Team Name and Cookie
You should create a team name for the one or two people in your group
of the following form:
- “name”
- where name is your username, if you are
working alone, or
- “name1+name2”
- where name1 is the username of the
first team member and name2 is the username of the second team member.
You should choose a consistent ordering of the IDs in the second form
of team name. Teams “bob+jane” and “jane+bob” are
considered distinct. You must follow this scheme for generating
your team name. Our grading program will only give credit to those
people whose username can be extracted from the team names.
A cookie is a string of eight hexadecimal digits that is (with high
probability) unique to your team. You can generate your cookie with
the makecookie program giving your team name as the
argument. For example:
unix> makecookie bob+jane
0x39297273
In three of your four buffer attacks, your objective will be to make
your cookie show up in places where it ordinarily would not.
The bufbomb Program
The bufbomb program reads a string from standard input with a
function getbuf having the following C code:
int getbuf()
{
char buf[NORMAL_BUFFER_SIZE];
Gets(buf);
return 1;
}
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, the
destination is an array buf having sufficient space for 32
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 31
characters long, it is clear that getbuf will return 1, as shown
by the following execution example:
unix> bufbomb -u bovik
Type string: howdy doody
Dud: getbuf returned 0x1
If we type a longer string, typically an error occurs:
unix> bufbomb -u bovik
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.
Note: This version of the lab has been specially modified to defeat the
stack randomization techniques used by newer versions of Linux. It
works by using mmap() and a bit of in-line assembly code to
move the stack pointed at by %esp to an otherwise unused part
of your address space. You may need to use gdb to
figure out where that is.
Bufbomb takes several command line arguments:
- –u TEAM
- Operate the bomb for the indicated team.
You should always provide this argument for several reasons:
- It is required to log your successful attacks.
-
Bufbomb determines the cookie you will be using based
on your team name, just as does the program makecookie.
- We have built features into
bufbomb so that some of
the key stack addresses you will need to use depend on your
team’s cookie.
- –h
- Print list of possible command line arguments
- –n
- Operate in “Nitro” mode, as is used in
Level 4 below.
- –s
- Submit your solution exploit string to the grading server.
Your exploit strings will typically contain byte values that do not
correspond to the ASCII values for printing characters. The program
hex2raw can help you generate these raw
strings.
It 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” since the ASCII code for decimal digit 0 is 0x30 and so
forth.
The hex characters you pass hex2raw should be separated by
whitespace (blanks or newlines). I recommend separating different
parts of your exploit string with newlines while you're working on
it. hex2raw also supports C-style block comments, so you can
mark off sections of your exploit string. For example:
bf 66 7b 32 78 /* mov $0x78327b66,%edi */
Be sure to leave space around both the starting and ending comment strings (/*, */)
so they will be properly ignored.
If you place a hex-formatted exploit string in the file
exploit.txt, you can apply the raw string to bufbomb
in at least two different ways:
- You can set up a series of pipes to pass the string through
hex2raw.
unix> cat exploit.txt | ./hex2raw | ./bufbomb -u bovik
- You can store the raw string in a file and use I/O redirection to
supply it to
bufbomb:
unix> ./hex2raw < exploit.txt > exploit-raw.txt
unix> ./bufbomb -u bovik < exploit-raw.txt
This second approach can also be used when running bufbomb
from within gdb:
unix> gdb bufbomb
(gdb) run -u bovik < exploit-raw.txt
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.
When you have correctly solved one of the levels, say level 0:
../hex2raw < smoke-bovik.txt | ../bufbomb -u bovik
Userid: bovik
Cookie: 0x1005b2b7
Type string:Smoke!: You called smoke()
VALID
NICE JOB!
then you can submit your solution to the grading server using the -s option:
./hex2raw < smoke-bovik.txt | ./bufbomb -u bovik -s
Userid: bovik
Cookie: 0x1005b2b7
Type string:Smoke!: You called smoke()
VALID
Sent exploit string to server to be validated.
NICE JOB!
The server will test your exploit string to make sure it really works, and
it will update the Buffer Lab scoreboard page indicating that your userid
(listed by your cookie for anonimity) has completed this level.
You can view the scoreboard by pointing your browser at http://cycle3.csug.rochester.edu:18213/scoreboard
Unlike the bomb lab, there is no penalty for making mistakes in this
lab. Feel free to fire away at bufbomb with any string you
like.
Levels
Levels may be done in any order.
Logistics
As noted above, you may work in a group of up to 2 people.
Any clarifications and revisions to the assignment will be
posted to the Blackboard forum.
Hand-in occurs automatically whenever you correctly solve a level and use the -s option.
Upon receiving your solution, the server will validate your string and
update the Buffer Lab scoreboard Web page, which you can view by pointing
your Web browser at http://cycle3.csug.rochester.edu:18213/scoreboard. You
should be sure to check this page after your submission to make
sure your string has been validated. (If you really solved the level, your string
should be valid.)
Note that each level is graded individually. You do not need to do
them in the specified order, but you will get credit only for the
levels for which the server receives a valid message.
Have fun!
“Trivia” Assignment
Before noon, Tuesday, Feb. 15, send email to
 containing answers to the
following questions (a single email per team is acceptable).
- Run the program
makecookie with your team name as a
parameter. What is the output?
- What is the purpose of the
hex2raw program?
- What is the very first instruction executed at the beginning of
most x86 functions? What is its purpose?
- Where is the return address of a calling function stored relative to
the callee function’s frame pointer?
- Where is the return value of a function stored when control returns to
the calling function?
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 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
0s to work around a shortcoming in objdump to be
described shortly.
We can now assemble and disassemble this file:
unix> gcc -c example.s
unix> 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 Objdump tries to interpret
9: ba dc fe 00 00 mov $0xfedc,%edx these 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 hex-formatted byte code
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 0xFEDCBA98. 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 it
only generated two of the four bytes at the end with value 00. Had
we not added this 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
0s) as:
68 ef cd ab 89 83 c0 11 98 ba dc fe
Turn-In
There is no explicit turn-in. The bomb will notify us automatically
after you have successfully defused it with the -s
option. You can keep track of how you
(and the other groups) are doing by looking at
http://cycle3.csug.rochester.edu:18213/scoreboard/.
This web page is updated continuously to show the progress of each
group.
DUE DATES:
For the “trivia” assignment: noon, Tuesday, February 15.
For the main assignment: 11:59pm, Wednesday, February 23.
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