Assignment #4: Attack Lab (due on Fri Mar 19, 2021 at 11:59pm)

Contents

Introduction

This assignment asks you to run buffer overflow attacks using two strategies: (1) loading your binary code on the stack and starting its execution by overwriting the return address, or (2) a return-oriented attack, where return addresses are used to jump to one or more “gadgets” (short sequences of instructions ending with ret).

Through this assignment:

  • You will get a better understanding of how to write programs that are more secure (i.e., using explicit checks on buffer sizes, or through features provided by compilers and operating systems).
  • You will gain a deeper understanding of the stack and parameter-passing mechanisms of x86-64 machine code.
  • You will learn debugging tools such as gdb and objdump even better!

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 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 the CS:APP3e book as reference material for this lab.

Instructions

A new repository will be created for you on GitHub, including the following files:

  • ctarget: a program vulnerable to code injection attacks;
  • rtarget: a program vulnerable to return-oriented programming attacks;
  • farm.c: the source code of the “gadget farm” present inside rtarget;
  • grade: a script to check your current grade;
  • hex2raw: a program to convert text files with hex sequences into their raw binary values (e.g., convert the text 48 65 6c 6c 6f to the binary sequence encoding the ASCII string Hello).

Both ctarget and rtarget read a string from stdin and store it inside the buffer array buf. They do so using the vulnerable getbuf function:

unsigned getbuf() {
  char buf[BUFFER_SIZE];
  Gets(buf);
  return 1;
}

The function Gets is similar to the standard library function gets: it reads bytes from stdin until it finds \n or EOF and stores them inside the input array buf, followed by a null terminator \0.

If the string is short, nothing interesting happens:

$ ./ctarget
Cookie: 0x1a7dd803
Type string: Keep it short!
No exploit. Getbuf returned 0x1

When the string typed by the user (or sourced from a text file with ctarget < attack.raw) is longer than the space allocated on the stack by the compiler, Gets will overwrite the return address of getbuf. Most likely, this will cause a segmentation fault:

$ ./ctarget
Cookie: 0x1a7dd803
Type string: This is not a very interesting string, but it has the property ...
Ouch!: You caused a segmentation fault!
Better luck next time

(Note that the magic cookie shown will differ from yours.)

Your goal is to craft attack strings that trigger the execution of functions target_f1/target_f2/target_f3 inside ctarget and inside rtarget, by “properly” overwriting return addresses.

If you enter the correct solution, the target program will save it in a text file named sol1.txt for level 1, sol2.txt for level 2, and so on. You must commit these files and push them to your repository:

$ git add sol1.txt
$ git commit -m "Solved level 1"
$ git push

You can solve multiple levels and commit/push these text files together, at the end. The push time of your solution files will be used to count late days.

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.

  • hex2raw expects two-digit hex values separated by one or more white spaces. So if you want to create a byte with a hex value of 0, you need to write it as 00.
  • To create the 4-byte word 0xdeadbeef (an int) you should pass ef be ad de to hex2raw (note the reversal required for little-endian byte ordering).
  • Your exploit string must not contain byte value 0x0a at any intermediate position, since this is the ASCII code for newline \n. If Gets encounters this byte, it will assume you intended to terminate the string.

Evaluation

You must complete the assignment using the class VM. Your virtual machine must be connected to the internet, as the program will connect to our server when you complete an attack.

Every attempt you make will be logged by the automated grading server. As in the Bomb Lab, run ./grade to view your current progress. Unlike the previous project, there is no penalty for making mistakes in this lab. Attempts to break or overload the server, however, are not allowed, and will be considered cheating.

Your solutions may not use attacks to circumvent the validation code in the programs. Specifically, any address you incorporate into an attack string for use by a ret instruction should be to one of the following destinations:

  • The addresses for functions target_f1, target_f2, or target_f3.
  • The address of your injected code.
  • The address of one of your gadgets from the gadget farm.

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

There are 6 attacks to complete:

  • target_f1 in ctarget (10 points).
  • target_f2 in ctarget (20 points).
  • target_f3 in ctarget (30 points).
  • target_f1 in rtarget (5 points).
  • target_f2 in rtarget (15 points).
  • target_f3 in rtarget (20 points).

The first three involve code-injection (CI) attacks on ctarget, while the last three involve return-oriented-programming (ROP) attacks on rtarget.

Logistics

This is an individual project. All handins are electronic. Clarifications and corrections will be posted on the course Piazza page.

Remember that:

  • You are not allowed to search for help online!
  • You are not allowed to ask other students for help, show them your solution, or discuss its specifics.
  • Reconsideration requests must be made within one week of our release of grades for the assignment.

Be aware that you may be asked to explain your solution to a member of our course staff.

Handout Instructions

Similarly to the previous assignment, we we will create a private GitHub repository for this assignment and share it with you. Be sure to clone the GitHub repository inside the class VM.

Hand-In Instructions

You must commit and push your solution files sol1.txt through sol6.txt to GitHub. Assignment collection will be automatic: after the assignment deadline, our grading system will fetch the most recent commit on the master branch of your repository.

Be sure to run ./grade and verify that we recorded your solutions. You can also check your solution files before submitting them with cat sol1.txt | ./hex2raw | ./ctarget and so on.

If you want to use late days, make a push after the deadline: we will use the push date (not the commit date) to determine your late days.

Attack Instructions: Code Injection

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 so 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.

Level 1: target_f1 in ctarget (10 points)

In the first attack, 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 the function test having the following C code:

void test() {
    int val = getbuf();
    printf("No exploit.  Getbuf returned 0x%x\n", val);
    fail();
}

When getbuf executes its return statement, the program ordinarily resumes execution within function test (with a call to fail). We want to change this behavior! Within the file ctarget, there is code for a function target_f1 having the following C representation:

void target_f1() {
    level = 1;
    printf("SUCCESS: You called target_f1()\n");
    validate();
}

Your task is to get ctarget to execute the code for target_f1 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 stage, but this will not cause a problem, since validate() causes the program to exit directly.

Some advice:

  • All the information you need to devise your exploit string for this level can be determined by examining a disassembled version of ctarget. Use objdump -d to get this dissembled version (layout asm inside gdb also works).
  • The idea is to position a byte representation of the starting address for target_f1 so that the ret instruction at the end of the code for getbuf will transfer control to target_f1.
  • Be careful about byte ordering (Intel CPUs are little-endian).
  • Use gdb to step the program through the last few instructions of getbuf to make sure it is doing the right thing.
  • The placement of buf within the stack frame for getbuf depends on the value of compile-time constant BUFFER_SIZE, as well the allocation strategy used by GCC. You will need to examine the disassembled code to determine its position.

Level 2: target_f2 in ctarget (20 points)

Level 2 involves injecting a small amount of code as part of your exploit string (see the section Generating Binary Instructions on how to generate the code to inject). Within the file ctarget there is code for a function target_f2 having the following C representation:

void target_f2(unsigned val) {
    level = 2;
    if (val == TARGET_ID) {
        printf("SUCCESS: You called target_f2(0x%.8x)\n", val);
        validate();
    } else {
        printf("Misfire: You called target_f2(0x%.8x)\n", val);
        fail();
    }
}

Your task is to get ctarget to execute the code for target_f2 rather than returning to test. In this case, however, you must make it appear to target_f2 as if you have passed your magic cookie number as the first argument.

Some advice:

  • You will want to position a byte representation of the address of your injected code in such a way that the ret instruction at the end of the code for getbuf will transfer control to it.
  • Recall that the first argument to a function is passed in register %rdi.
  • Your injected code should set this register to your magic cookie number, and then use a ret instruction to transfer control to the first instruction in target_f2.
  • Do not attempt to use jmp or call instructions in your exploit code. The encodings of destination addresses for these instructions are difficult to formulate. Use ret instructions for all transfers of control.
  • See the discussion at the end of this page on how to use tools to generate the byte-level representations of instructions.

Level 3: target_f3 in ctarget (30 points)

Level 3 also involves a code injection attack, but passing a string as argument. Within the file ctarget there is code for functions hexmatch and target_f3 having the following C representations:

int hexmatch(unsigned val, char *sval) {
    char cbuf[110];
    /* Make position of check string unpredictable */
    char *s = cbuf + random() % 100;
    sprintf(s, "%.8x", val);
    return strncmp(sval, s, 9) == 0;
}

void target_f3(char *sval) {
    level = 3;
    if (hexmatch(TARGET_ID, sval)) {
        printf("SUCCESS: You called target_f3(\"%s\")\n", sval);
        validate();
    } else {
        printf("Misfire: You called target_f3(\"%s\")\n", sval);
        fail();
    }
}

Your task is to get ctarget to execute the code for target_f3 rather than returning to test. You must make it appear to target_f3 as if you have passed a string representation of your magic value as its first argument.

Some advice:

  • You will need to include a string representation of your cookie in your exploit string. The string should consist of the eight hexadecimal digits (ordered from most to least significant) without a leading 0x and lowercase (e.g., if your cookie value is 0x1A7DD803 in hexadecimal, the string should be “1a7dd803”).
  • Recall that a string is represented in C as a sequence of bytes followed by a byte with value 0. Type man ascii on any Linux machine to see the byte representations of the characters you need.
  • Your injected code should set register %rdi to the address of this string representation of your magic number.
  • When functions hexmatch and strncmp are called, they push data onto the stack, overwriting portions of memory that held the buffer used by getbuf. As a result, you will need to be careful about the placement of the string representation of your magic cookie.

Attack Instructions: 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:

  • It uses randomization so that the stack positions differ from one run to another. This makes it impossible to determine where your injected code will be located.
  • It marks the section of memory holding the stack as nonexecutable, so even if you could set the program counter to the start of your injected code, the program would fail with a segmentation fault.

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 of 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 called a gadget. The following figure illustrates how the stack can be set up to execute a sequence of $n$ gadgets.

Return-Oriented Programming

  • 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 (at the end of the binary encoding of movl $0xc78948d4,(%rdi)) encodes the instruction movq %rax,%rdi.

This sequence is followed by the 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 Levels 2 and 3.

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

Level 4: target_f1 in rtarget (5 points)

For Level 4, you will repeat an attack similar to Level 1: you only need to overwrite the return address to move control to target_f1 inside rtarget.

Level 5: target_f2 in rtarget (15 points)

For Level 5, you will repeat the attack of Level 2 to target_f2, but in the 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 through %rdi).

  • movq: The codes for these are shown below. movq instruction codes
  • popq: The codes for these are shown below. popq instruction codes
  • ret: This instruction is encoded by the single byte 0xc3.
  • nop: 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.

Some advice:

  • All the gadgets you need can be found in the region of the code for rtarget demarcated by the functions start_farm and mid_farm.
  • You can complete this attack with just two gadgets.

Level 6: target_f3 in rtarget (20 points)

Before starting Level 6, pause to consider what you have accomplished so far! In Levels 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 Level 5, 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 also collected 80/100 points for the lab. That’s a good score. If you have other pressing obligations consider stopping right now.

Level 6 requires you to do an ROP attack on rtarget to invoke target_f3 with a pointer to a string representation of your magic cookie number.

That may not seem significantly more difficult than using an ROP attack to invoke target_f2, except that we have made it so through address space randomization. Moreover, Level 6 counts for 20 points, which is not a true measure of the effort it will require. Think of it as an extra credit problem for those who want to go beyond the normal expectations for the course.

  • To solve Level 6, 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 Level 5, this expanded farm includes encodings of movl instructions shown below. movl instruction codes
  • 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 instructions, shown below, operate on the low-order bytes of some of the registers but do not change their values. functional nop instruction codes

Some advice:

  • You’ll want to review the effect of movl on the upper 4 bytes of a register (page 183 of the textbook).
  • The official solution requires eight gadgets (not all of which are unique).
  • Remember address space randomization means you probably can’t hard-code a pointer.
  • There are some gadgets that might be useful to you as is (i.e., no hidden instructions… just call that function).

Using hex2raw

The program 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 attack string with newlines while you’re working on it. hex2raw supports C-style block comments, so you can mark off sections of your attack string. For example:

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

Be sure to leave space around both the starting and ending comment delimiters (/* and */), so that the comments are ignored.

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

  • You can set up a series of pipes to pass the string through hex2raw:
    $ cat attack.txt | ./hex2raw | ./ctarget
    
  • You can store the raw string in a file and use I/O redirection:
    $ ./hex2raw < attack.txt > attack.raw
    $ ./ctarget < attack.raw
    

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

    $ gdb ctarget
    (gdb) run < attack.raw
    

Generating Binary Instructions

For code-injection attacks, you need to save binary instructions on the stack. But how can you figure out the binary encoding of your attack instructions?

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:

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:

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

The generated file example.d contains the following:

$ objdump -d example.o
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.

Acknowledgements. This lab was developed by the authors of the course textbook and their staff. It has been customized for use by this course.