这段 C 代码中的缓冲区溢出漏洞在哪里？

Question

所以我正在学习 C 中的缓冲区溢出攻击。我了解它们是什么，并且可以在简单的 C 代码中找到缓冲区溢出漏洞。简单就好:).

但是这段代码似乎超出了我对'simple'的定义。

到目前为止，我了解到在这段 C 代码中，缓冲区溢出漏洞主要发生在以下行中： strcpy(retstr, "Process Error."); 但是在该行上方有一个 if 语句，我认为它可以防止该行的缓冲区溢出.

如果能帮助我找出这段代码中的缓冲区溢出漏洞，我将不胜感激。

#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;
}

Answer 1

get_time 中存在差异：strlen 用于检查传入缓冲区的“大小”，但 memcpy 与用户提供的 [=15] 一起使用=] 参数。在前 TIMEBUFSIZE 个字节中传递一个带有 NUL 字节的缓冲区就足够了。

如果您这样做，您可以直接在代码中触发崩溃：

received = 256;
memset(msgbuf, 'A', MSGBUFSIZE);
msgbuf[0] = 0;

这将用 256 个字节“填满”msgbuf，然后继续写入 128 个字节，将堆栈上的 return 地址覆盖为您选择的地址。因为第一个字节是 NUL，所以 strlen 检查通过。

如果你想在实际的二进制文件上触发它，你可能需要这样的东西：（假设它在 localhost:1234 上运行）

perl -MIO::Socket::IP -E '
  $buf = "[=11=]" . ("A"x255);
  my $s = IO::Socket::IP->new(PeerHost => "127.0.0.1", PeerPort => 1234, Type => SOCK_DGRAM);
  $s->autoflush(1);
  print $s $buf;
'

当然你需要修改缓冲区来执行实际的代码流

这段 C 代码中的缓冲区溢出漏洞在哪里？

Where is the Buffer Overflow vulnerability in this C code?

c

sockets

buffer-overflow