Tags: reversing z3 smt 

Rating: 5.0

# Intro

You Shall Not Pass

This rather strange binary needs a particular string as its input. Yet we have no idea what this string is. Do you?

This is one of the `reverse` challenges in VolgaCTF 2018.
The challenge is 200 points. What is interesting about it is:
* has 40 threads that check the input
* it is written in C++
* every thread does a mathematical calculation that is checked against a constant

You can't find more suitable challenge for using a SMT solver on.
My choice is `z3` with its Python binding. But before the solving lets do the
application analysis.

# Analysis

The app is a 64 bit ELF binary:

$ file ysnp
ysnp: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=efd7438ad3cc4c32c0ce7838e6f72d005b0ad18e, stripped

Using `radare2` there are two interesting functions to analyse:
* `main`
* `sub.__cxa_guard_acquire_4e0`

For the sake of convenience I usually dump all the interesting stuff in separate `.asm` files. It is easier to inspect the
assembly in `Sublime Text 3` for me. It let me search/highlight more freely:

r2> aaa
r2> pdf @ main > asm/main.asm
r2> pdf @ sub.__cxa_guard_acquire_4e0 > sub.__cxa_guard_acquire_4e0.asm

I spent some time there to grasp the whole scheme. I immediately noticed the launch of the 40 threads in `sub.__cxa_guard_acquire_4e0`. This code is repeated 40 times and it is easy to spot the start address of
the thread functions in `esi`:

```nasm
| ::| 0x00406509 be60204000 mov esi, 0x402060 ; "AVAUATUSH\x83\xec H\x8b\x1dmq "
| ::| 0x0040650e bf90926000 mov edi, 0x609290
| ::| 0x00406513 48c705c22c20. mov qword [0x006091e0], 0x609200 ; [0x6091e0:8]=0
| ::| 0x0040651e 48c744241010. mov qword [rsp + local_10h], 0x609110 ; [0x609110:8]=1
| ::| 0x00406527 48c704247092. mov qword [rsp], 0x609270 ; [0x609270:8]=0
| ::| 0x0040652f e83c0d0000 call sub.pthread_create_270
```

What I did is to search all the addresses and dump the assembly in separate files:

r2> pd 108 @ 0x402060 > asm.th/01-2060.asm
r2> pd 108 @ 0x403250 > asm.th/02-3250.asm
r2> pd 108 @ 0x403c90 > asm.th/03-3c90.asm
r2> pd 108 @ 0x405740 > asm.th/04-5740.asm
r2> pd 108 @ 0x403470 > asm.th/05-3470.asm
r2> pd 108 @ 0x402640 > asm.th/06-2640.asm
r2> pd 108 @ 0x401490 > asm.th/07-1490.asm
r2> pd 108 @ 0x403680 > asm.th/08-3680.asm
r2> pd 108 @ 0x4042a0 > asm.th/09-42a0.asm
r2> pd 108 @ 0x402840 > asm.th/10-2840.asm
r2> pd 108 @ 0x405b80 > asm.th/11-5b80.asm
r2> pd 108 @ 0x402e40 > asm.th/12-2e40.asm
r2> pd 108 @ 0x405fc0 > asm.th/13-5fc0.asm
r2> pd 108 @ 0x401a60 > asm.th/14-1a60.asm
r2> pd 108 @ 0x405da0 > asm.th/15-5da0.asm
r2> pd 108 @ 0x4061d0 > asm.th/16-61d0.asm
r2> pd 108 @ 0x4040b0 > asm.th/17-40b0.asm
r2> pd 108 @ 0x403050 > asm.th/18-3050.asm
r2> pd 108 @ 0x402a40 > asm.th/19-2a40.asm
r2> pd 108 @ 0x4048d0 > asm.th/20-48d0.asm
r2> pd 108 @ 0x405540 > asm.th/21-5540.asm
r2> pd 108 @ 0x401870 > asm.th/22-1870.asm
r2> pd 108 @ 0x404ef0 > asm.th/23-4ef0.asm
r2> pd 108 @ 0x405310 > asm.th/24-5310.asm
r2> pd 108 @ 0x401260 > asm.th/25-1260.asm
r2> pd 108 @ 0x403890 > asm.th/26-3890.asm
r2> pd 108 @ 0x402c50 > asm.th/27-2c50.asm
r2> pd 108 @ 0x403ea0 > asm.th/28-3ea0.asm
r2> pd 108 @ 0x405100 > asm.th/29-5100.asm
r2> pd 108 @ 0x4046b0 > asm.th/30-46b0.asm
r2> pd 108 @ 0x403a80 > asm.th/31-3a80.asm
r2> pd 108 @ 0x401e60 > asm.th/32-1e60.asm
r2> pd 108 @ 0x401c70 > asm.th/33-1c70.asm
r2> pd 108 @ 0x404cd0 > asm.th/34-4cd0.asm
r2> pd 108 @ 0x404ad0 > asm.th/35-4ad0.asm
r2> pd 108 @ 0x4044a0 > asm.th/36-44a0.asm
r2> pd 108 @ 0x402260 > asm.th/37-2260.asm
r2> pd 108 @ 0x402440 > asm.th/38-2440.asm
r2> pd 108 @ 0x405960 > asm.th/39-5960.asm
r2> pd 108 @ 0x401670 > asm.th/40-1670.asm

Radare could not recognize the functions at these addresses so I had to guess their lengths.
Looking at the code a similar structure emerges in all threads:

```nasm
; Boiler plate code ..
; ...
; Beginning of calculations. Note the use of 'r14'. As I discover later - it holds the input in an integer array
...-----> 0x004020f5 498b06 mov rax, qword [r14]
:::|||| 0x004020f8 488b88f00000. mov rcx, qword [rax + 0xf0] ; [0xf0:8]=-1 ; 240
:::|||| 0x004020ff 488b7008 mov rsi, qword [rax + 8] ; [0x8:8]=-1 ; 8
:::|||| 0x00402103 488b90980000. mov rdx, qword [rax + 0x98] ; [0x98:8]=-1 ; 152
; ...
; ...
; Some calculations
; ...
; ...
; The comparison
:::|||| 0x004021c1 483d77021400 cmp rax, 0x140277
; ...
; Exit code
```

I did some dynamic analysis in GDB also:

break *(0x402060+0x98)
break *(0x403250+0x98)
...
break *(0x401670+0x98)
run qwertyuiopasdfghjklzxcvbnm0123456789_=+-

I saw that `r14` holds the input (hexdump of `qword ptr [r14/rax]`):

pwndbg> hexdump $rax
+0000 0x61fcf0 71 00 00 00 00 00 00 00 77 00 00 00 00 00 00 00 │q...│....│w...│....│
+0010 0x61fd00 65 00 00 00 00 00 00 00 72 00 00 00 00 00 00 00 │e...│....│r...│....│
+0020 0x61fd10 74 00 00 00 00 00 00 00 79 00 00 00 00 00 00 00 │t...│....│y...│....│
+0030 0x61fd20 75 00 00 00 00 00 00 00 69 00 00 00 00 00 00 00 │u...│....│i...│....│
pwndbg>
+0040 0x61fd30 6f 00 00 00 00 00 00 00 70 00 00 00 00 00 00 00 │o...│....│p...│....│
+0050 0x61fd40 61 00 00 00 00 00 00 00 73 00 00 00 00 00 00 00 │a...│....│s...│....│
+0060 0x61fd50 64 00 00 00 00 00 00 00 66 00 00 00 00 00 00 00 │d...│....│f...│....│
+0070 0x61fd60 67 00 00 00 00 00 00 00 68 00 00 00 00 00 00 00 │g...│....│h...│....│
pwndbg>
+0080 0x61fd70 6a 00 00 00 00 00 00 00 6b 00 00 00 00 00 00 00 │j...│....│k...│....│
+0090 0x61fd80 6c 00 00 00 00 00 00 00 7a 00 00 00 00 00 00 00 │l...│....│z...│....│
+00a0 0x61fd90 78 00 00 00 00 00 00 00 63 00 00 00 00 00 00 00 │x...│....│c...│....│
+00b0 0x61fda0 76 00 00 00 00 00 00 00 62 00 00 00 00 00 00 00 │v...│....│b...│....│
pwndbg>
+00c0 0x61fdb0 6e 00 00 00 00 00 00 00 6d 00 00 00 00 00 00 00 │n...│....│m...│....│
+00d0 0x61fdc0 30 00 00 00 00 00 00 00 31 00 00 00 00 00 00 00 │0...│....│1...│....│
+00e0 0x61fdd0 32 00 00 00 00 00 00 00 33 00 00 00 00 00 00 00 │2...│....│3...│....│
+00f0 0x61fde0 34 00 00 00 00 00 00 00 35 00 00 00 00 00 00 00 │4...│....│5...│....│
pwndbg>
+0100 0x61fdf0 36 00 00 00 00 00 00 00 37 00 00 00 00 00 00 00 │6...│....│7...│....│
+0110 0x61fe00 38 00 00 00 00 00 00 00 39 00 00 00 00 00 00 00 │8...│....│9...│....│
+0120 0x61fe10 5f 00 00 00 00 00 00 00 3d 00 00 00 00 00 00 00 │_...│....│=...│....│
+0130 0x61fe20 2b 00 00 00 00 00 00 00 2d 00 00 00 00 00 00 00 │+...│....│-...│....│

Then I cleaned up the code and gathered it all in [all-asm.json](https://github.com/mr6r4y/re-write-ups/blob/master/crackmes/volgactf-2018/you-shall-not-pass/all-asm.json)

By searching all offsets in the snippets of all threads:

```nasm
mov rcx, qword [rax + 0xf0]
mov rsi, qword [rax + 8]
mov rdx, qword [rax + 0x98]
```

I came up with this:

```python
>>> indexes = [0xf0, 8, 0x98, ..., 0x10, 0xa8, 0x58, 0xc8]
>>> len(set(indexes))
3: 44
>>> max(set(indexes))
4: 352
>>> 352/8
5: 44
```

So the length of the input is 45 characters long. Expecting the input to have the format of a flag (`VolgaCTF{...}`)
I came up with the following start input in GDB:

pwndbg> run "VolgaCTF{qwertyuiopasdfghjklzxcvbnmQWERTYUIO}"

The analysis part finishes with:
* having length and format of the input/flag
* isolated `asm` code in [all-asm.json](https://github.com/mr6r4y/re-write-ups/blob/master/crackmes/volgactf-2018/you-shall-not-pass/all-asm.json)

What has left is to parse the code and cram into z3.

# Z3 Script

The script has the following main goals:
* parse the limited operations in `all-asm.json`
* extract a constraint for each thread
* cram all into `solve(..)`

Some tricky stuff include:
* usage of `BitVecVal` and `BitVec` to simulate machine registers
* use python globals (simulate registers) as processor state
* craft valid python statements from assembly code that can be `exec`-ed (lazy to think of more sophisticated parser/interpreter)

## `analysis.py`
```python
#!/usr/bin/env python

import json
import re
import struct
import binascii
from z3 import *

TABLE = [
BitVecVal(0x56, 64),
BitVecVal(0x6f, 64),
BitVecVal(0x6c, 64),
BitVecVal(0x67, 64),
BitVecVal(0x61, 64),
BitVecVal(0x43, 64),
BitVecVal(0x54, 64),
BitVecVal(0x46, 64),
BitVecVal(0x7b, 64),
]

TABLE = TABLE + [BitVec("x_%i" % i, 64) for i in range(9, 44)] + [BitVecVal(0x7d, 64)]

# TABLE = [BitVec("x_%i" % i, 64) for i in range(45)]

rax = None
rbx = None
rcx = None
rdx = None
rsi = None
rdi = None
r8 = None
r9 = None
r10 = None
r11 = None
r12 = None
r13 = None
r14 = TABLE

def reset():
global rax, rbx, rcx, rdx, rsi, rdi, r8, r9, r10, r11, r12, r13, r14
rax = None
rbx = None
rcx = None
rdx = None
rsi = None
rdi = None
r8 = None
r9 = None
r10 = None
r11 = None
r12 = None
r13 = None
r14 = TABLE

def asm_mov(a1, a2):
return "%s = %s" % (a1, a2)

def asm_lea(a1, a2):
return "%s = %s" % (a1, a2)

def asm_shl(a1, a2):
return "%s = %s << %s" % (a1, a1, a2)

def asm_add(a1, a2):
if a2.startswith("0x"):
a2 = "BitVecVal(%s, 64)" % a2
return "%s = %s + %s" % (a1, a1, a2)

def asm_sub(a1, a2):
if a2.startswith("0x"):
a2 = "BitVecVal(%s, 64)" % a2
return "%s = %s - %s" % (a1, a1, a2)

def asm_imul(a1, a2, a3=None):
if a3 is None:
if a2.startswith("0x"):
a2 = "BitVecVal(%s, 64)" % a2
return "%s = %s * %s" % (a1, a1, a2)
else:
if a3.startswith("0xffff"):
a3 = str(struct.unpack(">q", binascii.unhexlify(a3[2:]))[0])
a3 = "BitVecVal(%s, 64)" % a3
return "%s = %s * %s" % (a1, a2, a3)

def asm_neg(a1):
return "%s = -%s" % (a1, a1)

def asm_cmp(a1, a2):
if a2.startswith("0xffff"):
a2 = str(struct.unpack(">q", binascii.unhexlify(a2[2:]))[0])
return "%s == BitVecVal(%s, 64)" % (a1, a2)

def parse_assembly(s):
def _get_instr(s):
ins = re.search("^[a-z]+", s).group()
return ins

def _get_args(s):
raw_args = re.search("^[a-z]+ (.*)", s).group(1)
raw_args = [i.strip() for i in raw_args.split(",")]

args_expr = []
for a in raw_args:
if a in ["rax", "rbx", "rcx", "rdx", "rsi", "rdi",
"r8", "r9", "r10", "r11", "r12", "r13", "r14"]:
args_expr.append(a)
else:
r = re.search("^qword \\[(.+)\\]$", a)
if r:
r = r.group(1)
r = [i.strip() for i in r.split("+")]
if len(r) == 1:
if r[0] == "r14":
args_expr.append("%s" % r[0])
else:
args_expr.append("%s[0]" % r[0])
else:
r1, r2 = r
try:
e = int(r2) / 8
except ValueError:
e = int(r2, 16) / 8
args_expr.append("%s[%i]" % (r1, e))
else:
args_expr.append(a)

return args_expr

ins = _get_instr(s)
args = _get_args(s)

instructions = {
"mov": asm_mov,
"lea": asm_lea,
"shl": asm_shl,
"add": asm_add,
"sub": asm_sub,
"imul": asm_imul,
"neg": asm_neg,
"cmp": asm_cmp
}

c = instructions[ins](*args)
return ins, c

def eval_block(b):
global rax, rbx, rcx, rdx, rsi, rdi, r8, r9, r10, r11, r12, r13, r14
for i in b:
ins, c = parse_assembly(i)
if ins == "cmp":
print c
return eval(c)
else:
print c
exec c

def main():
allasm = json.load(open("all-asm.json", "r"))

constraints = []
for n, b in enumerate(allasm):
print "Block No: %i" % n
print "="*40
c = eval_block(b)
constraints.append(c)
reset()
print "-"*40

print solve(*constraints)

if __name__ == '__main__':
main()

```

At the end I get:

$ python analysis.py
...
...
----------------------------------------
[x_19 = 48,
x_38 = 101,
x_11 = 36,
x_20 = 117,
x_26 = 101,
x_17 = 95,
x_15 = 115,
x_39 = 95,
x_9 = 68,
x_34 = 117,
x_32 = 121,
x_29 = 110,
x_28 = 97,
x_43 = 101,
x_33 = 48,
x_27 = 95,
x_40 = 115,
x_13 = 117,
x_42 = 102,
x_35 = 95,
x_25 = 100,
x_18 = 121,
x_41 = 64,
x_31 = 95,
x_14 = 105,
x_37 = 114,
x_10 = 49,
x_12 = 103,
x_36 = 64,
x_30 = 100,
x_22 = 95,
x_24 = 111,
x_23 = 99,
x_21 = 114,
x_16 = 51]
None

and I then converted this to:

```python
x = [i for i in "VolgaCTF{qwertyuiopasdfghjklzxcvbnmQWERTYUIO}"]
x[19] = chr(48)
x[38] = chr(101)
x[11] = chr(36)
x[20] = chr(117)
x[26] = chr(101)
x[17] = chr(95)
x[15] = chr(115)
x[39] = chr(95)
x[9 ]= chr(68)
x[34] = chr(117)
x[32] = chr(121)
x[29] = chr(110)
x[28] = chr(97)
x[43] = chr(101)
x[33] = chr(48)
x[27] = chr(95)
x[40] = chr(115)
x[13] = chr(117)
x[42] = chr(102)
x[35] = chr(95)
x[25] = chr(100)
x[18] = chr(121)
x[41] = chr(64)
x[31] = chr(95)
x[14] = chr(105)
x[37] = chr(114)
x[10] = chr(49)
x[12] = chr(103)
x[36] = chr(64)
x[30] = chr(100)
x[22] = chr(95)
x[24] = chr(111)
x[23] = chr(99)
x[21] = chr(114)
x[16] = chr(51)
print "".join(x)
```

VolgaCTF{[email protected][email protected]}

Original writeup (https://github.com/mr6r4y/write-ups/tree/master/challenges/volgactf-2018%40you-shall-not-pass).