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172 lines
7.4 KiB
HTML
172 lines
7.4 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>Secure Usage of Libvirt</h1>
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<ul id="toc"></ul>
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<p>
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This page details information that application developers and
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administrators of libvirt should be aware of when working with
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libvirt, that may have a bearing on security of the system.
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</p>
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<h2><a name="diskimage">Disk image handling</a></h2>
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<h3><a name="diskimageformat">Disk image format probing</a></h3>
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<p>
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Historically there have been multiple flaws in QEMU and most
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projects using QEMU, related to handling of disk formats.
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The problems occur when a guest is given a virtual disk backed
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by raw disk format on the host. If the management application
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on the host tries to auto-detect / probe the disk format, it
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is vulnerable to a malicious guest which can write a qcow2
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file header into its raw disk. If the management application
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subsequently probes the disk, it will see it as a 'qcow2' disk
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instead of a 'raw' disk. Since 'qcow2' disks can have a copy
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on write backing file, such flaw can be leveraged to read
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arbitrary files on the host. The same type of flaw may occur
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if the management application allows users to upload pre-created
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raw images.
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</p>
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<p>
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<strong>Recommendation:</strong> never attempt to automatically
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detect the format of a disk image based on file contents which
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are accessible to / originate from an untrusted source.
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</p>
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<h3><a name="diskimagebacking">Disk image backing files</a></h3>
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<p>
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If a management application allows users to upload pre-created
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disk images in non-raw formats, it can be tricked into giving
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the user access to arbitrary host files via the copy-on-write
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backing file feature. This is because the qcow2 disk format
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header contains a filename field which can point to any location.
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It can also point to network protocols such as NBD, HTTP, GlusterFS,
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RBD and more. This could allow for compromise of almost arbitrary
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data accessible on the LAN/WAN.
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</p>
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<p>
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<strong>Recommendation:</strong> always validate that a disk
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image originating from an untrusted source has no backing
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file set. If a backing file is seen, reject the image.
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</p>
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<h3><a name="diskimagesize">Disk image size validation</a></h3>
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<p>
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If an application allows users to upload pre-created disk
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images in non-raw formats, it is essential to validate the
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logical disk image size, rather than the physical disk
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image size. Non-raw disk images have a grow-on-demand
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capability, so a user can provide a qcow2 image that may
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be only 1 MB in size, but is configured to grow to many
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TB in size.
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</p>
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<p>
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<strong>Recommendation:</strong> if receiving a non-raw disk
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image from an untrusted source, validate the logical image
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size stored in the disk image metadata against some finite
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limit.
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</p>
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<h3><a name="diskimageaccess">Disk image data access</a></h3>
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<p>
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If an untrusted disk image is ever mounted on the host OS by
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a management application or administrator, this opens an
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avenue of attack with which to potentially compromise the
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host kernel. Filesystem drivers in OS kernels are often very
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complex code and thus may have bugs lurking in them. With
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Linux, there are a large number of filesystem drivers, many
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of which attract little security analysis attention. Linux
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will helpfully probe filesystem formats if not told to use an
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explicit format, allowing an attacker the ability to target
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specific weak filesystem drivers. Even commonly used and
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widely audited filesystems such as <code>ext4</code> have had
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<a href="https://lwn.net/Articles/538898/">bugs lurking in them</a>
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undetected for years at a time.
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</p>
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<p>
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<strong>Recommendation:</strong> if there is a need to access
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the content of a disk image, use a single-use throwaway virtual
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machine to access the data. Never mount disk images on the host
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OS. Ideally make use of the <a href="http://libguestfs.org">libguestfs</a>
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tools and APIs for accessing disks
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</p>
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<h2><a name="migration">Guest migration network</a></h2>
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<p>
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Most hypervisors with support for guest migration between hosts
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make use of one (or more) network connections. Typically the source
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host will connect to some port on the target host to initiate the
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migration. There may be separate connections for co-ordinating the
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migration, transferring memory state and transferring storage.
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If the network over which migration takes place is accessible the
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guest, or client applications, there is potential for data leakage
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via packet snooping/capture. It is also possible for a malicious
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guest or client to make attempts to connect to the target host
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to trigger bogus migration operations, or at least inflict a denial
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of service attack.
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</p>
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<p>
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<strong>Recommendations:</strong> there are several things to consider
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when performing migration
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</p>
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<ul>
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<li>Use a specific address for establishing the migration
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connection which is accessible only to the virtualization
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hosts themselves, not libvirt clients or virtual guests.
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Most hypervisors allow the management application to provide
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the IP address of the target host as a way to
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determine which network migration takes place on. This is
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effectively the connect() socket address for the source host.</li>
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<li>Use a specific address for listening for incoming migration
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connections which is accessible only to the virtualization
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hosts themselves, not libvirt clients or virtual guests.
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Most hypervisors allow the management application to configure
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the IP address on which the target host listens. This is
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the bind() socket address for the target host.</li>
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<li>Use an encrypted migration protocol. Some hypervisors
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have support for encrypting the migration memory/storage
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data. In other cases it can be tunnelled over the libvirtd
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RPC protocol connections.</li>
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</ul>
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<h2><a name="storage">Storage encryption</a></h2>
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<p>
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Virtual disk images will typically contain confidential data
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belonging to the owner of the virtual machine. It is desirable
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to protect this against data center administrators as much as
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possible. For example, a rogue storage administrator may attempt
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to access disk contents directly from a storage host, or a network
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administrator/attack may attempt to snoop on data packets relating
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to storage access. Use of disk encryption on the virtualization
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host can ensure that only the virtualization host administrator
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can see the plain text contents of disk images.
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</p>
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<p>
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<strong>Recommendation:</strong> make use of storage encryption
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to protect non-local storage from attack by rogue network /
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storage administrators or external attackers. This is particularly
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important if the storage protocol itself does not offer any kind
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of encryption capabilities.
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</p>
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</body>
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</html>
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