AMD Secure Encrypted Virtualization (AMD-SEV) Guide

Encryption of computer data stored on disk is a widely deployed feature. However, data in RAM is stored in the clear. This can leave that data vulnerable to software or hardware probing by intruders on the host system, particularly in cloud computing environments where the physical resources are shared by many tenants. Consider a virtual machine of a hostile tenant escaping its sandbox because of a hypervisor bug and searching memory for sensitive data.

AMD's SEV (Secure Encrypted Virtualization) is a technology to protect Linux KVM virtual machines by transparently encrypting the memory of each VM with a unique key. SEV can also calculate a signature of the memory contents, which can be sent to the VM's owner as an attestation that the memory was encrypted correctly by the firmware. SEV is especially relevant to cloud computing environments, where VMs are hosted on remote servers which are not under the control of the VMs' owners. SEV can reduce the amount of trust VMs need to place in the hypervisor and administrator of their host system.

When a virtual machine is processing sensitive data, it can be present in CPU registers as well as memory. If the processing is halted, for example, to service an interrupt or share time with other processes, the virtual machine's CPU register contents are saved to hypervisor memory. This memory is readable by the hypervisor even if SEV is enabled. SEV-ES protects against this scenario by encrypting all CPU register contents when the processing of a virtual machine is halted. SEV-ES builds upon SEV to provide an even smaller attack surface for virtual machines running in a multi-tenant environment.

The VM host hardware must support AMD's SEV technology. To detect if the host hardware supports SEV, check that the sev attribute is in the capabilities of libvirt and that its value is set appropriately:

<domainCapabilities>
   ...
   <features>
   ...
   <sev supported='yes'/>
   ...
   </sev>
   </features>
</domainCapabilities>

Additionally, ensure that the kvm_amd kernel module has the sev parameter enabled:

/sys/module/kvm_amd/parameters/sev = 1

The VM must be the modern Q35 machine type and must use UEFI firmware. libvirt can automatically select an appropriate SEV or SEV-ES enabled UEFI firmware, or one can be specified manually. Currently, the only firmware supported are /usr/share/qemu/ovmf-x86_64-code.bin and /usr/share/qemu/ovmf-x86_64-4m-code.bin. See Section 10.3.1, “Advanced UEFI Configuration” for more details on using UEFI firmware and the auto-selection feature.

Note: No IDE support in Q35

The Q35 machine type does not have an IDE controller and does not support IDE disks.

All virtio-net devices need to be configured with the iPXE option ROM disabled. iPXE is currently not compatible with SEV and SEV-ES. All memory regions used by the VM must be locked for Direct Memory Access (DMA) and to prevent swapping. This includes memory for the VM and any memory regions allocated by QEMU to support running the VM, such as UEFI pflash for firmware and variable store, video RAM, etc.

<domain type='kvm'>
    <memory unit='KiB'>4194304</memory>
    <currentMemory unit='KiB'>4194304</currentMemory>
    <memoryBacking>
    <locked/> 1
    </memoryBacking>
    <os>
    <type arch='x86_64' machine='pc-q35-2.11'>hvm</type>
    <loader readonly='yes' type='pflash'>/usr/share/qemu/ovmf-x86_64-ms-4m-code.bin</loader>
    <nvram>/var/lib/libvirt/qemu/nvram/sles15-sev-guest_VARS.fd</nvram>
    <boot dev='hd'/>
    </os>
    <launchSecurity 2 type='sev'>
    <cbitpos>47</cbitpos> 3
    <reducedPhysBits>1</reducedPhysBits> 4
    <policy>0x0033</policy> 5
    <dhCert>AAAABBBB=CCCCCDDDDD</dhCert> 6
    <session>AAAABBBB=EEEEEFFFFF</session> 7
    </launchSecurity>
    <devices>
    <interface type='bridge'>
    <source bridge='br0'/>
    <model type='virtio'/>
    <rom enabled='no'/> 8
    </interface>
    ...
    </devices>
    ...
    </domain>

The guest policy is four unsigned bytes with the following definition:

virt-install supports the installation of SEV and SEV-ES virtual machines. In addition to your standard installation parameters, provide virt-install with options to satisfy the VM requirements and the --launchSecurity option.

The following example starts a network installation of a SLES 15 SP4 virtual machine protected with SEV-ES.

   virt-install --name sles15sp4-sev-es --location http://192.168.0.1/install/sles15sp4/x86_64 --disk size=20 --network=bridge=br0,model=virtio,rom.bar=off 1 --vcpus 4 --memory 4096 --noautoconsole --events on_reboot=destroy --machine q35 --memtune hard_limit=4563402 --launchSecurity sev,policy=0x07 2 --boot firmware=efi 3 --vnc --serial pty

VM attestation is the process of verifying that trusted software components are correctly instantiated on a trusted compute platform. The process typically involves starting a VM in a paused state, retrieving a launch measurement of the instantiated software components, verifying the measurement, then providing a disk password or other key to unlock the VM and resume the paused boot process. The launch measurement includes cryptographic artifacts provided by the VM owner, with the cryptographic root of trust being the AMD SEV platform. The VM owner can be confident their software components have not been compromised and are running on a trusted platform once the launch measurement has been verified.

The overall attestation process is moderately complex with plenty of opportunity for error. Care must be taken to ensure the process itself is secure. For example, a secure attestation process cannot be executed directly on the hypervisor running the VM, since the goal is to demonstrate the hypervisor has not acted maliciously and contaminated the VM.

Although all the information and APIs required for attestation exist in SLES 15 SP4, SLES 15 SP5 introduces a simple utility called virt-qemu-sev-validate that can be used to satisfy several attestation use cases. For example, automated tests in quality assurance and small libvirt+KVM deployments that are not managed by large, commercial management stacks.

virt-qemu-sev-vailidate is included in the libvirt-client-qemu package and supports both offline and online attestation modes. virt-qemu-sev-validate requires all input for attestation as command-line parameters. Assuming it is invoked on a trusted machine, the results of virt-qemu-sev-validate can be trusted since no information is retrieved from untrusted sources. Online mode is less secure, particularly when executed directly on the hypervisor running the VM.

Regardless of mode, the attestation process of a libvirt+KVM VM starts with creating a VM or Guest Owner (GO) certificate and session blob that is unique for each start of the VM. The certificate and blob can be created with the sevctl utility, available in the sevctl package. The following example illustrates the use of the sevctl session command to create all the prelaunch SEV-related artifacts. The NAME parameter is optional and allows a prefix to be specified for the artifact file names. Using the VM name as a prefix is convenient for matching artifacts with VMs later. The path to the platform Diffie-Hellman (DH) certificate and the desired SEV policy are required parameters.

# sevctl session --name test-sev /data/sev/pdh.cert 7

The sevctl session command produces four files: tik.bin, tek.bin, godh.b64 and session.b64. If the optional NAME parameter was used, the files are prefixed with the specified value. The transport integrity key (tik.bin) and transport encryption key (tek.bin) are used in the verification stage of the attestation process. The guest owner Diffie-Hellman key (godh.b64) and session blob (session.b64) are copied to the VM XML configuration before starting the VM. See the dhCert and session subelements of the launchSecurity element in the VM configuration section for more details.

After the VM session artifacts have been created and VM XML configuration updated, the VM can be started in a paused state, for example:

# virsh -c qemu+ssh://USER_NAME@HOST_NAME/system create --paused /path/to/vm.xml

Creating the VM in a paused state allows retrieving the launch measurement from the hypervisor and comparing it to a measurement calculated on a trusted host using trusted inputs. If the measurements compare, the VM owner can be confident the VM has been properly instantiated on the hypervisor and execution can safely be started. The following command demonstrates using the virsh domlaunchsecinfo command to retrieve the paused VM launch measurement and other SEV-related information from the hosting hypervisor.

# virsh -c qemu+ssh://username@hostname/system domlaunchsecinfo sevtest
sev-measurement: VZjxMSlu+UuYkWHN2mAxDVVYXRmL3wqTu84kwk+5QS+4OMii7hs6cMAmXNpmmyS/
sev-api-major  : 1
sev-api-minor  : 51
sev-build-id   : 3
sev-policy     : 7

The retrieved launch measurement can then be given to virt-qemu-sev-validate to verify the VM has been securely instantiated. The following example demonstrates a full offline attestation of the measurement.

# virt-qemu-sev-validate --api-major 1 --api-minor 51 --build-id 3 --policy 7 \
 --firmware /usr/share/qemu/ovmf-x86_64-4m.bin --tik sevtest_tik.bin --tek sevtest_tek.bin --num-cpus 4 \
 --cpu-family 25 --cpu-model 1 --cpu-stepping 1 \
 --measurement QJ0oDpFmWj+bGZzFoMPbAxTuC6QD44W5w88x/hQM8toVsB75ci7V1YDfYoI9GTk

It is also possible to use virt-qemu-sev-validate in an online mode, where information needed to perform the VM attestation is retrieved from the hosting hypervisor. The following example demonstrates an online attestation of the VM, where only the hosting hypervisor URI, VM name, and associated TIK and TEK are specified. virt-qemu-sev-validate retrieves the remaining information, including the measurement itself, from the hosting hypervisor:

# virt-qemu-sev-validate --tik sevtest_tik.bin --tek sevtest_tek.bin \
 --connect qemu+ssh://USER_NAME@HOST_NAME/system --domain sevtest

Once the VM launch measurement has been verified, the VM owner can optionally inject a secret in the VM and resume VM execution. An example of injecting a secret would be providing a key to unlock an encrypted root disk.

# virsh -c qemu+ssh://USER_NAME@HOST_NAME/system domsetlaunchsecstate sevtest \
 --secrethdr hdr-str --secret secret-str
# virsh -c qemu+ssh://USER_NAME@HOST_NAME/system resume sevtest

KubeVirt supports running SEV guests starting from the version 0.49.0. The functionality can be activated by enabling the WorkloadEncryptionSEV feature gate:

> kubectl edit kubevirt kubevirt -n kubevirt
[...]
spec:
  configuration:
    developerConfiguration:
      featureGates:
      - WorkloadEncryptionSEV
[...]

To run an SEV-encrypted guest, the virtual machine specification must include the entry sev: {} under the launchSecurity domain element. Additionally, you need to configure the firmware/bootloader parameters to use the efi option with the secureBoot flag set to disabled. The corresponding YAML snippet looks similar to the following:

[...]
spec:
  domain:
    firmware:
      bootloader:
        efi:
          secureBoot: false
    launchSecurity:
      sev: {}
[...]

SUSE does not recommend using the SEV and SEV-ES features with SUSE Linux products on the first generation AMD EPYC™ 7000 series of processors, code name Naples. It is recommended to use at least the second generation 7002 series processors, code name Rome. Additionally, the following limitations are placed on SEV and SEV-ES VMs.

These limitations will be removed in the future as the hardware, firmware and specific layers of software receive new features.

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