Assignment 3: Understanding Buffer Overflow Bugs

Due: Friday 11/10, 11:59 PM

Introduction

This assignment involves generating a total of five attacks on two programs having different security vulnerabilities. Outcomes you will gain from this lab include:

Note: In this lab, you will gain firsthand experience with methods 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 these security weaknesses so that you can avoid them when you write system code. We do not condone the use of this or any other form of attack to gain unauthorized access to any system resources.

You will want to study Sections 3.10.3 and 3.10.4 of your textbook as reference material for this lab.

Logistics

As usual, this is an individual project. You will generate attacks for target programs that are custom generated for you.

Getting Files

You can obtain your files by running the following commands:

cd ~/cs224
curl -o target.tar -s -L  "https://cs224.cs.vassar.edu/attacklab?username=$USER&usermail=$USER%40vassar.edu&submit=Submit"

The server will build your files and return them to your browser in a tar file called target.tar.

Note: It takes a few seconds to build and download your target, so please be patient.

To extract the tar file, run the following command:

tar -xf target.tar

This will extract a directory targetk, where k is the unique number of your target programs, containing the files described below.

You should only download one set of files. If for some reason you download multiple targets, choose one target to work on and delete the rest.

The files in targetk include:

Important Guidelines

Here is a summary of some important rules regarding valid solutions for this lab. These points may not make much sense the first time read this document. They are presented here as a central reference of rules once you get started.

You may only construct gadgets from file rtarget with addresses ranging between those for functions start_farm and end_farm.

Target Programs

Both ctarget and rgarget read strings from standard input. They do so with the function getbuf defined below:

1 unsigned getbuf()
2 {
3     char buf[BUFFER_SIZE];
4     Gets(buf);
5     return 1;
6 }

The function Gets is similar to the standard 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 this code, you can see that the destination is an array buf, declared as having BUFFER_SIZE bytes. At the time your targets were generated, BUFFER_SIZE was a compile-time constant specific to your version of the programs.

The functions Gets() and gets() have no way to determine whether their destination buffers are large enough to store the string they read. They simply copy a sequences of bytes, possibly overrunning the bounds of the storage allocated at the destinations.

If the string typed by the user and read by getbuf is sufficiently short, as to not overwrite the return address, getbuf will return 1, as shown by the following execution examples:

Linux> ./ctarget
Cookie: 0x1a7dd803
Type string: Keep it short!
No exploit. Getbuf returned 0x1
Normal return

Typically an error occurs if you type a long string:

Linux> ./ctarget
Cookie: 0x1a7dd803
Type string: This is not a very interesting string, but it has the property of being longer than the last one!
Ouch!: You caused a segmentation fault!
Better luck next time
FAIL

(Note that the value of the cookie shown will differ from yours.) Program rtarget will have the same behavior. As the error message indicates, overrunning the buffer typically causes the program state to be corrupted, leading to a memory access error (segmentation fault). Your task is to be more clever with the strings you feed ctarget and rtarget so that they do more interesting things. These are called exploit strings. Both ctarget and rtarget take several different command line arguments:

Your exploit strings will typically contain byte values that do not correspond to the ASCII values for printing characters. The program hex2raw will enable you to generate these raw strings. See Appendix A for more information on how to use hex2raw.

Very Important points:

When you have correctly solved one of the phases, your target program will automatically send a notification to the grading server. For example, if your exploit file for phase 2 was called exploit_p2.txt, a successful exploit would look something like the following:

Linux> ./hex2raw -i exploit_p2.txt | ./ctarget
Cookie: 0x1a7dd803
Type string:Touch2!: You called touch2(0x1a7dd803)
Valid solution for level 2 with target ctarget
PASSED: Sent exploit string to server to be validated.
NICE JOB!

The grading server will test your exploit string to make sure it really works, and it will update the scoreboard page indicating that your target has completed this phase.

You can view the scoreboard by pointing your Web browser http://cs224.cs.vassar.edu/attacklab/scoreboard. Unlike the Bomb Lab, there is no penalty for incorrect exploit strings.

Part 1: Code Injection Attacks

For the first three phases, your exploit strings will attack ctarget. This program is set up in a way that the stack positions will be consistent from one run to the next and that data on the stack can be treated as executable code. These features make the program vulnerable to attacks where the exploit strings contain the byte encodings of executable code (i.e., code injection attacks).

Phase 1

For Phase 1, you will not inject new code. Instead, your exploit string will redirect the program to execute an existing procedure.

Function getbuf is called within ctarget by a function called test, having the following C code:

1 void test()
2 {
3     int val;
4     val = getbuf();
5     printf("No exploit. Getbuf returned 0x%x\n", val);
6 }

When getbuf executes its return statement, the program ordinarily resumes execution within function test (at the printf in line 5). We want to change this behavior. Within the file ctarget, there is code for a function touch1 having the following C representation:

1 void touch1()
2 {
3     vlevel = 1; /* Part of validation protocol */
4     printf("Touch1!: You called touch1()\n");
5     validate(1);
6     exit(0);
7 }

Your task is to get ctarget to execute the code for touch1 when getbuf executes its return statement, rather than returning to test. Note that your exploit string may also corrupt parts of the stack not directly related to this phase, but this will not cause a problem since touch1 causes the program to exit.

Some Advice:

Phase 2

Phase 2 involves injecting a small amount of code as part of your exploit string.

Within the file ctarget there is code for a function touch2 having the following C representation:

1 void touch2(unsigned val)
2 {
3     vlevel = 2; /* Part of validation protocol */
4     if (val == cookie) {
5         printf("Touch2!: You called touch2(0x%.8x)\n", val);
6         validate(2);
7     } else {
8         printf("Misfire: You called touch2(0x%.8x)\n", val);
9         fail(2);
10    }
11    exit(0);
12 }

Your task is to get ctarget to execute the code for touch2 rather than returning to test. In this case, however, you must make it appear to touch2 that you have passed your cookie in as an argument. To do this your input string must perform a code injection exploit to set the argument to touch2 properly before passing control to touch2.

Some Advice:

Phase 3

Phase 3 also involves a code injection attack, but passing a string as argument.

Within the file ctarget there is code for functions touch3 having the following C representations:

01 void touch3(char *sval)
02 {
03     vlevel = 3; /* Part of validation protocol */
04     if (hexmatch(cookie, sval)) {
05         printf("Touch3!: You called touch3(\"%s\")\n", sval);
06         validate(3);
07     } else {
08         printf("Misfire: You called touch3(\"%s\")\n", sval);
09         fail(3);
10     }
11     exit(0);
12 }

Your task is to get ctarget to execute the code for touch3 rather than returning to test. You must make it appear to touch3 as if you have passed a string representation of your cookie as its argument.

Some Advice:

Part II: Return-Oriented Programming

Performing code-injection attacks on program rtarget is much more difficult than it is for ctarget, because it uses two techniques to thwart such attacks:

Fortunately, clever people have devised strategies for getting useful things done in a program by executing existing code, rather than injecting new code. The most general form of this is referred to as return-oriented programming (ROP). The strategy with ROP is to identify byte sequences within an existing program that consist of one or more instructions followed by the instruction ret. Such a segment is referred to as a gadget. The figure below illustrates how the stack can be set up to execute a sequence of n gadgets.

ROP Diagram

In this figure, the stack contains a sequence of gadget addresses. Each gadget consists of a series of instruction bytes, with the final one being 0xc3, encoding the ret instruction. When the program executes a ret instruction starting with this configuration, it will initiate a chain of gadget executions, with the ret instruction at the end of each gadget causing the program to jump to the beginning of the next.

A gadget can make use of code corresponding to assembly-language statements generated by the compiler, especially ones at the ends of functions. In practice, there may be some useful gadgets of this form, but not enough to implement many important operations. For example, it is highly unlikely that a compiled function would have popq %rdi as its last instruction before ret. Fortunately, with a byte-oriented instruction set, such as x86-64, a gadget can often be found by extracting patterns from other parts of the instruction byte sequence.

For example, one version of rtarget contains code generated for the following C function:

void setval_210(unsigned *p)
{
    *p = 3347663060U;
}

The chances of this function being useful for attacking a system seem pretty slim. But, the disassembled machine code for this function shows an interesting byte sequence:

0000000000400f15 <setval_210>:
  400f15: c7 07 d4 48 89 c7 movl $0xc78948d4, (%rdi)
  400f1b: c3 retq

The byte sequence 48 89 c7 encodes the instruction movq %rax, %rdi. This sequence is followed by byte value c3, which encodes the ret instruction. The function starts at address 0x400f15, and the sequence starts on the fourth byte of the function. Thus, this code contains a gadget, having a starting address of 0x400f18, that will copy the 64-bit value in register %rax to register %rdi.

Your code for rtarget contains a number of functions similar to the setval_210 function shown above in a region we refer to as the gadget farm. Your job will be to identify useful gadgets in the gadget farm and use these to perform attacks similar to those you did in Phases 2 and 3.

Important: The gadget farm is demarcated by functions start_farm, mid_farm, and end_farm in your copy of rtarget. Do not attempt to construct gadgets from other portions of the program code.

Phase 4

For Phase 4, you will repeat the attack of Phase 2, but do so on program rtarget using gadgets from your gadget farm. You can construct your solution using gadgets consisting of the following instruction types, and using only the first eight x86-64 registers (%rax–%rdi).

A. Encodings of movq instructions

movq encodings

B. Encodings of movl instructions

mov1 encodings

C. Encodings of popq instructions

popq encodings

D. The ret instruction. This instruction is encoded by the single byte 0xc3.

E. The nop instruction. This instruction (pronounced “no op,” which is short for “no operation”) is encoded by the single byte 0x90. Its only effect is to cause the program counter to be incremented by 1.

F. Encodings of 2-byte functional nop instructions

noop encodings

Some Advice:

Phase 5

Before you take on the Phase 5, pause to consider what you have accomplished so far. In Phases 2 and 3, you caused a program to execute machine code of your own design. If ctarget had been a network server, you could have injected your own code into a distant machine. In Phase 4, you circumvented two of the main devices modern systems use to thwart buffer overflow attacks. Although you did not inject your own code, you were able inject a type of program that operates by stitching together sequences of existing code. You have also gotten 95/100 points for the lab. That’s a great score! If you have other pressing obligations consider stopping right now.

Phase 5 requires you to do an ROP attack on rtarget to invoke function touch3 with a pointer to a string representation of your cookie. That may not seem significantly more difficult than using an ROP attack to invoke touch2, except that we have made it so. Moreover, Phase 5 counts for only 5 points, which is not a true measure of the effort it will require. Think of it as more an extra credit problem for those who want to go beyond the normal expectations for the course.

To solve Phase 5, you can use gadgets in the region of the code in rtarget demarcated by functions start_farm, and end_farm. In addition to the gadgets used in Phase 4, this expanded farm includes the encodings of different movl instructions. The byte sequences in this part of the farm also contain 2-byte instructions that serve as functional nops, i.e., they do not change any register or memory values. These include instructions, such as andb %al, %al, that operate on the low-order bytes of some of the registers but do not change their values.

Some Advice:

Good luck and have fun!

Appendix A: Using hex2raw

hex2raw takes as input a hex-formatted string. In this format, each byte value is represented by two hex digits. For example, the string 012345 could be entered in hex format as 30 31 32 33 34 35 00. (Recall that the ASCII code for decimal digit x is 0x3x, and that the end of a string is indicated by a null byte.)

The hex characters you pass to hex2raw should be separated by whitespace (blanks or newlines). We recommend separating different parts of your exploit string with newlines while you’re working on it. hex2raw supports C-style block comments, so you can mark off sections of your exploit string. For example:

48 c7 c1 f0 11 40 00 /* mov $0x40011f0, %rcx */

Important: be sure to leave a space around both the starting and ending comment delineators (/*, */) to make sure your comments are ignored by hex2raw.

If you generate a hex-formatted exploit string with the file exploit.txt, you can apply the raw string to ctarget or rtarget in a couple of different ways:

  1. You can specify an input file to hex2raw with the -i option and use I/O redirection to save the raw exploit string as a file.
Linux> ./hex2raw -i exploit.txt > exploit.raw
Linux> ./ctarget < exploit.raw

This approach can also be used when running from within GDB:

Linux> ./hex2raw -i exploit.txt > exploit.raw
Linux> gdb ctarget
(gdb) run < exploit.raw
  1. Or you can use a pipe to pass the converted string directly trough to your target:
Linux> ./hex2raw -i exploit.txt | ./ctarget

Appendix B: Generating Byte Codes

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

# Example of hand-generated assembly code
    pushq $0xabcdef      # Push value onto stack
    addq  $17, %rax      # Add 17 to %rax
    movl  %eax, %edx     # Copy lower 32 bits to %edx

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

You can now assemble and disassemble this file:

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

The generated file example.d contains the following:

example.o:       file format elf64-x86-64

Disassembly of section .text:

0000000000000000 <.text>:
    0: 68 ef cd ab 00      pushq $0xabcdef
    5: 48 83 c0 11         add $0x11, %rax
    9: 89 c2               mov %eax, %edx

The lines at the bottom show the machine code generated from the assembly language instructions. Each line has a hexadecimal number on the left indicating the instruction’s 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 push $0xABCDEF has hex-formatted byte code 68 ef cd ab 00.

From this file, you can get the byte sequence for the code:

68 ef cd ab 00 48 83 c0 11 89 c2

This string can then be passed through hex2raw to generate an input string for the target programs.. Alternatively, you can edit example.d to omit extraneous values and to contain C-style comments for readability, yielding:

68 ef cd ab 00        /* pushq $0xabcdef */
48 83 c0 11           /* add $0x11, %rax */
89 c2                 /* mov %eax, %edx */

This is also a valid input you can pass through hex2raw before sending to one of the target programs.