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Building your own disassembly tooling for — that’s right — fun and profit

While solving complex reversing challenges, we often use established tools like radare2 or IDA for disassembling and debugging. But there are times when you need to dig in a little deeper and understand how things work under the hood.

Rolling your own disassembly scripts can be immensely helpful when it comes to automating certain processes, and eventually build your own homebrew reversing toolchain of sorts. At least, that’s what I’m attempting anyway.

Setup

As the title suggests, you’re going to need a Python 3 interpreter before
anything else. Once you’ve confirmed beyond reasonable doubt that you do,
in fact, have a Python 3 interpreter installed on your system, run

$ pip install capstone pyelftools

where capstone is the disassembly engine we’ll be scripting with and pyelftools to help parse ELF files.

With that out of the way, let’s start with an example of a basic reversing
challenge.

/* chall.c */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
   char *pw = malloc(9);
   pw[0] = 'a';
   for(int i = 1; i <= 8; i++){
       pw[i] = pw[i  1] + 1;
   }
   pw[9] = '';
   char *in = malloc(10);
   printf("password: ");
   fgets(in, 10, stdin);        // 'abcdefghi'
   if(strcmp(in, pw) == 0) {
       printf("haha yes!n");
   }
   else {
       printf("nah duden");
   }
}

Compile it with GCC/Clang:

$ gcc chall.c -o chall.elf

Scripting

For starters, let’s look at the different sections present in the binary.

# sections.py

from elftools.elf.elffile import ELFFile

with open('./chall.elf', 'rb') as f:
    e = ELFFile(f)
    for section in e.iter_sections():
        print(hex(section[sh_addr]), section.name)

This script iterates through all the sections and also shows us where it’s loaded. This will be pretty useful later. Running it gives us

› python sections.py
0x238 .interp
0x254 .note.ABI-tag
0x274 .note.gnu.build-id
0x298 .gnu.hash
0x2c0 .dynsym
0x3e0 .dynstr
0x484 .gnu.version
0x4a0 .gnu.version_r
0x4c0 .rela.dyn
0x598 .rela.plt
0x610 .init
0x630 .plt
0x690 .plt.got
0x6a0 .text
0x8f4 .fini
0x900 .rodata
0x924 .eh_frame_hdr
0x960 .eh_frame
0x200d98 .init_array
0x200da0 .fini_array
0x200da8 .dynamic
0x200f98 .got
0x201000 .data
0x201010 .bss
0x0 .comment
0x0 .symtab
0x0 .strtab
0x0 .shstrtab

Most of these aren’t relevant to us, but a few sections here are to be noted. The .text section contains the instructions (opcodes) that we’re after. The .data section should have strings and constants initialized at compile time. Finally, the .plt which is the Procedure Linkage Table and the .got, the Global Offset Table. If you’re unsure about what these mean, read up on the ELF format and its internals.

Since we know that the .text section has the opcodes, let’s disassemble the binary starting at that address.

# disas1.py

from elftools.elf.elffile import ELFFile
from capstone import *

with open('./bin.elf', 'rb') as f:
    elf = ELFFile(f)
    code = elf.get_section_by_name('.text')
    ops = code.data()
    addr = code['sh_addr']
    md = Cs(CS_ARCH_X86, CS_MODE_64)
    for i in md.disasm(ops, addr):        
        print(f'0x{i.address:x}:t{i.mnemonic}t{i.op_str}')

The code is fairly straightforward (I think). We should be seeing this, on running

› python disas1.py | less      
0x6a0: xor ebp, ebp
0x6a2: mov r9, rdx
0x6a5: pop rsi
0x6a6: mov rdx, rsp
0x6a9: and rsp, 0xfffffffffffffff0
0x6ad: push rax
0x6ae: push rsp
0x6af: lea r8, [rip + 0x23a]
0x6b6: lea rcx, [rip + 0x1c3]
0x6bd: lea rdi, [rip + 0xe6]
**0x6c4: call qword ptr [rip + 0x200916]**
0x6ca: hlt
... snip ...

The line in bold is fairly interesting to us. The address at [rip + 0x200916] is equivalent to [0x6ca + 0x200916], which in turn evaluates to 0x200fe0. The first call being made to a function at 0x200fe0? What could this function be?

For this, we will have to look at relocations. Quoting linuxbase.org

Relocation is the process of connecting symbolic references with symbolic definitions. For example, when a program calls a function, the associated call instruction must transfer control to the proper destination address at execution. Relocatable files must have “relocation entries’’ which are necessary because they contain information that describes how to modify their section contents, thus allowing executable and shared object files to hold the right information for a process’s program image.

To try and find these relocation entries, we write a third script.

# relocations.py

import sys
from elftools.elf.elffile import ELFFile
from elftools.elf.relocation import RelocationSection

with open('./chall.elf', 'rb') as f:
    e = ELFFile(f)
    for section in e.iter_sections():
        if isinstance(section, RelocationSection):
            print(f'{section.name}:')
            symbol_table = e.get_section(section['sh_link'])
            for relocation in section.iter_relocations():
                symbol = symbol_table.get_symbol(relocation['r_info_sym'])
                addr = hex(relocation['r_offset'])
                print(f'{symbol.name} {addr}')

Let’s run through this code real quick. We first loop through the sections, and check if it’s of the type RelocationSection. We then iterate through the relocations from the symbol table for each section. Finally, running this gives us

› python relocations.py
.rela.dyn:
 0x200d98
 0x200da0
 0x201008
_ITM_deregisterTMCloneTable 0x200fd8
**__libc_start_main 0x200fe0**
__gmon_start__ 0x200fe8
_ITM_registerTMCloneTable 0x200ff0
__cxa_finalize 0x200ff8
stdin 0x201010
.rela.plt:
puts 0x200fb0
printf 0x200fb8
fgets 0x200fc0
strcmp 0x200fc8
malloc 0x200fd0

Remember the function call at 0x200fe0 from earlier? Yep, so that was a call to the well known __libc_start_main. Again, according to linuxbase.org

The __libc_start_main() function shall perform any necessary initialization of the execution environment, call the main function with appropriate arguments, and handle the return from main(). If the main() function returns, the return value shall be passed to the exit() function.

And its definition is like so

int __libc_start_main(int *(main) (int, char * *, char * *), 
int argc, char * * ubp_av, 
void (*init) (void), 
void (*fini) (void), 
void (*rtld_fini) (void), 
void (* stack_end));

Looking back at our disassembly

0x6a0: xor ebp, ebp
0x6a2: mov r9, rdx
0x6a5: pop rsi
0x6a6: mov rdx, rsp
0x6a9: and rsp, 0xfffffffffffffff0
0x6ad: push rax
0x6ae: push rsp
0x6af: lea r8, [rip + 0x23a]
0x6b6: lea rcx, [rip + 0x1c3]
**0x6bd: lea rdi, [rip + 0xe6]**
0x6c4: call qword ptr [rip + 0x200916]
0x6ca: hlt
... snip ...

but this time, at the lea or Load Effective Address instruction, which loads some address [rip + 0xe6] into the rdi register. [rip + 0xe6] evaluates to 0x7aa which happens to be the address of our main() function! How do I know that? Because __libc_start_main(), after doing whatever it does, eventually jumps to the function at rdi, which is generally the main() function. It looks something like this

To see the disassembly of main, seek to 0x7aa in the output of the script we’d written earlier (disas1.py).

From what we discovered earlier, each call instruction points to some function which we can see from the relocation entries. So following each call into their relocations gives us this

printf 0x650
fgets  0x660
strcmp 0x670
malloc 0x680

Putting all this together, things start falling into place. Let me highlight the key sections of the disassembly here. It’s pretty self-explanatory.

0x7b2: mov edi, 0xa  ; 10
0x7b7: call 0x680    ; malloc

The loop to populate the *pw string

0x7d0:  mov     eax, dword ptr [rbp - 0x14]
0x7d3:  cdqe    
0x7d5:  lea     rdx, [rax - 1]
0x7d9:  mov     rax, qword ptr [rbp - 0x10]
0x7dd:  add     rax, rdx
0x7e0:  movzx   eax, byte ptr [rax]
0x7e3:  lea     ecx, [rax + 1]
0x7e6:  mov     eax, dword ptr [rbp - 0x14]
0x7e9:  movsxd  rdx, eax
0x7ec:  mov     rax, qword ptr [rbp - 0x10]
0x7f0:  add     rax, rdx
0x7f3:  mov     edx, ecx
0x7f5:  mov     byte ptr [rax], dl
0x7f7:  add     dword ptr [rbp - 0x14], 1
0x7fb:  cmp     dword ptr [rbp - 0x14], 8
0x7ff:  jle     0x7d0

And this looks like our strcmp()

0x843:  mov     rdx, qword ptr [rbp - 0x10] ; *in
0x847:  mov     rax, qword ptr [rbp - 8]    ; *pw
0x84b:  mov     rsi, rdx             
0x84e:  mov     rdi, rax
0x851:  call    0x670                       ; strcmp  
0x856:  test    eax, eax                    ; is = 0? 
0x858:  jne     0x868                       ; no? jump to 0x868
0x85a:  lea     rdi, [rip + 0xae]           ; "haha yes!" 
0x861:  call    0x640                       ; puts
0x866:  jmp     0x874
0x868:  lea     rdi, [rip + 0xaa]           ; "nah dude"
0x86f:  call    0x640                       ; puts  

I’m not sure why it uses puts here? I might be missing something; perhaps printf calls puts. I could be wrong. I also confirmed with radare2 that those locations are actually the strings “haha yes!” and “nah dude”.

Conclusion

Wew, that took quite some time. But we’re done. If you’re a beginner, you might find this extremely confusing, or probably didn’t even understand what was going on. And that’s okay. Building an intuition for reading and grokking disassembly comes with practice. I’m no good at it either.

All the code used in this post is here: https://github.com/icyphox/asdf/tree/master/reversing-elf

Ciao for now, and I’ll see ya in #2 of this series — PE binaries. Whenever that is.