Lab 3: Buffer Overflows?|?d??|?d?Segmentation fault: 11
Assigned | Wednesday, November 2, 2011 |
---|---|
Due Date | Monday, November 14, 2011 at 11:59p |
Files | lab3.tar |
Overview
This assignment helps you develop a detailed understanding of the
calling stack organization on an IA32 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.
For this lab, you should use attu32
and
not attu
. The lab is designed to work on a 32-bit
architecture, but attu
is comprised of a series of 64-bit
machines. attu32
provides the 32-bit architecture that
we'll need for this lab. You should avoid using your own computer even
if it has a 32-bit architecture because of the possibility that your
solutions may not work on attu32
.
Instructions
Start on attu32
by
extracting lab3.tar to a
directory in which you plan to do your work:
tar xvf lab3.tar
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 attu32
. The rest of the
instructions assume that you will be performing your work
on attu32
.
Be sure to read this write-up in its entirety before beginning your work.
Making Your Cookie
A cookie is a string of eight 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
0x12a05b96
As an example, if your CSE email address
is thecookiemonster42@cs.washington.edu
, you would
run ./makecookie thecookiemonster42
.
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()
:
int getbuf() {
char buf[12];
Gets(buf);
return 1;
}
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 12 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 11 characters long, it is clear that getbuf()
will
return 1, as shown by the following execution example:
$ ./bufbomb
Type string: howdy doody
Dud: getbuf returned 0x1
Typically an error occurs if we type a longer string:
$ ./bufbomb
Type string: This string is too long
Ouch!: You caused a segmentation fault!
As the error message indicates, overrunning the buffer typically
causes the program state 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:
$ 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
at any intermediate position, since this is the
ASCII code for newline ('\n
'). When Gets()
encounters this 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
pushl $0x89abcdef # Push value onto stack
addl $17,%eax # Add 17 to %eax
.align 4 # Following will be aligned on multiple of 4
.long 0xfedcba98 # A 4-byte constant
.long 0x00000000 # Padding
The code can contain a mixture of instructions and data. Anything
to the right of a '#
' character is a comment. We have
added an extra word of all 0
s to work around a
shortcoming in objdump
to be described shortly.
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: 68 ef cd ab 89 push $0x89abcdef
5: 83 c0 11 add $0x11,%eax
8: 98 cwtl These two lines are really the data which
9: ba dc fe 00 00 mov $0xfedc,%edx objdump tries to interpret as instructions.
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 pushl
$0x89abcdef
has a hex-formatted byte code of 68 ef cd ab
89
.
Starting at address 8, the disassembler gets confused. It tries to
interpret the bytes in the file example.o
as
instructions, but these bytes actually correspond to data. Note,
however, that if we read off the 4 bytes starting at address 8 we
get: 98 ba dc fe
. This is a byte-reversed version of the
data word 0xfedcba8
. 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. Note also
that objdump
only generates two of the four bytes at the
end with value 00
. If we do not add the
padding, objdump
gets even more confused and does not
emit all of the bytes we want.
Finally, we can read off the byte sequence for our code (omitting the
final 0
's) as:
68 ef cd ab 89 83 c0 11 98 ba dc fe
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()
{
int val;
volatile int local = 0xdeadbeef;
entry_check(3); /* Make sure entered this function properly */
val = getbuf();
/* Check for corrupted stack */
if (local != 0xdeadbeef)
{
printf("Sabotaged!: the stack has been corrupted\n");
}
else if (val == cookie)
{
printf("Boom!: getbuf returned 0x%x\n", val);
validate(3);
}
else
{
printf("Dud: getbuf returned 0x%x\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:
- All the information you need to devise your exploit string for
this level can be determined by examining a disassembled version
of
bufbomb
. - Be careful about byte ordering.
- You might want to use
gdb
to step the program through the last few instructions ofgetbuf()
to make sure it is doing the right thing. - The placement of
buf
within the stack frame forgetbuf()
depends on which version ofgcc
was used to compilebufbomb
. You will need to pad the beginning of your exploit string with the proper number of bytes to overwrite the return pointer. The values of these bytes can be arbitrary.
Level 1: Sparkler
Within the file bufbomb
there is also a
function fizz()
:
void fizz(int val)
{
entry_check(1); /* Make sure entered this function properly */
if (val == cookie)
{
printf("Fizz!: You called fizz(0x%x)\n", val);
validate(1);
}
else
{
printf("Misfire: You called fizz(0x%x)\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:
- Note that the program won't really
call
fizz()
—it will simply execute its code. This has important implications for where on the stack you want to place your cookie.
Level 2: Firecracker
For level 2, you will need to run your exploit
within gdb
for it to succeed. (attu32
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.)
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.
Within the file bufbomb
there is a
function bang()
:
int global_value = 0;
void bang(int val)
{
entry_check(2); /* Make sure entered this function properly */
if (global_value == cookie)
{
printf("Bang!: You set global_value to 0x%x\n", global_value);
validate(2);
}
else
{
printf("Misfire: global_value = 0x%x\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 ret
instruction to cause a jump to the code
for bang()
.
Advice:
- You can use
gdb
to get the information you need to construct your exploit string. Set a breakpoint withingetbuf()
and run to this breakpoint. Determine parameters such as the address ofglobal_value
and the location of the buffer. - Determining the byte encoding of instruction sequences by hand
is tedious and prone to errors. You can let tools do all of the work
by writing an assembly code file containing the instructions and
data you want to put on the stack. Assemble this file
with
gcc
and disassemble it withobjdump
. You should be able to get the exact byte sequence that you will type at the prompt. (A brief example of how to do this is included in the Generating Byte Codes section above.) - Keep in mind that your exploit string depends on your machine,
your compiler, and even your cookie. Do all of your work
on
attu32
, and make sure you include your CSE username on the command line tobufbomb
. - Our solution requires 16 bytes of exploit code. Fortunately,
there is sufficient space on the stack, because we can overwrite the
stored value of
%ebp
. This stack corruption will not cause any problems, becausebang()
causes the program to exit directly. - Watch your use of address modes when writing assembly code. Note
that
movl $0x4, %eax
moves the value0x00000004
into register%eax
; whereasmovl 0x4, %eax
moves the value at memory location0x00000004
into%eax
. Because that memory location is usually undefined, the second instruction will cause a segmentation fault! - Do not attempt to use either a
jmp
or acall
instruction to jump to the code forbang()
. These instructions use PC-relative addressing, which is very tricky to set up correctly. Instead, push an address on the stack and use theret
instruction.
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 %ebp
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:
- In order to overwrite the return pointer, you must also
overwrite the saved value of
%ebp
. However, it is important that this value is correctly restored before you return totest()
. You can do this by either (1) making sure that your exploit string contains the correct value of the saved%ebp
in the correct position, so that it never gets corrupted, or (2) restore the correct value as part of your exploit code. You'll see that the code fortest()
has some explicit tests to check for a corrupted stack. - You can use
gdb
to get the information you need to construct your exploit string. Set a breakpoint withingetbuf()
and run to this breakpoint. Determine parameters such as the saved return address and the saved value of%ebp
. - Again, let tools such as
gcc
andobjdump
do all of the work of generating a byte encoding of the instructions. - Keep in mind that your exploit string depends on your machine,
your compiler, and even your cookie. Do all of your work
on
attu32
, and make sure you include your CSE username on the command line tobufbomb
.
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.
Submitting Your Work
You should submit the following files:
smoke.txt
fizz.txt
bang.txt
dynamite.txt
(if you did the extra credit)CSE_ID.txt
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.