Secure Usage of Libvirt

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.

Disk image handling

Disk image format probing

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.

Recommendation: never attempt to automatically detect the format of a disk image based on file contents which are accessible to / originate from an untrusted source.

Disk image backing files

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.

Recommendation: 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.

Disk image size validation

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.

Recommendation: 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.

Disk image data access

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 ext4 have had bugs lurking in them undetected for years at a time.

Recommendation: 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 libguestfs tools and APIs for accessing disks

Guest migration network

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.

Recommendations: there are several things to consider when performing migration

Storage encryption

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.

Recommendation: 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.