly code as well as gaining familiarity with the debugger, we highly recommend a mixture of the two.

This section is intended to give you the tools to follow both approaches, as desired, in analyzing your bomb, and hints on how to use them:

• gdb: The GNU debugger is a powerful command line debugger tool. Some of its many features include: setting breakpoints, line-by-line code execution, examining memory and registers, and viewing of the source code and assembly code (we are not giving you the source code for most of your bomb).
  • See our GDB section of the debugging tips page for guides, examples, and other resources to get started.
  • For this lab, you will find the following GDB commands most useful:
    • disas <function> will display the disassembly of the specified function.
    • break <line>, where <line> can be specified as a line number, a function name, or an instruction address, will create and set a breakpoint.
    • run defuser.txt will run the bomb using defuser.txt as the command-line argument until it encounters a breakpoint or terminates.
    • stepi <#> and nexti <#> will move forward by <#> assembly instructions (stepi will enter functions whereas nexti will go over functions). If omitted, <#> will default to 1.
    • print /<f> <expr> will evaluate the expression <expr> and print out its value according to the format string <f>. The expression can use variable or register names. The format string can be omitted; see documentation for more details.
  • For other documentation, you can (1) type help at the gdb command prompt, (2) type man gdb or info gdb at the shell prompt, or (3) run gdb under gdb-mode in Emacs.
• objdump -t bomb > bomb_sym: This will print out the bomb's symbol table into a file called bomb_sym. 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!
• strings -t x bomb > bomb_strings: This will print the printable strings in your bomb and their offsets within the bomb into into a file called bomb_strings.

Tips and Hints

x86-64 assembly instructions

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.

• Intel Instruction Reference
• AMD Instruction Reference

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& format". You can read more about these differences in the recommended textbook (CSPP 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, and %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);

• After printf prints the shown prompt, scanf will wait for the user to enter a number and hit [Enter] before storing the input 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);

• The first argument, mystring, is the input string.
• The second argument, "%d, %d" is the format string that contains format specifiers to parse the input string with.
• After matching the input string to the format string, the extracted values are stored at the addresses given in the additional arguments. After this code is run, a = 123 and b = 456.

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

Phase Context 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:

1. Comparison: Dr. Evil is a fan of inspiring (or not so inspiring) quotes.
2. Loops: Phase 2 calls a function, then starts a loop. What inputs are required by the function? How is the loop using those inputs?
3. Switch statements: Figure out what inputs are required. For each input, what values would cause a call to explode_bomb? Avoid those!
4. Recursion: What are the initial arguments of the recursive function? How are they manipulated before the recursive call occurs? What do we do with the final value once we exit out of the recursive function?
5. Pointers and arrays: Where are the arrays stored? What are their contents, and how are they being manipulated?
6. Linked lists (extra credit)