Assigned | February 6, 2013 |
---|---|
Due Date | February 13, 2013 at 5:00pm |
Files | lab3.tar.gz |
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.
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.
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.
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.
bufbomb
ProgramThe 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.
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 at any intermediate position,
since these are the ASCII codes for newline ('0x00
\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.
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
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.
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.
bufbomb
.gdb
to step the program
through the last few instructions of getbuf()
to make
sure it is doing the right thing.buf
within the stack frame
for getbuf()
depends on which version
of gcc
was used to compile bufbomb
. 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.hexdump -C smoke.txt
.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.
gdb
to get the information you need to
construct your exploit string. Set a breakpoint
within getbuf()
and run to this breakpoint. Determine
parameters such as the address of global_value
and the
location of the buffer.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()
.
gcc
and disassemble it
with objdump
. 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.)attu
, and make sure you include your CSE
username on the command line to bufbomb
.movq $0x4, %rax
moves the
value 0x0000000000000004
into register %rax
;
whereas movq 0x4, %rax
moves the value at
memory location 0x0000000000000004
into %rax
. Because that memory location is usually
undefined, the second instruction will cause a segmentation
fault!jmp
or
a call
instruction to jump to the code
for bang()
. These instructions use PC-relative
addressing, which is very tricky to set up correctly. Instead, push
an address on the stack and use the retq
instruction.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()
.
%rbp
. However, it is
important that this value is correctly restored before you return
to test()
. You can do this by either (1) making sure that
your exploit string contains the correct value of the
saved %rbp
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 for test()
has
some explicit tests to check for a corrupted stack.gdb
to get the information you need to
construct your exploit string. Set a breakpoint
within getbuf()
and run to this breakpoint. Determine
parameters such as the saved return address and the saved value
of %rbp
.gcc
and objdump
do all of the work of generating a byte
encoding of the instructions.attu
, and make sure you include your CSE username on the
command line to bufbomb
.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.
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.