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SUSE Linux Enterprise Server 15 SP4

AMD Secure Encrypted Virtualization (AMD-SEV) Guide

Publication Date: December 13, 2024

AMD's Secure Encrypted Virtualization (SEV) allows the memory of virtual machines to be encrypted. SEV with Encrypted State (SEV-ES) goes one step further by encrypting the virtual machine's CPU register content. These technologies increase system security and are ideal for multi-tenant environments such as cloud computing. They enable protection from a variety of cross-VM and hypervisor-based attacks. As an example, a hostile VM that has escaped its hypervisor-enforced confines and is able to read arbitrary memory is unable to steal sensitive data from an SEV or SEV-ES VM.

This document aims to provide a basic understanding of how SEV and SEV-ES work, and how to enable and configure these features. It also mentions certain limitations and restrictions that the use of SEV and SEV-ES causes as compared to non-encrypted virtualization.

1 Introducing SEV

  

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.

2 VM host requirements

  

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

3 VM requirements

  

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
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.

4 VM configuration

  
Example 1: Sample configuration file
  

As an example, an SEV-encrypted VM configured with 4 GB of memory would contain the following XML configuration: 

<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>

1

The memoryBacking element, along with its child element locking, is used to ease memory limit restrictions libvirt places on the VM's cgroup. Otherwise, VM creation would fail when QEMU attempts to lock the VM's memory regions along with other regions used to support the VM operation. See https://libvirt.org/kbase/launch_security_sev.html#memory for more information on VM memory configuration requirements for SEV VMs.

2

The launchSecurity type='sev' element and its contents enable encryption of the VM's memory contents.

3

When memory encryption is enabled, one of the physical address bits (also known as the "C-bit") is used to mark if a memory page is protected. The required cbitpos element provides the location of the C-bit in a guest page table entry. For example, the value 47 indicates that bit position 47 in a page table entry will determine whether that page is encrypted or not. The C-bit number is read from the host's CPUID and is thus hardware-dependent. The value of cbitpos is hypervisor-dependent, and can be obtained through the sev element in the capabilities of the domain.

4

When memory encryption is enabled, we lose certain bits of the physical address space. The required reducedPhysBits element provides this physical address bit reduction. Similarly to cbitpos, the value of reducedPhysBits is processor-family-dependent and can be obtained through the sev element in the domain capabilities.

5

The required policy element provides the guest policy which must be maintained by the SEV firmware. This policy is enforced by the firmware, and restricts what configuration and operational commands can be performed on the VM by the hypervisor. The guest policy provided when starting the VM is bound to that VM and cannot be changed throughout its lifetime.

6

The optional dhCert element provides the guest owner's base64-encoded Diffie-Hellman (DH) key. The key is used to negotiate a master secret key between the SEV firmware and guest owner. This master secret key is then used to establish a trusted channel between the SEV firmware and guest owner.

7

The optional session element provides the guest owner's base64-encoded session blob, as defined in the SEV API specification. See the LAUNCH_START section of the SEV specification for the session-blob format.

8

In addition to the launchSecurity settings, SEV-encrypted VMs must have the iPXE option ROM disabled on all virtio-net devices. Currently, iPXE is not compatible with SEV-encrypted VMs.

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

Table 1: Guest policy definitions
  

Bit(s)

Definition

0

If set, debugging of the guest is disallowed

1

If set, sharing keys with other guests is disallowed

2

If set, SEV-ES is required

3

If set, sending the guest to another platform is disallowed

4

If set, the guest must not be transmitted to another platform that is not in the domain

5

If set, the guest must not be transmitted to another platform that is not SEV-capable

6-15

Reserved

16-32

The guest must not be transmitted to another platform with a lower firmware version

5 VM installation

  

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

1

The iPXE option ROM is not compatible with SEV-encrypted VMs and must be disabled on all virtio-net devices. While libvirt supports disabling option ROMs using either the enabled or bar attributes of the rom element, virt-install only supports disabling option ROMs using the bar attribute.

2

The launchSecurity option specifies the type and policy to be enforced by the SEV firmware. The policy setting is described in Table 1, “Guest policy definitions”.

3

The boot option allows specifying many boot-related settings, including the firmware used by the virtual machine. Specifying a firmware type efi allows libvirt's firmware auto-selection feature to select an appropriate SEV capable firmware for the virtual machine.

6 SEV with KubeVirt

  

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: {}
[...]

7 Current limitations

  

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.

  • The guest operating system running inside an SEV-encrypted VM must contain SEV support. SUSE Linux Enterprise Server 12 SP4 and newer, and all SUSE Linux Enterprise Server 15 releases support SEV.

  • Any operations that involve saving and restoring the memory and state of an instance are currently not supported. This means that SEV-encrypted VMs cannot be resumed from snapshots, saved/restored, or live migrated. Encrypted VMs can be shutdown and restarted on another host as normal.

  • SEV-encrypted VMs cannot contain directly-accessible host devices (that is, PCI passthrough).

  • SEV-encrypted VMs are not compatible with Secure Boot. UEFI firmware containing Secure Boot support will not work with SEV or SEV-ES VMs.

  • SEV-ES VMs cannot be rebooted from within using reboot, shutdown -r now, etc. A reboot must be done by shutting down the VM and starting it again. This limitation does not apply to SEV VMs, only SEV-ES.

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

8 More information

  

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