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<h1>Secure Usage of Libvirt</h1>
<ul id="toc"></ul>
<p>
This page details information that application developers and
administrators of libvirt should be aware of when working with
libvirt, that may have a bearing on security of the system.
</p>
<h2><a id="diskimage">Disk image handling</a></h2>
<h3><a id="diskimageformat">Disk image format probing</a></h3>
<p>
Historically there have been multiple flaws in QEMU and most
projects using QEMU, related to handling of disk formats.
The problems occur when a guest is given a virtual disk backed
by raw disk format on the host. If the management application
on the host tries to auto-detect / probe the disk format, it
is vulnerable to a malicious guest which can write a qcow2
file header into its raw disk. If the management application
subsequently probes the disk, it will see it as a 'qcow2' disk
instead of a 'raw' disk. Since 'qcow2' disks can have a copy
on write backing file, such flaw can be leveraged to read
arbitrary files on the host. The same type of flaw may occur
if the management application allows users to upload pre-created
raw images.
</p>
<p>
<strong>Recommendation:</strong> never attempt to automatically
detect the format of a disk image based on file contents which
are accessible to / originate from an untrusted source.
</p>
<h3><a id="diskimagebacking">Disk image backing files</a></h3>
<p>
If a management application allows users to upload pre-created
disk images in non-raw formats, it can be tricked into giving
the user access to arbitrary host files via the copy-on-write
backing file feature. This is because the qcow2 disk format
header contains a filename field which can point to any location.
It can also point to network protocols such as NBD, HTTP, GlusterFS,
RBD and more. This could allow for compromise of almost arbitrary
data accessible on the LAN/WAN.
</p>
<p>
<strong>Recommendation:</strong> always validate that a disk
image originating from an untrusted source has no backing
file set. If a backing file is seen, reject the image.
</p>
<h3><a id="diskimagesize">Disk image size validation</a></h3>
<p>
If an application allows users to upload pre-created disk
images in non-raw formats, it is essential to validate the
logical disk image size, rather than the physical disk
image size. Non-raw disk images have a grow-on-demand
capability, so a user can provide a qcow2 image that may
be only 1 MB in size, but is configured to grow to many
TB in size.
</p>
<p>
<strong>Recommendation:</strong> if receiving a non-raw disk
image from an untrusted source, validate the logical image
size stored in the disk image metadata against some finite
limit.
</p>
<h3><a id="diskimageaccess">Disk image data access</a></h3>
<p>
If an untrusted disk image is ever mounted on the host OS by
a management application or administrator, this opens an
avenue of attack with which to potentially compromise the
host kernel. Filesystem drivers in OS kernels are often very
complex code and thus may have bugs lurking in them. With
Linux, there are a large number of filesystem drivers, many
of which attract little security analysis attention. Linux
will helpfully probe filesystem formats if not told to use an
explicit format, allowing an attacker the ability to target
specific weak filesystem drivers. Even commonly used and
widely audited filesystems such as <code>ext4</code> have had
<a href="https://lwn.net/Articles/538898/">bugs lurking in them</a>
undetected for years at a time.
</p>
<p>
<strong>Recommendation:</strong> if there is a need to access
the content of a disk image, use a single-use throwaway virtual
machine to access the data. Never mount disk images on the host
OS. Ideally make use of the <a href="http://libguestfs.org">libguestfs</a>
tools and APIs for accessing disks
</p>
<h2><a id="migration">Guest migration network</a></h2>
<p>
Most hypervisors with support for guest migration between hosts
make use of one (or more) network connections. Typically the source
host will connect to some port on the target host to initiate the
migration. There may be separate connections for co-ordinating the
migration, transferring memory state and transferring storage.
If the network over which migration takes place is accessible the
guest, or client applications, there is potential for data leakage
via packet snooping/capture. It is also possible for a malicious
guest or client to make attempts to connect to the target host
to trigger bogus migration operations, or at least inflict a denial
of service attack.
</p>
<p>
<strong>Recommendations:</strong> there are several things to consider
when performing migration
</p>
<ul>
<li>Use a specific address for establishing the migration
connection which is accessible only to the virtualization
hosts themselves, not libvirt clients or virtual guests.
Most hypervisors allow the management application to provide
the IP address of the target host as a way to
determine which network migration takes place on. This is
effectively the connect() socket address for the source host.</li>
<li>Use a specific address for listening for incoming migration
connections which is accessible only to the virtualization
hosts themselves, not libvirt clients or virtual guests.
Most hypervisors allow the management application to configure
the IP address on which the target host listens. This is
the bind() socket address for the target host.</li>
<li>Use an encrypted migration protocol. Some hypervisors
have support for encrypting the migration memory/storage
data. In other cases it can be tunnelled over the libvirtd
RPC protocol connections.</li>
</ul>
<h2><a id="storage">Storage encryption</a></h2>
<p>
Virtual disk images will typically contain confidential data
belonging to the owner of the virtual machine. It is desirable
to protect this against data center administrators as much as
possible. For example, a rogue storage administrator may attempt
to access disk contents directly from a storage host, or a network
administrator/attack may attempt to snoop on data packets relating
to storage access. Use of disk encryption on the virtualization
host can ensure that only the virtualization host administrator
can see the plain text contents of disk images.
</p>
<p>
<strong>Recommendation:</strong> make use of storage encryption
to protect non-local storage from attack by rogue network /
storage administrators or external attackers. This is particularly
important if the storage protocol itself does not offer any kind
of encryption capabilities.
</p>
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