Project 2: Return-oriented Programming

Due: Friday, November 3, 2017, 23:59

Goal

The goal of this project is to practice writing return-oriented programs and implement several different types of shellcode.

Specifically, there is one target binary which is a statically-linked, 32-bit x86 Linux binary. You will write five different Python programs which will exploit the binary in different ways. Unlike Project 1, this target is a server program which you will treat as if it were running on a remote machine. (In reality, both the target and the exploits will run on the same machine, but they will communicate over TCP.)

Skeleton code for each Python program has been provided, but each will need substantial modification.

Collaboration

You may work on this project in collaboration with a single partner as described on the main page.

You must not discuss the project with anyone other than your partner and course staff. You may use online resources for general reference, but not to search for solutions to specific questions posed in this project.

The Environment

You should be able to work in any 32- or 64-bit Linux environment that has Python 2.7 (and netcat is helpful). In particular, you may use bertvm or ernievm.

When exploiting the target (see below), you should not assume anything about stack addresses (in particular, you should not assume anything about environment variables).

The Target

The target binary has been compiled from target.c and statically linked with PCRE2. You must not change target or attempt to recompile it from target.c. If you do so, your exploits will almost certainly not work against the unmodified target.

The target is a simple server program that listens on the specified port for a connection. Once connected, a client can send one of two commands:

PUT SECRET <password> <secret>\r\n

and

GET SECRET <password>\r\n

The first command directs the server to remember a secret and its associated password. (The password may not contain white space.) The second command directs the server to reply with the secret, if the password matches. See target.c for the precise workings.

The Assignment

There are five tasks to be completed. Each task involves modifying a Python exploit program to construct a return-oriented exploit to exploit target in the manner specified.

WARNING: Although care has been taken to limit connections to localhost, as part of this project, you will be connecting sockets to shells. You must ensure that all sockets you create, either in the Python exploit programs or in target are bound to 127.0.0.1.

FURTHER WARNING: Any attempt to connect to other students’ open ports is strictly prohibited. Violations will result in a 0 for the project and academic integrity hearings. So please, just don’t do it.

Unlike Project 1, code injection will not be possible due to standard DEP protections. Instead, you will need to implement return-oriented programs. This will be tricky, but it is how modern software exploitation actually works.

To find gadgets in target, you may use Jonathan Salwan’s ROPgadget tool. ROPgadget requires the capstone disassembler to run. This is not installed on the departmental servers, but you should be able to install it on your own machines. We have included a gadgets.txt file that is the result of running:

$ ./ROPgadget.py --binary target --ropchain > gadgets.txt

You may still find it helpful to run it yourself with other options.

Each task description below contains an example which consists of running ./target <port> in one shell (with <port> replaced by a real port number) and running the Python program for the task. In addition, each task description includes the partial output of running the target using strace(1). Your code should induce the same behavior in target. That is, replacing ./target <port> with ./strace target <port> and following the provided example should produce the same system calls, with the same arguments, in the same order as shown. (File descriptor numbers may be different from those shown, but should be self-consistent.)

You may not use any of the system calls that allocate writable and executable memory nor any system calls that change memory page permissions. In particular, all of the exploits you write must be entirely in the return-oriented style; you should not inject any x86 code.

The Tasks

Task 1 Local shell (local.py)

The first task is to exploit target and have it exec a shell. Conveniently, ROPgadget has constructed such an exploit for us! Look at the bottom of gadgets.txt (or run ROPgadget.py with the --ropchain option).

The constructed exploit uses struct.pack to build a binary string. See the documentation for details. You’re going to be using this function extensively.

Let’s take a look at the first few calls.

p += pack('<I', 0x0808522a) # pop edx ; ret
p += pack('<I', 0x08139060) # @ .data
p += pack('<I', 0x080f1016) # pop eax ; ret
p += '/bin'
p += pack('<I', 0x080c219d) # mov dword ptr [edx], eax ; ret

Each 4-byte word in the p string is either the address of code to return to or some data. The first word, 0x0808522a is the address of the instructions pop edx ; ret, as noted in the comment. When target returns to this address, it will pop 0x08139060 into edx.

But what is that value and what does @ .data mean? If you run readelf -S target, you’ll see that the .data section starts at address 0x08139060. ROPgadget’s shellcode has decided to use target’s writable data section as a place to write some data. In particular, this will write /bin at 0x08139060. You’ll want to keep this in mind for some of the other parts.

At this point, you should try to figure out what the rest of the code is doing. You can probably get away with not understanding this, but it’ll make the whole rest of the project easier if you figure this out now.

One final point about the generated shellcode: The final 13 words clear eax, increment eax 11 times, and then run int 0x80. ROPgadget is avoiding zero-bytes. Since you are using sockets which can handle binary data, this isn’t a concern. Go ahead and replace the xor and the incs with a pop eax.

To complete this task, modify local.py to:

  1. Exploit target and make it exec /bin/sh by overwriting the saved eip and the subsequent words on the stack with this return-oriented program. (Disassembling target using objdump -d target can help you figure out where the saved eip is relative to the start of the array.)

To test that everything works, run ./target <port> in one shell and in another shell, run ./local.py <port> (replace <port> which an actual port number). You should see something like the following.

user@bertvm:~/project2$ ./target <port>
$

Running strace ./target <port>, with the example above prints out (in part)

user@bertvm:~/project2$ strace ./target <port>
...
send(4, "INVALID COMMAND\r\n", 17, 0)   = 17
execve("/bin//sh", [], [/* 0 vars */])  = 0
...

Task 2 Dup shell (dup.py)

The first exploit was fun to do (I hope), but not terribly useful. After all, it opened a shell on the “remote” machine with no way to communicate with it! You’re going to fix that right now.

You need to connect target’s stdin, stdout, and stderr to the socket before you exec /bin/sh. Fortunately, that’s easy to do with the dup2(2) system call.

To make a system call, you need to know what to put in each register. Fortunately, kernelgrok is a great resource. Search for dup to see what goes in each register.

One tricky aspect is you need to put the socket file descriptor in register ebx, but you can’t know what value to use until the exploit connects. Looking at the disassembly for target, you’ll see that the return value from accept(2) is stored in ebx in main and also on the stack just above the saved eip as the first argument to handle_connection. Unfortunately, you’re going to trash the saved ebx as well as the argument. All hope is not lost, run target in gdb and break near the end of handle_connection. Luckily, the socket file descriptor is available!

In essence, you want to get the socket file descriptor that was returned from accept(2) in target—call it sock—and make the three system calls that correspond to

dup2(sock, 0);
dup2(sock, 1);
dup2(sock, 2);

and then exec /bin/sh as you did in local.py.

To complete this task, modify dup.py to:

  1. Exploit target and have it perform the dup2(2) system calls and exec the shell as described above;
  2. Read from stdin and write to the socket and read from the socket and write to stdout. You may find the console function in console.py useful for this task. Simply import the function using from console import console, pass the socket to console, and it should take care of everything. The prompt will not appear, but you can still enter commands and see the result.

To test that everything works, run ./target <port> in one shell and in another shell, run

user@bertvm:~/project2$ ./dup.py <port>.
INVALID COMMAND
date
Sun Oct 16 03:28:26 CDT 2016
exit

Running strace ./target as described in the hints, with the example above prints out (in part)

user@bertvm:~/project2$ strace ./target <port>
...
send(4, "INVALID COMMAND\r\n", 17, 0)   = 17
dup2(4, 0)                              = 0
dup2(4, 1)                              = 1
dup2(4, 2)                              = 2
execve("/bin//sh", [], [/* 3 vars */])  = 0
...
read(0, "date\n", 8192)                 = 5
...
read(0, "exit\n", 8192)                 = 5
...

Task 3 Reverse shell (reverse.py)

The exploit used in dup.py connected the shell to the socket we used to connect to target initially. For this task, the exploit will cause target to make a connection to remote server, connect the resultant socket to stdin/stdout/stderr (as was done in dup.py), and exec a shell.

Creating a new socket and making a connection involves making several system calls.

There are several ways to call these functions. Since they appear in target, it’s possible to return to them with the arguments on the stack. However, the first argument to connect(2) is the return value from the socket(2) which makes making returning to the libc implementations difficult. Instead, you should make the corresponding system calls directly; just as you did with dup2(2).

See the associated manual pages for example usage and see the hints below for suggestions on making these system calls. And see the example below for the arguments to the system calls.

The connection should be to 127.0.0.1 and the port should be specified as an argument to reverse.py (see example below).

To complete this task, modify reverse.py to:

  1. Open a socket to listen on 127.0.0.1 with the port specified as a command line parameter to reverse.py (see example below).
  2. Exploit target and have it make a new connection to 127.0.0.1 with the same port used in step 1. Once connected, target should exec a shell with stdin/stdout/stderr connected to the new socket.
  3. Read from stdin and write to the newly opened socket and read from the socket and write to stdout. Again, the console function may be helpful.

To test that everything works, run ./target <port> in one shell and in another shell, run

user@bertvm:~/project2$ ./reverse.py <port> <connect_port>.
INVALID COMMAND
date
Sun Oct 16 03:47:33 CDT 2016
exit

Running strace ./target <port>, with the example above prints out (in part)

user@bertvm:~/project2$ strace ./target <port>
...
send(4, "INVALID COMMAND\r\n", 17, 0)   = 17
socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 5
connect(5, {sa_family=AF_INET, sin_port=htons(<connect_port>), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
dup2(5, 0)                              = 0
dup2(5, 1)                              = 1
dup2(5, 2)                              = 2
execve("/bin//sh", [], [/* 6 vars */])  = 0
...
read(0, "date\n", 8192)                 = 5
...
read(0, "exit\n", 8192)                 = 5
...

Task 4 Bind shell (bind.py)

The exploit used in dup.py connected the shell to the socket we used to connect to target initially; the exploit used in reverse.py made a connection back to the attacker. For this task, the exploit will cause target to start listening on a specified port and when a connection happens, connect stdin/stdout/stderr to the newly connected socket and exec a shell. This is called a bind shell.

Listening on a port and accepting new connections involves making several system calls.

As was the case for reverse.py, there are several ways to call these functions. Since they’re used in target, it’s possible to return to them with the arguments on the stack. However, the first argument to each of the last three require system calls requires the return value from the first which makes making returning to the libc implementations more difficult. Instead, you should make the corresponding system calls directly.

See target.c and the associated manual pages for example usage and see the hints below for suggestions on making these system calls. And see the example below for the arguments to the system calls.

Note that the bind(2) call must bind the socket to address 127.0.0.1 as mentioned in the warning above.

To complete this task, modify bind.py to:

  1. Exploit target and have it listen for a new connection on a different port (specified as a command line parameter to bind.py—see example below). Once a connection happens, target should exec a shell with stdin/stdout/stderr connected to the socket.
  2. Make a new connection to 127.0.0.1:<listen_port> where <listen_port> is the port specified as the second command line parameter (see example).
  3. Read from stdin and write to the newly opened socket and read from the socket and write to stdout. Again, the console function may be helpful.

To test that everything works, run ./target <port> in one shell and in another shell, run

user@bertvm:~/project2$ ./bind.py <port> <listen_port>.
INVALID COMMAND
date
Sun Oct 16 03:47:33 CDT 2016
exit

Running strace ./target <port>, with the example above prints out (in part)

user@bertvm:~/project2$ strace ./target <port>
...
send(4, "INVALID COMMAND\r\n", 17, 0)   = 17
socket(PF_INET, SOCK_STREAM, IPPROTO_TCP) = 5
bind(5, {sa_family=AF_INET, sin_port=htons(<listen_port>), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
listen(5, 135499884)                    = 0
accept(5, 0, NULL)                      = 6
dup2(6, 0)                              = 0
dup2(6, 1)                              = 1
dup2(6, 2)                              = 2
execve("/bin//sh", [], [/* 3 vars */])  = 0
...
read(0, "date\n", 8192)                 = 5
...
read(0, "exit\n", 8192)                 = 5
...

Task 5 Data exfiltration (secret.py)

The final task is to convince target to write its secret to the socket without knowing the password. No need to get a shell this time.

It’s possible to write to a socket using send(2) or write(2). As such, there are several possible approaches that one could take: (1) return to the __libc_send function, (2) return to the __libc_write function, (3) make a send(2) system call, or (4) make a write(2) system call. Each approach requires passing the socket file descriptor. Since the file descriptor is not constant, returning to the __libc_send or __libc_write functions is actually more difficult than making a system call. As you’ve seen, making socket system calls is more complicated than other system calls, so go with approach (4).

You also need to know how much data to write. To do this, you’re going to need to implement a return-oriented loop. This is easily the most complicated part of writing a return-oriented program. There are many ways that one could implement it. The method I used was to implement the following StringLength gadget:

  1. Set edi to 0xffffffff.
  2. Set esi to point to a 0 byte. (I set esi to point to .data and wrote a 0 there.)
  3. Set ecx to point to the start of the secret array.
  4. Set edx to the number of bytes to conditionally move the stack pointer to start the next iteration of the loop (this should be a negative number).
  5. Use edi as the length counter and ecx as a pointer to the next byte to test.
  6. Use eax and ebp as scratch registers to perform the conditional update of esp.
  7. After a short setup, the body of the loop increments the ecx pointer and edi length, compares the byte pointed to by ecx to 0, and if it is nonzero, branch back to the top of the loop by updating esp.
  8. After the loop exits, edi holds the length of the string.

The sequences of code I used to implement step 6 of the StringLength gadget (organized by functionality, not order in which I used them) were

# Increment/decrement pointer and length.
0x080545a1 # dec ecx ; ret
0x0807703f # inc ecx ; mov ebp, 0xa0b80810 ; ret
0x0808d7c5 # inc edi ; ret

# Compare to 0.
0x08048ac1 # xor eax, eax ; ret
0x08127ded # cmp byte ptr [ecx], al ; ret

# Conditional update of esp.
0x0808522a # pop edx ; ret
0x0809c909 # cmovne eax, edx ; ret
0x080485f4 # pop ebp ; ret
0x0807c05f # add ebp, eax ; retf
0x0812c918 # add esp, ebp ; add cl, byte ptr [esi] ; adc al, 0x43 ; ret

There a few things to notice about these code sequences.

To complete this task, modify secret.py to:

  1. Exploit target and have it write the secret value to the socket by making a write(2) system call. Afterward, exit cleanly by making an exit system call with return value 0. (See the hints below for suggestions on making multiple system calls.)
  2. Read the secret from the socket and print it out. (The secret won’t have a newline, so you’ll probably want to add that yourself.)

You may assume that a secret has been set.

To test that everything works, run ./target <port> in one shell and in another shell, run

user@bertvm:~/project2$ echo 'PUT SECRET password1 Secret value!' | nc -C 127.0.0.1 <port>
SECRET STORED
user@bertvm:~/project2$ ./secret.py <port>.
INVALID COMMAND
Secret value!

Running strace ./target <port>, with the example above prints out (in part)

user@bertvm:~/project2$ strace ./target <port>
...
accept(3, 0, NULL)                      = 4
recv(4, "PUT SECRET password1 Secret valu"..., 1024, 0) = 36
send(4, "SECRET STORED\r\n", 15, 0)     = 15
close(4)                                = 0
accept(3, 0, NULL)                      = 4
...
send(4, "INVALID COMMAND\r\n", 17, 0)   = 17
write(4, "Secret value!", 13)           = 13
_exit(0)                                = ?

Hints

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