如何使用缓冲区溢出攻击找到要利用的程序的漏洞

Question

作为实验室的任务，我想首先找到以下代码的漏洞，然后作为示例时间服务器运行，然后使用缓冲区溢出对其进行攻击。 但第一个问题是不知道从哪里开始。 我知道 C 程序中的某些功能可能有害（例如“strftime”、“memcoy”或“strcpy”），但我不能 select 开始使用它。

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <netdb.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <time.h>


#define CANBUFSIZE 106
#define MSGBUFSIZE 256
#define TIMEBUFSIZE 128

char msgbuf[MSGBUFSIZE];
char canarybuf[CANBUFSIZE];

void get_time(char* format, char* retstr, unsigned received)
{
  // memory for our local copy of the timestring
  char timebuf[TIMEBUFSIZE];
  time_t curtime;

  // if the format string esceeds our local buffer ...
  if(strlen(format) > TIMEBUFSIZE)
  {
    strcpy(retstr,"Process Error.");
    return;
  }

  // otherwise create a local working copy
  memcpy(timebuf,format,received);

  // Get the current time.
  curtime = time (NULL);

  // Convert it to local time representation.
  // and convert the format string to the real timestring
  struct tm *loctime = localtime (&curtime);
  strftime(retstr,TIMEBUFSIZE,timebuf,loctime);

  return;
}


int main(int argc, char** argv)
{
  int port;                     // the portnumber of our service
  struct in_addr bind_addr;     // bind address of the server
  int sd;                       // the socketdescriptor
  struct sockaddr_in addr;      // address of our service
  struct sockaddr_in addr_from; //address of the client
  int addrlen = sizeof(addr_from);
  int pid;                      // our process id
  int sid;                      // our session id
  unsigned received;            // number of bytes received from network


  // resolve command line arguments
  if(argc != 3)
  {
    printf("Usage: timeservice <bind address> <portnum>\n");
    return 1;
  }
  
  if (inet_aton(argv[1], &bind_addr) == 0)
  {
       fprintf(stderr, "Invalid bind address\n");
       exit(EXIT_FAILURE);
  }
  
  port = atoi(argv[2]); 
  if ((port < 1024) || (port > 65535))
  {
    printf("Portrange has to be between 1024 and 65535.\n");
    exit(EXIT_FAILURE);
  }


  // forking to background
  pid = fork();
  if(pid < 0)
  {
    printf("fork() failed\n");
    exit(EXIT_FAILURE);
  }
  // we are parent
  else if(pid > 0)
  {
    return 0;
  }

  /*
   * we are the child process
   * because of the termination of our parent, we need a new session id,
   * else we are zombie
   */
  sid = setsid();
  if (sid < 0) {
    return 1;
  }

  /*
   * since we are a system service we have to close all standard file 
   * descriptors
   */
  close(STDIN_FILENO);
  close(STDOUT_FILENO);
  close(STDERR_FILENO);

  // create an udp socket
  if((sd = socket(PF_INET,SOCK_DGRAM,IPPROTO_UDP)) < 0)
  {
    return 1;
  }

  // clear the memory of our addr struct
  memset(&addr,0,sizeof(addr));

  // Protocol Family = IPv4
  addr.sin_family = PF_INET; 
  
  // Listen on bindAddr and bindPort only
  addr.sin_addr.s_addr = bind_addr.s_addr;
  addr.sin_port = htons(port);

  // bind to the udp socket
  if(bind(sd,(struct sockaddr*)&addr,sizeof(addr)) != 0)
  {
    return 1;
  }

  for(;;)
  {
    // prepare memory
    memset(&msgbuf, 0, sizeof(msgbuf));

    received = recvfrom(sd,msgbuf,MSGBUFSIZE,MSG_WAITALL,
      (struct sockaddr*)&addr_from,(socklen_t*) &addrlen);

    // fork a new child
    pid = fork();

    // we are parent
    if (pid > 0)
    {
      // wait for the child to finish
      waitpid(pid,NULL,0);
    }
    else
    {
      /*
       * we are inside the child process
       */

      // reserve some memory for our response
      char * returnstr = (char*) malloc(TIMEBUFSIZE);

      // analyse the client request and format the time string
      get_time(msgbuf, returnstr, received);

      // send our response to the client
      sendto(sd,returnstr,strlen(returnstr)+1,MSG_DONTWAIT,
        (struct sockaddr *) &addr_from, addrlen);

      free(returnstr);
      return EXIT_SUCCESS;
    }
  }

  close(sd);

  return 0;
}

我使用 gcc 编译文件并使用 ./timeserver 127.0.0.1 2222 运行本地服务器，然后使用 nc -u 127.0.0.1 2222 连接到该文件。然后我现在可以向服务器输入一些格式字符串并获取一些结果回来了。 例如，如果我在命令行中使用 %d，它将以特殊格式显示日期/时间。 我知道这是我应该强制一些字符串并使程序崩溃的地方，但我找不到确切的字符串。

Answer 1

没有检查recvfrom的返回值，这是一个错误。 如果 recvfrom 出错，则返回值为 -1。 由于received的类型是无符号的，这是第二个错误，这看起来是一个很大的正值。 memcpy 调用将导致程序失败。