mirror of
https://gitlab.com/libvirt/libvirt.git
synced 2024-11-03 11:51:11 +00:00
b1c81567c7
The HTML5 doctype is simply <!DOCTYPE html> no DTD is present because HTML5 is no longer defined as an extension of SGML. XSL has no way to natively output a doctype without a public or system identifier, so we have to use an <xsl:text> hack instead. See also https://dev.w3.org/html5/html-author/#doctype-declaration Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
172 lines
7.3 KiB
XML
172 lines
7.3 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
|
|
<!DOCTYPE html>
|
|
<html xmlns="http://www.w3.org/1999/xhtml">
|
|
<body>
|
|
|
|
<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>
|
|
|
|
</body>
|
|
</html>
|