Custom shellcode in C? - c

Question

I am trying to develop a shellcode that would execute the following command /bin/ls / (list root directory).
So, I first try to develop a script in assembly to do the job for me, below is the script:
SECTION .data
buf: db "./", 0
SECTION .text
global _start
_start:
xor eax, eax
xor edx, edx
push eax
push long 0x736c2f2f ; "sl/"
push long 0x6e69622f ; "nib/"
mov ebx, esp
push eax
push byte 0x2f
mov esi, esp
push eax
push esi
push ebx
mov ecx, esp
mov eax, 0x0b
int 0x80
mov eax, 1
int 0x80
I then compile with following: nasm -f elf -g shell.asm && ld -s -o shell shell.o -m elf_i386. This works perfectly, If executed, it list root directory.
Then, I disassemble the binary using objdump to get the op-codes:
#objdump -d ./shell
./shell: file format elf32-i386
Disassembly of section .text:
08049000 <.text>:
8049000: 31 c0 xor %eax,%eax
8049002: 31 d2 xor %edx,%edx
8049004: 50 push %eax
8049005: 68 2f 2f 6c 73 push $0x736c2f2f
804900a: 68 2f 62 69 6e push $0x6e69622f
804900f: 89 e3 mov %esp,%ebx
8049011: 50 push %eax
8049012: 6a 2f push $0x2f
8049014: 89 e6 mov %esp,%esi
8049016: 50 push %eax
8049017: 56 push %esi
8049018: 53 push %ebx
8049019: 89 e1 mov %esp,%ecx
804901b: b8 0b 00 00 00 mov $0xb,%eax
8049020: cd 80 int $0x80
8049022: b8 01 00 00 00 mov $0x1,%eax
8049027: cd 80 int $0x80
To make life easy, I did the following: objdump -d ./shell | awk -F " " '{print $1}' |awk -F ":" '{print $2}' | tr -d " " | tr -d "\n" | tr -d "\t" | xclip -selection c. Basically it copies the opcodes to the clipboard. 31c031d250682f2f6c73682f62696e89e3506a2f89e650565389e1b80b000000cd80b801000000cd80.
I then to test my shellcode, I inject it to a c program as below:
#include<stdio.h>
#include<string.h>
unsigned char shellcode[] = "\x31\xc0\x31\xd2\x50\x68\x2f\x2f\x6c\x73\x68\x2f\x62\x69\x6e\x89\xe3\x50\x6a\x2f\x89\xe6\x50\x56\x53\x89\xe1\xb8\x0b\x00\x00\x00\xcd\x80\xb8\x01\x00\x00\x00\xcd\x80";
int main(int argc, char **argv) {
int *ret;
ret = (int *)&ret + 2;
(*ret) = (int)shellcode;
}
# gcc -g -o foo foo.c -z execstack -fno-stack-protector
foo.c: In function ‘main’:
foo.c:9:11: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
9 | (*ret) = (int)shellcode;
|
with that done, If I execute my foo binary, it execute the program but do not list the root directory.
I am using nasm with the following arc: Linux kali 5.2.0-kali2-amd64 #1 SMP Debian 5.2.9-2kali1 (2019-08-22) x86_64 GNU/Linux