Shellcode C Program Harness

Question

Having a hard time getting a basic shellcode harness/test C program to work. The assembly program (given below) seems to work just fine when I run it on its own (nasm and ld) --- in that it opens a file called key, reads 1 byte at a time and displays that to stdout.

I've taken that asm file and extracted the hex encoding to hardcode into a char[] named assn1[] within my C harness program and use function pointers to execute the code but end up segfaulting. To make sure I wasn't crazy I added another char[] named shellcode[] which holds hex for popping a shell (aleph1) and this seems to work just fine. I'm calling execstack -s before running my C executable.

Any help would be greatly appreciated! I'm spinning on this for hours now...

assn1.asm-------

jmp short bottom    ; relative addressing start

top:    
; setup params for open()
pop ebx         ; param1 ebx now holds 'key' 
xor ecx, ecx        ; param2 ecx corresponds to flag O_RDONLY
            ; param3 edx not required for existing file
xor eax, eax        ; clear eax to 0
mov al, 5       ; syscall open()
int 0x80        ; software interrupt to call open()
            ; returns int filedescriptor in eax

; setup params for read() and write()
mov ebx, eax        ; param1 ebx now holds filedescriptor   
sub esp, 1      ; allocate buffer of 1 bytes on stack
mov ecx, esp        ; param2 ecx now points to buffer
xor edx, edx        ; clear edx
inc edx         ; param3 edx set to 1 byte to be read   
rwloop:
xor eax, eax        ; clear eax
mov al, 3       ; syscall code for read()
int 0x80        ; read() 1 byte into buffer
test eax,eax        ; if eax=0, read() reached EoF
jz end          ; and stop reading/writing

; else get ready to write
push ebx        ; store filedescriptor for KEY onto stack 
xor ebx, ebx        ; clear ebx
inc ebx         ; param1 ebx = 1 for stdout
            ; param2 and param3 same from read()
xor eax, eax        ; clear eax
mov al, 4       ; syscall for write()
int 0x80
pop ebx         ; restore filedescriptor to ebx
jmp rwloop      

end:
; place esp back to original point on stack
; add esp, 1

; exit cleanly
xor ebx,ebx     ; retcode = 1
xor eax,eax     ; eax = 0
inc eax         ; eax = 1, syscall exit(1)
int 0x80

bottom:
call top        ; address of key pushed on stack
db 'key', 0

assn1_harness.c ---

#include <sys/mman.h>
#include <stdio.h>
#include <stdlib.h>


// The trailing null byte is implied because this is a string
char shellcode[] = {
"\xeb\x16\x5e\x31\xd2\x52\x56\x89\xe1\x89\xf3\x31\xc0\xb0\x0b\xcd"
"\x80\x31\xdb\x31\xc0\x40\xcd\x80\xe8\xe5\xff\xff\xff\x2f\x62\x69"
"\x6e\x2f\x73\x68"
};

char assn1[] = {
  "\xeb\x43\x66\x5b\x66\x31\xc9\x66\x31\xc0\xb0\x05\xcd\x80\x66\x89"
  "\xc3\x66\x83\xec\x01\x66\x89\xe1\x66\x31\xd2\x66\x42\x66\x31\xc0"
  "\xb0\x03\xcd\x80\x66\x85\xc0\x74\x12\x66\x53\x66\x31\xdb\x66\x43"
  "\x66\x31\xc0\xb0\x04\xcd\x80\x66\x5b\xeb\xe2\x66\x31\xdb\x66\x31"
  "\xc0\x66\x40\xcd\x80\xe8\xba\xff\x6b\x65\x79"
};

int main (int argc, char **argv) {

int (*func)();

//func = (int (*)()) shellcode;
func = (int (*)()) assn1;

(*func)();  

return 0;
}

Answer 1

Your machine code is wrong, it seems to be assembled as 16 bit code. You forgot bits 32.

This works better:

char assn1[] = {
    "\xeb\x31\x5b\x31\xc9\x31\xc0\xb0\x05\xcd\x80\x89\xc3\x83\xec\x01"
    "\x89\xe1\x31\xd2\x42\x31\xc0\xb0\x03\xcd\x80\x85\xc0\x74\x0d\x53"
    "\x31\xdb\x43\x31\xc0\xb0\x04\xcd\x80\x5b\xeb\xe9\x31\xdb\x31\xc0"
    "\x40\xcd\x80\xe8\xca\xff\xff\xff\x6b\x65\x79"
};

Learn to use a debugger.