pwnable.kr-fd-Writeup

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pwnable.kr-fd-Writeup

• 根据题目描述Mommy! what is a file descriptor in Linux? 知该题与Linux系统下的文件描述有关；
• ssh远程登录如下：
  
• 根据题目提示，ls -l查看文件及权限如下,由下图，用户fd只具有读文件fd.c的权限（尝试sudo chmod增加权限，但失败）：
  
• cat fd.c读取fd.c中的内容，可得到如下代码：

 fd@ubuntu:~$ cat fd.c
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 char buf[];
 int main(int argc, char* argv[], char* envp[]){
         if(argc<){
                 printf("pass argv[1] a number\n");
                 return ;
         }
         int fd = atoi( argv[] ) - 0x1234;
         int len = ;
         len = read(fd, buf, );
         if(!strcmp("LETMEWIN\n", buf)){
                 printf("good job :)\n");
                 system("/bin/cat flag");
                 exit();
         }
         printf("learn about Linux file IO\n");
         return ;

 }

 fd@ubuntu:~$

• 分析上述代码，执行system("/bin/cat flag");语句，即可获得flag;
• 执行system("/bin/cat flag");需要使buf == "LETMEWIN";
• 继续分析，需要buf通过read函数读入"LETMEWIN\n"，有read函数的定义，需要使fd==0；
  
• 则有atoi( argv[1] ) == 0x1234(即十进制下的4660），由atoi函数的定义，需要argv[1] == "4660";
  
• 进而由LinuxC下argv[]的相应定义可以构造输入，LinuxC下argc，argv[]的意义请参考http://www.cnblogs.com/WangAoBo/p/6366600.html

• echo "LETMEWIN" | ./fd 4660

如下，flag为：mommy! I think I know what a file descriptor is!!

2017-2-4 22:24;39