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 Book “Virtualization Guide”, Chapter 6 “Installation of virtualization components”, Section 6.3 “Installing UEFI support” 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. In addition, 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 SLES15 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

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 will look 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 various layers of software receive new features.

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