AppArmor sVirt security driver for libvirt November 3, 2009

Background
Ubuntu has been using libvirt as part of its recommended virtualization management toolkit since Ubuntu 8.04 LTS. In short, libvirt provides a set of tools and APIs for managing virtual machines (VMs) such as creating a VM, modifying its hardware configuration, starting and stopping VMs, and a whole lot more. For more information on using libvirt in Ubuntu, see http://doc.ubuntu.com/ubuntu/serverguide/C/libvirt.html.

Libvirt greatly eases the deployment and management of VMs, but due to the fact that it has traditionally been limited to using POSIX ACLs and sometimes needs to perform privileged actions, using libvirt (or any virtualization technology for that matter) can create a security risk, especially when the guest VM isn’t trusted. It is easy to imagine a bug in the hypervisor which would allow a compromised VM to modify other guests or files on the host. Considering that when using qemu:///system the guest VM process runs as a root (this is configurable in 0.7.0 and later, but still the default in Fedora and Ubuntu 9.10), it is even more important to contain what a guest can do. To address these issues, SELinux developers created a fork of libvirt, called sVirt, which when using kvm/qemu, allows libvirt to add SELinux labels to files required for the VM to run. This work was merged back into upstream libvirt in version 0.6.1, and it’s implementation, features and limitations can be seen in a blog post by Dan Walsh and an article on LWN.net. This is inspired work, and the sVirt folks did a great job implementing it by using a plugin framework so that others could create different security drivers for libvirt.

AppArmor Security Driver for Libvirt
While Ubuntu has SELinux support, by default it uses AppArmor. AppArmor is different from SELinux and some other MAC systems available on Linux in that it is path-based, allows for mixing of enforcement and complain mode profiles, uses include files to ease development, and typically has a far lower barrier to entry than other popular MAC systems. It has been an important security feature of Ubuntu since 7.10, where CUPS was confined by AppArmor in the default installation (more profiles have been added with each new release).

Since virtualization is becoming more and more prevalent, improving the security stance for libvirt users is of primary concern. It was very natural to look at adding an AppArmor security driver to libvirt, and as of libvirt 0.7.2 and Ubuntu 9.10, users have just that. In terms of supported features, the AppArmor driver should be on par with the SELinux driver, where the vast majority of libvirt functionality is supported by both drivers out of the box.

Implementation
First, the libvirtd process is confined with a lenient profile that allows the libvirt daemon to launch VMs, change into another AppArmor profile and use virt-aa-helper to manipulate AppArmor profiles. virt-aa-helper is a helper application that can add, remove, modify, load and unload AppArmor profiles in a limited and restricted way. Specifically, libvirtd is not allowed to adjust anything in /sys/kernel/security directly, or modify the profiles for the virtual machines directly. Instead, libvirtd must use virt-aa-helper, which is itself run under a very restrictive AppArmor profile. Using this architecture helps prevent any opportunities for a subverted libvirtd to change its own profile (especially useful if the libvirtd profile is adjusted to be restrictive) or modify other AppArmor profiles on the system.

Next, there are several profiles that comprise the system:

• /etc/apparmor.d/usr.sbin.libvirtd
• /etc/apparmor.d/usr.bin.virt-aa-helper
• /etc/apparmor.d/abstractions/libvirt-qemu
• /etc/apparmor.d/libvirt/TEMPLATE
• /etc/apparmor.d/libvirt/libvirt-<uuid>
• /etc/apparmor.d/libvirt/libvirt-<uuid>.files

/etc/apparmor.d/usr.sbin.libvirtd and /etc/apparmor.d/usr.bin.virt-aa-helper define the profiles for libvirtd and virt-aa-helper (note that in libvirt 0.7.2, virt-aa-helper is located in /usr/lib/libvirt/virt-aa-helper). /etc/apparmor.d/libvirt/TEMPLATE is consulted when creating a new profile when one does not already exist. /etc/apparmor.d/abstractions/libvirt-qemu is the abstraction shared by all running VMs. /etc/apparmor.d/libvirt/libvirt-<uuid> is the unique base profile for an individual VM, and /etc/apparmor.d/libvirt/libvirt-<uuid>.files contains rules for the guest-specific files required to run this individual VM.

The confinement process is as follows (assume the VM has a libvirt UUID of ‘a22e3930-d87a-584e-22b2-1d8950212bac’):

1. When libvirtd is started, it determines if it should use a security driver. If so, it checks which driver to use (eg SELinux or AppArmor). If libvirtd is confined by AppArmor, it will use the AppArmor security driver
2. When a VM is started, libvirtd decides whether to ask virt-aa-helper to create a new profile or modify an existing one. If no profile exists, libvirtd asks virt-aa-helper to generate the new base profile, in this case /etc/apparmor.d/libvirt/libvirt-a22e3930-d87a-584e-22b2-1d8950212bac, which it does based on /etc/apparmor.d/libvirt/TEMPLATE. Notice, the new profile has a profile name that is based on the guest’s UUID. Once the base profile is created, virt-aa-helper works the same for create and modify: virt-aa-helper will determine what files are required for the guest to run (eg kernel, initrd, disk, serial, etc), updates /etc/apparmor.d/libvirt/libvirt-a22e3930-d87a-584e-22b2-1d8950212bac.files, then loads the profile into the kernel.
3. libvirtd will proceed as normal at this point, until just before it forks a qemu/kvm process, it will call aa_change_profile() to transition into the profile ‘libvirt-a22e3930-d87a-584e-22b2-1d8950212bac’ (the one virt-aa-helper loaded into the kernel in the previous step)
4. When the VM is shutdown, libvirtd asks virt-aa-helper to remove the profile, and virt-aa-helper unloads the profile from the kernel

It should be noted that due to current limitations of AppArmor, only qemu:///system is confined by AppArmor. In practice, this is fine because qemu:///session is run as a normal user and does not have privileged access to the system like qemu:///system does.

Basic Usage
By default in Ubuntu 9.10, both AppArmor and the AppArmor security driver for libvirt are enabled, so users benefit from the AppArmor protection right away. To see if libvirtd is using the AppArmor security driver, do:

$ virsh capabilities
Connecting to uri: qemu:///system
<capabilities>
 <host>
  ...
  <secmodel>
    <model>apparmor</model>
    <doi>0</doi>
  </secmodel>
 </host>
 ...
</capabilities>


Next, start a VM and see if it is confined:

$ virsh start testqemu
Connecting to uri: qemu:///system
Domain testqemu started

$ virsh domuuid testqemu
Connecting to uri: qemu:///system
a22e3930-d87a-584e-22b2-1d8950212bac

$ sudo aa-status
apparmor module is loaded.
16 profiles are loaded.
16 profiles are in enforce mode.
...
  /usr/bin/virt-aa-helper
  /usr/sbin/libvirtd
  libvirt-a22e3930-d87a-584e-22b2-1d8950212bac
...
0 profiles are in complain mode.
6 processes have profiles defined.
6 processes are in enforce mode :
...
  libvirt-a22e3930-d87a-584e-22b2-1d8950212bac (6089)
...
0 processes are in complain mode.
0 processes are unconfined but have a profile defined.

$ ps ww 6089
PID TTY STAT TIME COMMAND
6089 ? R 0:00 /usr/bin/qemu-system-x86_64 -S -M pc-0.11 -no-kvm -m 64 -smp 1 -name testqemu -uuid a22e3930-d87a-584e-22b2-1d8950212bac -monitor unix:/var/run/libvirt/qemu/testqemu.monitor,server,nowait -boot c -drive file=/var/lib/libvirt/images/testqemu.img,if=ide,index=0,boot=on -drive file=,if=ide,media=cdrom,index=2 -net nic,macaddr=52:54:00:86:5b:6e,vlan=0,model=virtio,name=virtio.0 -net tap,fd=17,vlan=0,name=tap.0 -serial none -parallel none -usb -vnc 127.0.0.1:1 -k en-us -vga cirrus


Here is the unique, restrictive profile for this VM:

$ cat /etc/apparmor.d/libvirt/libvirt-a22e3930-d87a-584e-22b2-1d8950212bac
#
# This profile is for the domain whose UUID
# matches this file.
#
 
#include <tunables/global>
 
profile libvirt-a22e3930-d87a-584e-22b2-1d8950212bac {
   #include <abstractions/libvirt-qemu>
   #include <libvirt/libvirt-a22e3930-d87a-584e-22b2-1d8950212bac.files>
}

$ cat /etc/apparmor.d/libvirt/libvirt-a22e3930-d87a-584e-22b2-1d8950212bac.files
# DO NOT EDIT THIS FILE DIRECTLY. IT IS MANAGED BY LIBVIRT.
  "/var/log/libvirt/**/testqemu.log" w,
  "/var/run/libvirt/**/testqemu.monitor" rw,
  "/var/run/libvirt/**/testqemu.pid" rwk,
  "/var/lib/libvirt/images/testqemu.img" rw,


Now shut it down:

$ virsh shutdown testqemu
Connecting to uri: qemu:///system
Domain testqemu is being shutdown

$ virsh domstate testqemu
Connecting to uri: qemu:///system
shut off

$ sudo aa-status | grep 'a22e3930-d87a-584e-22b2-1d8950212bac'
[1]


Advanced Usage
In general, you can forget about AppArmor confinement and just use libvirt like normal. The guests will be isolated from each other and user-space protection for the host is provided. However, the design allows for a lot of flexibility in the system. For example:

• If you want to adjust the profile for all future, newly created VMs, adjust /etc/apparmor.d/libvirt/TEMPLATE
• If you need to adjust access controls for all VMs, new or existing, adjust /etc/apparmor.d/abstractions/libvirt-qemu
• If you need to adjust access controls for a single guest, adjust /etc/apparmor.d/libvirt-<uuid>, where <uuid> is the UUID of the guest
• To disable the driver, either adjust /etc/libvirt/qemu.conf to have ‘security_driver = “none”‘ or remove the AppArmor profile for libvirtd from the kernel and restart libvirtd

Of course, you can also adjust the profiles for libvirtd and virt-aa-helper if desired. All the files are simple text files. See AppArmor for more information on using AppArmor in general.

Limitations and the Future
While the sVirt framework provides good guest isolation and user-space host protection, the framework does not provide protection against in-kernel attacks (eg, where a guest process is able to access the host kernel memory). The AppArrmor security driver as included in Ubuntu 9.10 also does not handle access to host devices as well as it could. Allowing a guest to access a local pci device or USB disk is a potentially dangerous operation anyway, and the driver will block this access by default. Users can work around this by adjusting the base profile for the guest.

There are few missing features in the sVirt model, such as labeling state files. The AppArmor driver also needs to better support host devices. Once AppArmor provides the ability for regular users to define profiles, then qemu:///session can be properly supported. Finally, it will be great when distributions take advantage of libvirt’s recently added ability to run guests as non-root when using qemu:///system (while the sVirt framework largely mitigates this risk, it is good to have security in depth).

Summary
While cloud computing feels like it is talked about everywhere and virtualization becoming even more important in the data center, leveraging technologies like libvirt and AppArmor is a must. Virtualization removes the traditional barriers afforded to stand-alone computers, thus increasing the attack surface for hostile users and compromised guests. By using the sVirt framework in libvirt, and in particular AppArmor on Ubuntu 9.10, administrators can better defend themselves against virtualization-specific attacks. Have fun and be safe!

More Information
http://libvirt.org/
https://wiki.ubuntu.com/AppArmor
https://wiki.ubuntu.com/SecurityTeam/Specifications/AppArmorLibvirtProfile

Serving up Sftp and AppArmor September 9, 2009

Recently I decided to replace NFS on a small network with something that was more secure and more resistant to network failures (as in, Nautilus wouldn’t hang because of a symlink to a directory in an autofs mounted NFS share). Most importantly, I wanted something that was secure, simple and robust. I naturally thought of SFTP, but there were at least two problems with a naive SFTP implementation, both of which I decided I must solve to meet the ‘secure’ criteria:

• shell access to the file server
• SFTP can’t restrict users to a particular directory

Of course, there are other alternatives that I could have pursued: sshfs, shfs, SFTP with scponly, patching OpenSSH to support chrooting (see ‘Update’ below), NFSv4, IPsec, etc. Adding all the infrastructure to properly support NFSv4 and IPsec did not meet the simplicity requirement, and neither did running a patched OpenSSH server. sshfs, shfs, and SFTP with scponly did not really fit the bill either.

What did I come up with? A combination of SFTP, a hardened sshd configuration and AppArmor on Ubuntu.

SFTP setup
This was easy because sftp is enabled by default on Ubuntu and Debian systems. This should be enough:

$ sudo apt-get install openssh-server
Just make sure you have the following set in /etc/ssh/sshd_config (see man sshd_config for details).

Subsystem sftp /usr/lib/openssh/sftp-server

With that in place, all I needed to do was add users with strong passwords:

$ sudo adduser sftupser1
$ sudo adduser sftpuser2

Then you can test if it worked with:

$ sftp sftpuser1@server
sftp> ls /
/bin ...


Hardened sshd
Now that SFTP is working, we need to limit access. One way to do this is via a Match rule that uses a ForceCommand. Combined with AllowUsers, adding something like this to /etc/ssh/sshd_config is pretty powerful:

AllowUsers adminuser sftpuser1@192.168.0.10 sftpuser2
Match User sftpuser1,sftpuser2
    AllowTcpForwarding no
    X11Forwarding no
    ForceCommand /usr/lib/openssh/sftp-server -l INFO

Remember to restart ssh with ‘/etc/init.d/ssh restart’. The above allows normal shell access for the adminuser, SFTP-only access to sftpuser1 from 192.168.0.10 and to sftupser2 from anywhere. One can imagine combining this with ‘PasswordAuthentication no’ or GSSAPI to enforce more stringent authentication so access is even more tightly controlled.

AppArmor
The above does a lot to increase security over the standard NFS shares that I had before. Good encryption, strong authentication and reliable UID mappings for DAC (POSIX discretionary access controls) are all in place. However, it doesn’t have the ability to confine access to a certain directory like NFS does. A simple AppArmor profile can achieve this and give even more granularity than just DAC. Imagine the following directory structure:

• /var/exports (top-level ‘exported’ filesystem (where all the files you want to share are))
• /var/exports/users (user-specific files, only to be accessed by the users themselves)
• /var/exports/shared (a free-for-all shared directory where any user can put stuff. The ‘shared’ directory has ‘2775’ permissions with group ‘shared’)

Now add to /etc/apparmor.d/usr.lib.openssh.sftp-server (and enable with ‘sudo apparmor_parser -r /etc/apparmor.d/usr.lib.openssh.sftp-server’):

#include <tunables/global>
/usr/lib/openssh/sftp-server {
  #include <abstractions/base>
  #include <abstractions/nameservice>
 
  # Served files
  # Need read access for every parent directory
  / r,
  /var/ r,
  /var/exports/ r,
 
  /var/exports/**/ r,
  owner /var/exports/** rwkl,
 
  # don't require ownership to read shared files
  /var/exports/shared/** rwkl,
}

This is a very simple profile for sftp-server itself, with access to files in /var/exports. Notice that by default the owner must match for any files in /var/exports, but in /var/exports/shared it does not. AppArmor works in conjunction with DAC, so that if DAC denies access, AppArmor is not consulted. If DAC permits access, then AppArmor is consulted and may deny access.

Summary
This is but one man’s implementation for a simple, secure and robust file service. There are limitations with the method as described, notably managing the sshd_config file and not supporting some traditional setups such as $HOME on NFS. That said, with a little creativity, a lot of possibilities exist for file serving with this technique. For my needs, the combination of standard OpenSSH and AppArmor on Ubuntu was very compelling. Enjoy!

Update
OpenSSH 4.8 and higher (available in Ubuntu 8.10 and later) contains the ChrootDirectory option, which may be enough for certain environments. It is simpler to setup (ie AppArmor is not required), but doesn’t have the same granularity and sftp-server protection that the AppArmor method provides. See comment 32 and comment 34 for details. Combining ChrootDirectory and AppArmor would provide even more defense in depth. It’s great to have so many options for secure file sharing! :)

AppArmor now available in Karmic — testing needed July 15, 2009

After a lot of hard work by John Johansen and the Ubuntu kernel team, bug #375422 is well on its way to be fixed. More than just forward ported for Ubuntu, AppArmor has been reworked to use the updated kernel infrastructure for LSMs. As seen in #apparmor on Freenode a couple of days ago:

11:24 < jjohansen> I am working to a point where I can try upstreaming again, base off of the security_path_XXX patches instead of the vfs patches
11:24 < jjohansen> so the module is mostly self contained again

These patches are in the latest 9.10 kernel, and help testing AppArmor in Karmic is needed. To get started, verify you have at least 2.6.31-3.19-generic:

$ cat /proc/version_signature
Ubuntu 2.6.31-3.19-generic

AppArmor will be enabled by default for Karmic just like in previous Ubuntu releases, but it is off for now until a few kinks are worked out. To test it right away, you’ll need to reboot, adding ‘security=apparmor’ to the kernel command line. Then fire up ‘aa-status’ to see if it is enabled. A fresh install of 9.10 as of today should look something like:

$ sudo aa-status
apparmor module is loaded.
8 profiles are loaded.
8 profiles are in enforce mode.
/usr/lib/connman/scripts/dhclient-script
/usr/share/gdm/guest-session/Xsession
/usr/sbin/tcpdump
/usr/lib/cups/backend/cups-pdf
/sbin/dhclient3
/usr/sbin/cupsd
/sbin/dhclient-script
/usr/lib/NetworkManager/nm-dhcp-client.action
0 profiles are in complain mode.
2 processes have profiles defined.
2 processes are in enforce mode :
/sbin/dhclient3 (3271)
/usr/sbin/cupsd (2645)
0 processes are in complain mode.
0 processes are unconfined but have a profile defined.

Please throw all your crazy profiles at it as well as testing the packages with existing profiles, then file bugs:

• For the kernel, add your comments (positive and negative) to bug #375422
• AppArmor tools bugs should be filed with ‘ubuntu-bug apparmor’
• Profile bugs should be filed against the individual source package with ‘ubuntu-bug <source package name>’. See DebuggingApparmor for details.

Thank you Ubuntu Kernel team and especially John for all the hard work on getting the “MAC system for human beings” (as I like to call it) not only working again, but upstreamable — this is really great stuff! :)