CSE 351: The Hardware/Software Interface

Winter 2013 Course Website Return home »

Lab 3: Buffer Overflows?|?d??|?d?Segmentation fault: 11

Assigned February 6, 2013
Due Date February 13, 2013 at 5:00pm
Files lab3.tar.gz

Overview

This assignment helps you develop a detailed understanding of the calling stack organization on an x86-64 processor. It involves applying a series of buffer overflow attacks on an executable file called bufbomb. (For some reason the textbook authors have a penchant for pyrotechnics.)

In this lab, you will gain firsthand experience with one of the methods commonly used to exploit security weaknesses in operating systems and network servers. Our purpose is to help you learn about the runtime operation of programs and to understand the nature of this form of security weakness so that you can avoid it when you write system code. We do not condone the use of these or any other form of attack to gain unauthorized access to any system resources. There are criminal statutes governing such activities.

Instructions

Start on attu by extracting lab3.tar.gz to a directory in which you plan to do your work:

tar xzvf lab3.tar.gz

This will cause a number of files to be unpacked in a directory called lab3:

File Name Description
makecookie Generates a "cookie" based on some string (which will be your CSE username).
bufbomb The executable you will attack.
bufbomb.c The important bits of C code used to compile bufbomb.
sendstring A utility to help convert between string formats.
Makefile For getting your exploits ready for submission.

All of these programs are compiled to run on attu. The rest of the instructions assume that you will be performing your work on attu. You should do this assignment on a lab Linux machine or on a 64-bit CSE Linux VM and be sure it works there or on attu (test your solution there before submitting it!)

Be sure to read this write-up in its entirety before beginning your work.

A Note about Line Endings

Linux (and UNIX machines in general) use a different line ending from Windows and traditional MacOS in text files. The reason for this difference is historical, early printers need more time to move the print head back to the beginning of the next line that to print a single character, so someone introduced the idea of separate line feed \n and carriage return \r characters. Windows and HTTP use the \r\n pairs, MacOS uses \r and Linux uses \n. In this lab it is important that your lines end with line feed (\n) not any of the alternative line endings. If you are working on attu or another Linux system this will probably not be a problem, but if you working across systems check your line endings.

Making Your Cookie

A cookie is a string of eight bytes (or 16 hexadecimal digits) that is (with high probability) unique to you. You can generate your cookie with the makecookie program giving your CSE username as the argument:

$ ./makecookie your_cse_username
0x5e57e63274f39587

As an example, if your CSE email address is thecookiemonster42@cs.washington.edu, you would run ./makecookie thecookiemonster42.

While you are doing this, you might as well prepare the first file you need to turn in: CSE_ID.txt

$ echo your_cse_username > CSE_ID.txt

Where you replaced your_cse_username with your real username as above, this will generate a text file containing your CSE username followed by a single new line. You could also use a text editor, but you have to be careful about line endings.

In most of the attacks in this lab, your objective will be to make your cookie show up in places where it ordinarily would not.

Using the bufbomb Program

The bufbomb program reads a string from standard input with the function getbuf():

unsigned long long getbuf() {
   char buf[36];
   volatile char* variable_length;
   int i;
   unsigned long long val = (unsigned long long)Gets(buf);
   variable_length = alloca((val % 40) < 36 ? 36 : val % 40);
   for(i = 0; i < 36; i++) {
      variable_length[i] = buf[i];
   }
   return val % 40;
}

Don't worry about what's going on with variable_length and val and alloca() for now; all you need to know is that getbuf() calls the function Gets() and returns some arbitrary value.

The function Gets() is similar to the standard C library function gets()—it reads a string from standard input (terminated by '\n' or end-of-file) and stores it (along with a null terminator) at the specified destination. In the above code, the destination is an array buf having sufficient space for 36 characters.

Neither Gets() nor gets() has any way to determine whether there is enough space at the destination to store the entire string. Instead, they simply copy the entire string, possibly overrunning the bounds of the storage allocated at the destination.

If the string typed by the user to getbuf() is no more than 36 characters long, it is clear that getbuf() will return some value less than 0x28, as shown by the following execution example:

$ ./bufbomb
Type string: howdy doody
Dud: getbuf returned 0x20

It's possible that the value returned might differ for you, since the returned value is derived from the location on the stack that Gets() is writing to. The returned value will also be different depending on whether you run the bomb inside gdb or run it outside of gdb for the same reason.

Typically an error occurs if we type a longer string:

$ ./bufbomb
Type string: This string is too long and it starts overwriting things.
Ouch!: You caused a segmentation fault!

As the error message indicates, overrunning the buffer typically causes the program state (e.g., the return addresses and other data structured that were stored on the stack) to be corrupted, leading to a memory access error. Your task is to be more clever with the strings you feed bufbomb so that it does more interesting things. These are called exploit strings.

bufbomb must be run with the -u your_cse_username flag, which operates the bomb for the indicated CSE username. (We will feed bufbomb your CSE username with the -u flag when grading your solutions.) bufbomb determines the cookie you will be using based on this flag value, just as the program makecookie does. Some of the key stack addresses you will need to use depend on your cookie.

Formatting Your Exploit Strings

Your exploit strings will typically contain byte values that do not correspond to the ASCII values for printing characters. The program sendstring can help you generate these raw strings. sendstring takes as input a hex-formatted string and prints the raw string to standard output. In a hex-formatted string, each byte value is represented by two hex digits. Byte values are separated by spaces. For example, the string "012345" could be entered in hex format as 30 31 32 33 34 35. (The ASCII code for decimal digit Z is 0x3Z. Run man ascii for a full table.) Non-hex digit characters are ignored, including the blanks in the example shown.

If you generate a hex-formatted exploit string in a file named exploit.txt, you can send it to bufbomb through a couple of pipes (see CSE390A course notes if you are unfamiliar with unix pipes that take the output of one program and direct it as input to another program):

$ cat exploit.txt | ./sendstring | ./bufbomb -u your_cse_username

Or you can store the raw bytes in a file and use I/O redirection to supply it to bufbomb:

$ ./sendstring < exploit.txt > exploit.bytes
$ ./bufbomb -u your_cse_username < exploit.bytes

With the above method, when running bufbomb from within gdb, you can pass in the exploit string as follows:

$ gdb ./bufbomb
(gdb) run -u your_cse_username < exploit.bytes

One important point: your exploit string must not contain byte value 0x0A or 0x00at any intermediate position, since these are the ASCII codes for newline ('\n') and the end of string sentinel value. When Gets() encounters either of these byte, it will assume you intended to terminate the string. sendstring will warn you if it encounters this byte value.

When using gdb, you may find it useful to save a series of gdb commands to a text file and then use the -x commands.txt flag. This saves you the trouble of retyping the commands every time you run gdb. You can read more about the -x flag in gdb's man page.

Generating Byte Codes

You may wish to come back and read this section later after looking at the problems.

Using gcc as an assembler and objdump as a disassembler makes it convenient to generate the byte codes for instruction sequences. For example, suppose we write a file example.s containing the following assembly code:

# Example of hand-generated assembly code
movq $0x1234abcd,%rax    # Move 0x1234abcd to %rax
pushq $0x401080          # Push 0x401080 on to the stack
retq                     # Return

The code can contain a mixture of instructions and data. Anything to the right of a '#' character is a comment.

We can now assemble and disassemble this file:

$ gcc -c example.s
$ objdump -d example.o > example.d

The generated file example.d contains the following lines


   0:	48 c7 c0 cd ab 34 12 	mov    $0x1234abcd,%rax
   7:	68 80 10 40 00       	pushq  $0x401080
   c:	c3                   	retq

Each line shows a single instruction. The number on the left indicates the starting address (starting with 0), while the hex digits after the ':' character indicate the byte codes for the instruction. Thus, we can see that the instruction pushq $0x401080 has a hex-formatted byte code of 68 80 10 40 00.

If we read off the 4 bytes starting at address 8 we get: 80 10 40 00. This is a byte-reversed version of the data word 0x00401080. This byte reversal represents the proper way to supply the bytes as a string, since a little-endian machine lists the least significant byte first.

Finally, we can read off the byte sequence for our code (omitting the final 0's) as:

48 c7 c0 cd ab 34 12 68 80 10 40 00 c3

The Exploits

There are three functions that you must exploit for this lab. The exploits increase in difficulty. For those of you looking for a challenge, there is a fourth function you can exploit for extra credit.

Level 0: Candle

The function getbuf() is called within bufbomb by a function test():

void test()
{
   volatile unsigned long long val;
   volatile unsigned long long local = 0xdeadbeef;
   char* variable_length;
   entry_check(3);  /* Make sure entered this function properly */
   val = getbuf();
   if (val <= 40) {
      variable_length = alloca(val);
   }
   entry_check(3);
   /* Check for corrupted stack */
   if (local != 0xdeadbeef) {
      printf("Sabotaged!: the stack has been corrupted\n");
   }
   else if (val == cookie) {
      printf("Boom!: getbuf returned 0x%llx\n", val);
      if (local != 0xdeadbeef) {
         printf("Sabotaged!: the stack has been corrupted\n");
      }
      if (val != cookie) {
         printf("Sabotaged!: control flow has been disrupted\n");
      }
      validate(3);
   }
   else {
      printf("Dud: getbuf returned 0x%llx\n", val);
   }
}

When getbuf() executes its return statement, the program ordinarily resumes execution within function test(). Within the file bufbomb, there is a function smoke():

void smoke()
{
    entry_check(0); /* Make sure entered this function properly */
    printf("Smoke!: You called smoke()\n");
    validate(0);
    exit(0);
}

Your task is to get bufbomb to execute the code for smoke() when getbuf() executes its return statement, rather than returning to test(). You can do this by supplying an exploit string that overwrites the stored return pointer in the stack frame for getbuf() with the address of the first instruction in smoke. Note that your exploit string may also corrupt other parts of the stack state, but this will not cause a problem, because smoke() causes the program to exit directly.

Advice:

Level 1: Sparkler

Within the file bufbomb there is also a function fizz():

void fizz(int arg1, char arg2, long arg3,
    char* arg4, short arg5, short arg6, unsigned long long val)
{
    entry_check(1); /* Make sure entered this function properly */
    if (val == cookie)
    {
        printf("Fizz!: You called fizz(0x%llx)\n", val);
        validate(1);
    }
    else
    {
        printf("Misfire: You called fizz(0x%llx)\n", val);
    }
    exit(0);
}

Similar to Level 0, your task is to get bufbomb to execute the code for fizz() rather than returning to test. In this case, however, you must make it appear to fizz as if you have passed your cookie as its argument. You can do this by encoding your cookie in the appropriate place within your exploit string.

Advice:

Level 2: Firecracker

A much more sophisticated form of buffer attack involves supplying a string that encodes actual machine instructions. The exploit string then overwrites the return pointer with the starting address of these instructions. When the calling function (in this case getbuf) executes its ret instruction, the program will start executing the instructions on the stack rather than returning. With this form of attack, you can get the program to do almost anything. The code you place on the stack is called the exploit code. This style of attack is tricky, though, because you must get machine code onto the stack and set the return pointer to the start of this code.

For level 2, you will need to run your exploit within gdb for it to succeed. (attu has special memory protection that prevents execution of memory locations in the stack. Since gdb works a little differently, it will allow the exploit to succeed.)

Within the file bufbomb there is a function bang():

unsigned long long global_value = 0;

void bang(unsigned long long val)
{
    entry_check(2); /* Make sure entered this function properly */
    if (global_value == cookie)
    {
       printf("Bang!: You set global_value to 0x%llx\n", global_value);
       validate(2);
    }
    else
    {
        printf("Misfire: global_value = 0x%llx\n", global_value);
    }
    exit(0);
}

Similar to Levels 0 and 1, your task is to get bufbomb to execute the code for bang() rather than returning to test(). Before this, however, you must set global variable global_value to your cookie. Your exploit code should set global_value, push the address of bang() on the stack, and then execute a retq instruction to cause a jump to the code for bang().

Advice:

Extra Credit – Level 3: Dynamite

For level 3, you will need to run your exploit within gdb for it to succeed.

Our preceding attacks have all caused the program to jump to the code for some other function, which then causes the program to exit. As a result, it was acceptable to use exploit strings that corrupt the stack, overwriting the saved value of register %rbp and the return pointer.

The most sophisticated form of buffer overflow attack causes the program to execute some exploit code that patches up the stack and makes the program return to the original calling function (test() in this case). The calling function is oblivious to the attack. This style of attack is tricky, though, since you must: (1) get machine code onto the stack, (2) set the return pointer to the start of this code, and (3) undo the corruptions made to the stack state.

Your job for this level is to supply an exploit string that will cause getbuf() to return your cookie back to test(), rather than the value 1. You can see in the code for test() that this will cause the program to go "Boom!". Your exploit code should set your cookie as the return value, restore any corrupted state, push the correct return location on the stack, and execute a ret instruction to really return to test().

Advice:

Reflect on what you have accomplished. You caused a program to execute machine code of your own design. You have done so in a sufficiently stealthy way that the program did not realize that anything was amiss.

execve is system call that replaces the currently running program with another program inheriting all the open file descriptors. What are the limitations the exploits you have preformed so far? How could calling execve allow you to circumvent this limitation? If you have time, try writing an additional exploit that uses execve and another program to print a message.

Submitting Your Work

You should submit the following files:

The first four files correspond to the different exploits. Each of these files should only contain the hex-formatted exploit string. The CSE_ID.txt file should contain your CSE username (without the @cs.washington.edu part) followed by an empty line. Please follow the formatting specified here. Our grading scripts won't be nice if you don't name the files like we've asked or if you include additional text in any of the files.

Before submitting your exploits, you can check them by placing them in the same directory as bufbomb and running make test. This will output a summary of your exploits (the Makefile looks for all the files ending with .txt and sends the contents of each to bufbomb, one by one) and whether they succeed.

Once you're satisfied with your solutions, submit them through the Catalyst Drop Box for this assignment. There will not be partial credit.