CS 642: Computer Security - Homework One

This homework assignment tasks you with understanding vulnerabilities in five target programs. The first four are required; if you do the fifth you can earn extra credit. We strongly encourage you to work with a partner.

It is due February 21, 2019 by 9pm PM local time.

The Environment

You will test your exploit programs within a virtual machine (VM) we provide, which is configured with Debian stable ("Lenny"), with ASLR (address space layout randomization) turned off.

See: vulnerability-demo for information about VMs and the VM image.

The Targets

Download hw1_s2019.tgz to get the files for the homework. The 'targets' directory contains the source code for the targets, along with a Makefile for building them.

Your exploits should assume that the compiled target programs are installed setuid-root in /tmp -- /tmp/target1, /tmp/target2, etc.

To build the targets, change to the targets/ directory and type make on the command line; the Makefile will take care of building the targets.

To install the target binaries in /tmp, run:

  make install

To make the target binaries setuid-root, run:

  make install
  su
  make setuid

Once you've run make setuid use exit to return to your user shell. Alternatively, you can keep a separate terminal or virtual console open with a root login, and run make setuid (in the ~user/hw1/targets directory!) in that terminal or console.

Keep in mind that it'll be easier to debug the exploits if the targets aren't setuid. (See below for more on debugging.) If an exploit succeeds in getting a user shell on a non-setuid target in /tmp, it should succeed in getting a root shell on that target when it is setuid. (But be sure to test that way, too, before submitting your solutions!)

The Exploits

The sploits/contains skeleton source for the exploits which you are to write, along with a Makefile for building them. Also included is shellcode.h, which gives Aleph One's shellcode.

The Assignment

You are to write exploits, one per target. Each exploit, when run in the virtual machine with its target installed setuid-root in /tmp, should yield a root shell (/bin/sh).

Your task is to attack these targets. Targets 1-4 are required; target5 is extra credit.

Hints

Read the Phrack articles suggested below. Read Aleph One's paper carefully, in particular.

To understand what's going on, it is helpful to run code through gdb. See the GDB tips section below.

Make sure that your exploits work within the provided virtual machine.

Start early! Theoretical knowledge of exploits does not readily translate into the ability to write working exploits. Target1 is relatively simple and the other problems are quite challenging.

GDB tips

Notice the disassemble and stepi commands.

You may find the x command useful to examine memory (and the different ways you can print the contents such as /a /i after x). The info register command is helpful in printing out the contents of registers such as ebp and esp.

A useful way to run gdb is to use the -e and -s command line flags; for example, the command gdb -e sploit3 -s /tmp/target3 in the VM tells gdb to execute sploit3 and use the symbol file in target3. These flags let you trace the execution of the target3 after the sploit's memory image has been replaced with the target's through the execve system call.

When running gdb using these command line flags, you should follow the following procedure for setting breakpoints and debugging memory:

Tell gdb to notify you on exec(), by issuing the command catch exec
Run the program. gdb will execute the sploit until the execve syscall, then return control to you
Set any breakpoints you want in the target
Resume execution by telling gdb continue (or just c).

If you try to set breakpoints before the exec boundary, you will get a segfault.

If you wish, you can instrument the target code with arbitrary assembly using the __asm__ () pseudofunction, to help with debugging. Be sure, however, that your final exploits work against the unmodified targets, since these we will use these in grading.

Warnings

Aleph One gives code that calculates addresses on the target's stack based on addresses on the exploit's stack. Addresses on the exploit's stack can change based on how the exploit is executed (working directory, arguments, environment, etc.); in my testing, I do not guarantee to execute your exploits exactly the same way bash does.

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.

(In other words, during grading the exploits may be run with a different environment and different working directory than one would get by logging in as user, changing directory to ~/hw1/sploits, and running ./sploit1, etc.; your exploits must work even so.)

Your exploit programs should not take any command-line arguments.

Collaboration Policy

You are encouraged to use the internet, the Piazza discussion board for this class, and classmates for information about tools, reference material, VM setup, and gdb. Please don't discuss solution specifics with anyone beyond your project partner.

Deliverables

You will have three deliverables.

You will need to submit the source code for your exploits (sploit1 through sploit4 or sploit5), along with any files (Makefile, shellcode.h) necessary for building them.
Along with each exploit, include a text file (sploit1.txt, sploit2.txt, and so on). In this text file, explain how your exploit works: what the bug is in the corresponding target, how you exploit it, and where the various constants in your exploit come from.
Finally, you must include file called ID, in the sploits director, which contains, on each line: <netid of Person 1>,<CS login>,<Last Name>,<First Name>. Two lines, one for each group member. An example:

 mmswift,  swift, swift, michael 
 zliuy676, zifan, liu, zifan

Put the ID file, exploits and explanation files in the 'sploits/' directory and package them into a tarball with the following command:

 tar -cf hw1.tar sploits/*

To submit, copy hw1.tar to your handin directory: From a department managed machine:

 cp hw1.tar ~cs642-1/handin/<cslogin>/hw1

From another machine

 scp hw1.tar emperor-01.cs.wisc.edu:~cs642-1/handin/<cslogin>/hw1

As a team, only one person should submit the homework tarfile.

Grading

You will receive two points for each exploit that yields a root shell in our testing. The extra credit target, target5, is also worth two points.

There will not normally be partial credit, but we may make an exception depending on your explanatory writeup. We may also ask you to explain to us how and why each exploit works. Be sure that each group member understands every exploit you turn in!

If you cannot get the exploit working in C code, but get it working from the command shell, you can submit a transcript of your exploit for partial credit in the file describing your exploit. Showing a GDB transcript demonstrating how the attack manipulates memory will be helpful.

Acknowledgements

This assignment is based in part on materials from Prof. Hovav Shacham at UC San Diego as well as Prof. Dan Boneh at Stanford. Thanks for their hard work.

HW1