In the beginning of this chapter we discuss the use of both physical and virtual hardware by instances, the associated security risks, and some recommendations for mitigating those risks. We conclude the chapter with a discussion of sVirt, an open source project for integrating SELinux mandatory access controls with the virtualization components.
Many hypervisors offer a functionality known as PCI passthrough. This allows an instance to have direct access to a piece of hardware on the node. For example, this could be used to allow instances to access video cards offering the compute unified device architecture (CUDA) for high performance computation. This feature carries two types of security risks: direct memory access and hardware infection.
Direct memory access (DMA) is a feature that permits certain hardware devices to access arbitrary physical memory addresses in the host computer. Often video cards have this capability. However, an instance should not be given arbitrary physical memory access because this would give it full view of both the host system and other instances running on the same node. Hardware vendors use an input/output memory management unit (IOMMU) to manage DMA access in these situations. Therefore, cloud architects should ensure that the hypervisor is configured to utilize this hardware feature.
![[Note]](../common/images/admon/note.png) | Note |
|---|---|
The IOMMU feature is marketed as VT-d by Intel and AMD-Vi by AMD. |
A hardware infection occurs when an instance makes a malicious modification to the firmware or some other part of a device. As this device is used by other instances, or even the host OS, the malicious code can spread into these systems. The end result is that one instance can run code outside of its security domain. This is a potential problem in any hardware sharing scenario. The problem is specific to this scenario because it is harder to reset the state of physical hardware than virtual hardware.
Solutions to the hardware infection problem are domain specific. The strategy is to identify how an instance can modify hardware state then determine how to reset any modifications when the instance is done using the hardware. For example, one option could be to re-flash the firmware after use. Clearly there is a need to balance hardware longevity with security as some firmwares will fail after a large number of writes. TPM technology, described in link:Management/Node Bootstrapping, provides a solution for detecting unauthorized firmware changes. Regardless of the strategy selected, it is important to understand the risks associated with this kind of hardware sharing so that they can be properly mitigated for a given deployment scenario.
Additionally, due to the risk and complexities associated with PCI passthrough, it should be disabled by default. If enabled for a specific need, you will need to have appropriate processes in place to ensure the hardware is clean before re-issue.
When running a virtual machine, virtual hardware is a software layer that provides the hardware interface for the virtual machine. Instances use this functionality to provide network, storage, video, and other devices that may be needed. With this in mind, most instances in your environment will exclusively use virtual hardware, with a minority that will require direct hardware access. The major open source hypervisors use QEMU for this functionality. While QEMU fills an important need for virtualization platforms, it has proven to be a very challenging software project to write and maintain. Much of the functionality in QEMU is implemented with low-level code that is difficult for most developers to comprehend. Furthermore, the hardware virtualized by QEMU includes many legacy devices that have their own set of quirks. Putting all of this together, QEMU has been the source of many security problems, including hypervisor breakout attacks.
For the reasons stated above, it is important to take proactive steps to harden QEMU. We recommend three specific steps: minimizing the code base, using compiler hardening, and using mandatory access controls, such as sVirt, SELinux, or AppArmor.
One classic security principle is to remove any unused components from your system. QEMU provides support for many different virtual hardware devices. However, only a small number of devices are needed for a given instance. Most instances will use the virtio devices. However, some legacy instances will need access to specific hardware, which can be specified using glance metadata:
glance image-update \ --property hw_disk_bus=ide \ --property hw_cdrom_bus=ide \ --property hw_vif_model=e1000 \ f16-x86_64-openstack-sda
A cloud architect should decide what devices to make available to cloud users. Anything that is not needed should be removed from QEMU. This step requires recompiling QEMU after modifying the options passed to the QEMU configure script. For a complete list of up-to-date options simply run ./configure --help from within the QEMU source directory. Decide what is needed for your deployment, and disable the remaining options.
The next step is to harden QEMU using compiler hardening options. Modern compilers provide a variety of compile time options to improve the security of the resulting binaries. These features, which we will describe in more detail below, include relocation read-only (RELRO), stack canaries, never execute (NX), position independent executable (PIE), and address space layout randomization (ASLR).
Many modern linux distributions already build QEMU with compiler hardening enabled, so you may want to verify your existing executable before proceeding with the information below. One tool that can assist you with this verification is called checksec.sh.
RELocation Read-Only (RELRO): Hardens the data sections of an executable. Both full and partial RELRO modes are supported by gcc. For QEMU full RELRO is your best choice. This will make the global offset table read-only and place various internal data sections before the program data section in the resulting executable.
Stack Canaries: Places values on the stack and verifies their presence to help prevent buffer overflow attacks.
Never eXecute (NX): Also known as Data Execution Prevention (DEP), ensures that data sections of the executable can not be executed.
Position Independent Executable (PIE): Produces a position independent executable, which is necessary for ASLR.
Address Space Layout Randomization (ASLR) : This ensures that both code and data regions will be randomized. Enabled by the kernel (all modern linux kernels support ASLR), when the executable is built with PIE.
Putting this all together, and adding in some additional useful protections, we recommend the following compiler options for gcc when compiling QEMU:
CFLAGS="-arch x86_64 -fstack-protector-all -Wstack-protector --param ssp-buffer-size=4 -pie -fPIE -ftrapv -D_FORTIFY_SOURCE=2 O2 -Wl,-z,relro,-z,now"
We recommend testing your QEMU executable file after it is compiled to ensure that the compiler hardening worked properly.
Most cloud deployments will not want to build software such as QEMU by hand. It is better to use packaging to ensure that the process is repeatable and to ensure that the end result can be easily deployed throughout the cloud. The references below provide some additional details on applying compiler hardening options to existing packages.
DEB packages: Hardening Walkthrough
RPM packages: How to create an RPM package
Compiler hardening makes it more difficult to attack the QEMU process. However, if an attacker does succeed, we would like to limit the impact of the attack. Mandatory access controls accomplish this by restricting the privileges on QEMU process to only what is needed. This can be accomplished using sVirt / SELinux or AppArmor. When using sVirt, SELinux is configured to run every QEMU process under a different security context. AppArmor can be configured to provide similar functionality. We provide more details on sVirt in the instance isolation section below.
With unique kernel-level architecture and National Security Agency (NSA) developed security mechanisms, KVM provides foundational isolation technologies for multi tenancy. With developmental origins dating back to 2002, the Secure Virtualization (sVirt) technology is the application of SELinux against modern day virtualization. SELinux, which was designed to apply separation control based upon labels, has been extended to provide isolation between virtual machine processes, devices, data files and system processes acting upon their behalf.
OpenStack's sVirt implementation aspires to protect hypervisor hosts and virtual machines against two primary threat vectors:
Hypervisor threats A compromised application running within a virtual machine attacks the hypervisor to access underlying resources. For example, the host OS, applications, or devices within the physical machine. This is a threat vector unique to virtualization and represents considerable risk as the underlying real machine can be compromised due to vulnerability in a single virtual application.
Virtual Machine (multi-tenant) threats A compromised application running within a VM attacks the hypervisor to access/control another virtual machine and its resources. This is a threat vector unique to virtualization and represents considerable risk as a multitude of virtual machine file images could be compromised due to vulnerability in a single application. This virtual network attack is a major concern as the administrative techniques for protecting real networks do not directly apply to the virtual environment.
Each KVM-based virtual machine is a process which is labeled by SELinux, effectively establishing a security boundary around each virtual machine. This security boundary is monitored and enforced by the Linux kernel, restricting the virtual machine's access to resources outside of its boundary such as host machine data files or other VMs.
As shown above, sVirt isolation is provided regardless of the guest Operating System running inside the virtual machine -- Linux or Windows VMs can be used. Additionally, many Linux distributions provide SELinux within the operating system, allowing the virtual machine to protect internal virtual resources from threats.
KVM-based virtual machine instances are labelled with their own SELinux data type, known as svirt_image_t. Kernel level protections prevent unauthorized system processes, such as malware, from manipulating the virtual machine image files on disk. When virtual machines are powered off, images are stored as svirt_image_t as shown below:
system_u:object_r:svirt_image_t:SystemLow image1 system_u:object_r:svirt_image_t:SystemLow image2 system_u:object_r:svirt_image_t:SystemLow image3 system_u:object_r:svirt_image_t:SystemLow image4
The svirt_image_t label uniquely identifies image files on disk, allowing for the SELinux policy to restrict access. When a KVM-based Compute image is powered on, sVirt appends a random numerical identifier to the image. sVirt is technically capable of assigning numerical identifiers to 524,288 virtual machines per hypervisor node, however OpenStack deployments are highly unlikely to encounter this limitation.
This example shows the sVirt category identifier:
system_u:object_r:svirt_image_t:s0:c87,c520 image1 system_u:object_r:svirt_image_t:s0:419,c172 image2
To ease the administrative burden of managing SELinux, many enterprise Linux platforms utilize SELinux Booleans to quickly change the security posture of sVirt.
Red Hat Enterprise Linux-based KVM deployments utilize the following sVirt booleans:
sVirt SELinux Boolean |
Description |
virt_use_common |
Allow virt to use serial/parallel communication ports. |
virt_use_fusefs |
Allow virt to read FUSE mounted files. |
virt_use_nfs |
Allow virt to manage NFS mounted files. |
virt_use_samba |
Allow virt to manage CIFS mounted files. |
virt_use_sanlock |
Allow confined virtual guests to interact with the sanlock. |
virt_use_sysfs |
Allow virt to manage device configuration (PCI). |
virt_use_usb |
Allow virt to use USB devices. |
virt_use_xserver |
Allow virtual machine to interact with the X Window System. |
