Assignment 2 75 pts + 25pts extra for sploit4¶
Deadline: Wednesday, January 29, 2020 by 1:59:59 PM
Early Turn-In: Sunday, January 26, 2020 by 11:59:59 PM (10% bonus)
The goal of this assignment is to gain hands-on experience with the effects of buffer overflows and other memory-safety bugs.
sploit1-sploit3 are required. sploit4 is extra credit!
- You will be provided a skeleton for implementing these exploits in C.
- You must not discuss your solution with other students until three days after the assignment deadline.
- You may consult any online references you wish.
- If you use any code in your answer that you did not write yourself, you must document that fact. Failure to do so will be considered a violation of the academic integrity policy.
To complete this assignment you will be provided with a VirtualBox VM and a set of files.
In order to match the environment in which your submission will be graded, all work for this assignment must be done on the VirtualBox VM we have provided, named
pa2box. You can download the VM image here.
pa2box.vbox file into VirtualBox via the
Add menu item. Don't create a brand-new VirtualBox VM or import the
pa2box-data.vmdk file on its own: if you do this, you won't have the correct configuration.
After importing the VM, select it in the VirtualBox sidebar and start it with the green arrow button or the
Start menu item. The VM is configured with two users:
student, with password
root, with password
hackallthethings. Instead of using the command line through VirtualBox, we recommend that you SSH into your VM from another client. The VM image is configured to expose an SSH server on port 2222 of your system's loopback address (
To SSH into the VM (from your host system running VirtualBox, not from inside the VM):
ssh -p 2222 firstname.lastname@example.org
To copy files from your computer to the VM:
scp -P 2222 -r /path/to/files/ email@example.com:/home/student
To copy files from the VM to your computer:
scp -P 2222 firstname.lastname@example.org:/path/to/files/ /destination/path
Please note that SSH is disabled for
root, so you can only SSH in as the
student user. You can still log in as
su or by logging into the VM directly.
Starter files are included inside the
student home directory of the VM image.
targets directory contains a
Makefile to generate target executables specific to your PID (as well as a folder called
base, which you should not modify, used to generate the targets).
Exploit starter code can be found in the
sploits4 directories: one per each of the four vulnerable target programs. Each of these contains a
shellcode.h header file providing Aleph One's shellcode, and a
sploitN.c file in which you should write your exploit code (in addition to an
assignment.toml file, which should not be modified).
You will be writing an exploit for each of the four vulnerable programs provided in the assignment. Each exploit, when run in the VM with its target installed setuid-root in
/tmp, should yield a root shell (
/bin/sh). You can verify that the shell has been launched as
root by typing
whoami, to which you should see the response
root. You must use Aleph One’s shellcode in
shellcode.h, as this will be used in the grading scripts.
For each exploit, in addition to your
.c file, please also create a corresponding file
writeup.txt containing a brief description of how the exploit works.
To complete the assignment, you will need to: generate targets specific to your PID; use GDB to find vulnerabilities in the targets; and, finally, craft your exploit programs.
Generating the Targets¶
make generate in the
targets directory to create the four target source files specific to you. This will prompt you for your student ID (
U########), which will be used to randomize portions of the contents of
make to build the target binaries
sudo make install to copy the binaries into the
sudo make setuid to mark the binaries as setuid-root. If you forget this step, then your exploits will end up launching a normal shell instead of a root shell!
For this assignment you should read and have a solid understanding of Aleph One’s “Smashing the Stack for Fun and Profit”.
Aleph One gives code that calculates addresses on the target’s stack based on addresses on the exploit’s stack. However, addresses on the exploit’s stack can change based on how the exploit is executed (working directory, arguments, environment, etc.). You must therefore hard-code target stack locations in your exploits.
You should not use a function such as
get_sp() in the exploits you hand in. You should only modify the
sploitN.c files and corresponding
writeup.txt files; i.e., your exploits should work with unmodified Makefiles, targets, etc. In grading, the exploits may be run with a different working directory. Your exploits must work in these cases also. If you hard-code the target's stack locations into your exploit you should be fine. Your exploit programs should not take any command-line arguments.
Shutting down the VM removes the files in /tmp; if you want to keep them, pause the VM and save its state!
IMPORTANT: Using the shellcode¶
The shellcode is defined for you in
shellcode.h inside each
sploitN/ directory. To use the shellcode in your sploit buffers, please import the shellcode with
#include and copy the contents programatically. You can do this with
strcpy, or even just a
for loop with array assignments. Please do NOT manually copy-paste the shellcode string literal into your sploitN.c file. While the sploit will still work, it may break grading.
To run an exploit in GDB, run, e.g.,
gdb sploit1 to execute
Note that there are two executables when we run these programs, since, e.g.,
sploit1 will execute
target1. Because of this, you can use the following sample workflow to get started in GDB:
b main--- break at
r--- run the executable
c--- continue until
b foo--- set a breakpoint at
Note that you will not be able to set breakpoints in
targetN until the program has started executing
That is, e.g., if you tried to run
b foo before step 3, you would get the error
Function "foo" not defined.
Here are some additional GDB commands that may come in handy:
p var--- prints the value of a variable named
p buf--- (if
bufis an array) prints the contents of an array named
p &buf--- prints the starting address of an array or variable named
i r--- view registers
i frame--- view info about the current stack frame
i stack--- view high level info about the stack
help <command>--- get help inside GDB about a particular command
Also refer back to the sample commands in Assignment 1.
Note that this is only a cursory overview of GDB; much more info is available from online resources.
- Create a file named
PIDin the student home directory on the VM, and put your PID in it (ex:
- Run the following command from the student home directory
tar -czvf pa2.tar.gz PID sploit[1-4]/ targets/This will create a
pa2.tar.gzin the home directory with all your files in it. You can see what files were zipped by running
tar -tf pa2.tar.gz
pa2.tar.gzfile to your local computer and submit to gradescope.
Note: The autograder will take time to set up, as it'll have to redirect your submissions somewhere else, where your sploits can be tested in the same VirtualBox environment. We'll post an update when it's up. In the meanwhile, you'll know your answer is correct if you followed the instructions above and you were able to get a root shell.