AMD Secure Encrypted Virtualization (SEV) — QEMU documentation

# AMD Secure Encrypted Virtualization (SEV)

Secure Encrypted Virtualization (SEV) is a feature found on AMD processors.

SEV is an extension to the AMD-V architecture which supports running encrypted virtual machines (VMs) under the control of KVM. Encrypted VMs have their pages (code and data) secured such that only the guest itself has access to the unencrypted version. Each encrypted VM is associated with a unique encryption key; if its data is accessed by a different entity using a different key the encrypted guests data will be incorrectly decrypted, leading to unintelligible data.

Key management for this feature is handled by a separate processor known as the AMD secure processor (AMD-SP), which is present in AMD SOCs. Firmware running inside the AMD-SP provides commands to support a common VM lifecycle. This includes commands for launching, snapshotting, migrating and debugging the encrypted guest. These SEV commands can be issued via KVM_MEMORY_ENCRYPT_OP ioctls.

Secure Encrypted Virtualization - Encrypted State (SEV-ES) builds on the SEV support to additionally protect the guest register state. In order to allow a hypervisor to perform functions on behalf of a guest, there is architectural support for notifying a guest’s operating system when certain types of VMEXITs are about to occur. This allows the guest to selectively share information with the hypervisor to satisfy the requested function.

## Launching (SEV and SEV-ES)

Boot images (such as bios) must be encrypted before a guest can be booted. The `MEMORY_ENCRYPT_OP` ioctl provides commands to encrypt the images: `LAUNCH_START`, `LAUNCH_UPDATE_DATA`, `LAUNCH_MEASURE` and `LAUNCH_FINISH`. These four commands together generate a fresh memory encryption key for the VM, encrypt the boot images and provide a measurement than can be used as an attestation of a successful launch.

For a SEV-ES guest, the `LAUNCH_UPDATE_VMSA` command is also used to encrypt the guest register state, or VM save area (VMSA), for all of the guest vCPUs.

`LAUNCH_START` is called first to create a cryptographic launch context within the firmware. To create this context, guest owner must provide a guest policy, its public Diffie-Hellman key (PDH) and session parameters. These inputs should be treated as a binary blob and must be passed as-is to the SEV firmware.

The guest policy is passed as plaintext. A hypervisor may choose to read it, but should not modify it (any modification of the policy bits will result in bad measurement). The guest policy is a 4-byte data structure containing several flags that restricts what can be done on a running SEV guest. See SEV API Spec ([SEVAPI]) section 3 and 6.2 for more details.

The guest policy can be provided via the `policy` property:

    # ${QEMU} \
       sev-guest,id=sev0,policy=0x1...\

Setting the “SEV-ES required” policy bit (bit 2) will launch the guest as a SEV-ES guest:

    # ${QEMU} \
       sev-guest,id=sev0,policy=0x5...\

The guest owner provided DH certificate and session parameters will be used to establish a cryptographic session with the guest owner to negotiate keys used for the attestation.

The DH certificate and session blob can be provided via the `dh-cert-file` and `session-file` properties:

    # ${QEMU} \
         sev-guest,id=sev0,dh-cert-file=<file1>,session-file=<file2>

`LAUNCH_UPDATE_DATA` encrypts the memory region using the cryptographic context created via the `LAUNCH_START` command. If required, this command can be called multiple times to encrypt different memory regions. The command also calculates the measurement of the memory contents as it encrypts.

`LAUNCH_UPDATE_VMSA` encrypts all the vCPU VMSAs for a SEV-ES guest using the cryptographic context created via the `LAUNCH_START` command. The command also calculates the measurement of the VMSAs as it encrypts them.

`LAUNCH_MEASURE` can be used to retrieve the measurement of encrypted memory and, for a SEV-ES guest, encrypted VMSAs. This measurement is a signature of the memory contents and, for a SEV-ES guest, the VMSA contents, that can be sent to the guest owner as an attestation that the memory and VMSAs were encrypted correctly by the firmware. The guest owner may wait to provide the guest confidential information until it can verify the attestation measurement. Since the guest owner knows the initial contents of the guest at boot, the attestation measurement can be verified by comparing it to what the guest owner expects.

`LAUNCH_FINISH` finalizes the guest launch and destroys the cryptographic context.

See SEV API Spec ([SEVAPI]) ‘Launching a guest’ usage flow (Appendix A) for the complete flow chart.

To launch a SEV guest:

    # ${QEMU} \
        -machine ...,confidential-guest-support=sev0 \
        -object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1

To launch a SEV-ES guest:

    # ${QEMU} \
        -machine ...,confidential-guest-support=sev0 \
        -object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1,policy=0x5

An SEV-ES guest has some restrictions as compared to a SEV guest. Because the guest register state is encrypted and cannot be updated by the VMM/hypervisor, a SEV-ES guest:

> - Does not support SMM - SMM support requires updating the guest register state.

> - Does not support reboot - a system reset requires updating the guest register state.

> - Requires in-kernel irqchip - the burden is placed on the hypervisor to manage booting APs.

## Calculating expected guest launch measurement

In order to verify the guest launch measurement, The Guest Owner must compute it in the exact same way as it is calculated by the AMD-SP. SEV API Spec ([SEVAPI]) section 6.5.1 describes the AMD-SP operations:

> GCTX.LD is finalized, producing the hash digest of all plaintext data imported into the guest.

> The launch measurement is calculated as:

> HMAC(0x04 || API_MAJOR || API_MINOR || BUILD || GCTX.POLICY || GCTX.LD || MNONCE; GCTX.TIK)

> where “||” represents concatenation.

The values of API_MAJOR, API_MINOR, BUILD, and GCTX.POLICY can be obtained from the `query-sev` qmp command.

The value of MNONCE is part of the response of `query-sev-launch-measure`: it is the last 16 bytes of the base64-decoded data field (see SEV API Spec ([SEVAPI]) section 6.5.2 Table 52: LAUNCH_MEASURE Measurement Buffer).

The value of GCTX.LD is `SHA256(firmware_blob || kernel_hashes_blob || vmsas_blob)`, where:

- `firmware_blob` is the content of the entire firmware flash file (for example, `OVMF.fd`). Note that you must build a stateless firmware file which doesn’t use an NVRAM store, because the NVRAM area is not measured, and therefore it is not secure to use a firmware which uses state from an NVRAM store.

- if kernel is used, and `kernel-hashes=on`, then `kernel_hashes_blob` is the content of PaddedSevHashTable (including the zero padding), which itself includes the hashes of kernel, initrd, and cmdline that are passed to the guest. The PaddedSevHashTable struct is defined in `target/i386/sev.c`.

- if SEV-ES is enabled (`policy & 0x4 != 0`), `vmsas_blob` is the concatenation of all VMSAs of the guest vcpus. Each VMSA is 4096 bytes long; its content is defined inside Linux kernel code as `struct vmcb_save_area`, or in AMD APM Volume 2 ([APMVOL2]) Table B-2: VMCB Layout, State Save Area.

If kernel hashes are not used, or SEV-ES is disabled, use empty blobs for `kernel_hashes_blob` and `vmsas_blob` as needed.

## Launching (SEV-SNP)

Boot images (such as bios) must be encrypted before a guest can be booted. The `MEMORY_ENCRYPT_OP` ioctl provides commands to encrypt the images: `SNP_LAUNCH_START`, `SNP_LAUNCH_UPDATE`, and `SNP_LAUNCH_FINISH`. These three commands communicate with SEV-SNP firmware to generate a fresh memory encryption key for the VM, encrypt the boot images for a successful launch. For more details on the SEV-SNP firmware interfaces used by these commands please see the SEV-SNP Firmware ABI.

`SNP_LAUNCH_START` is called first to create a cryptographic launch context within the firmware. To create this context, the guest owner must provide a guest policy and other parameters as described in the SEV-SNP firmware specification. The launch parameters should be specified as described in the QAPI schema for the sev-snp-guest object.

The `SNP_LAUNCH_START` uses the following parameters, which can be configured by the corresponding parameters documented in the QAPI schema for the ‘sev-snp-guest’ object.

key | type | default | meaning
policy | hex | 0x30000 | a 64-bit guest policy
guest-visible-workarounds | string| 0 | 16-byte base64 encoded string | for guest OS visible | workarounds.

`SNP_LAUNCH_UPDATE` encrypts the memory region using the cryptographic context created via the `SNP_LAUNCH_START` command. If required, this command can be called multiple times to encrypt different memory regions. The command also calculates the measurement of the memory contents as it encrypts.

`SNP_LAUNCH_FINISH` finalizes the guest launch flow. Optionally, while finalizing the launch the firmware can perform checks on the launch digest computing through the `SNP_LAUNCH_UPDATE`. To perform the check the user must supply the id block, authentication blob and host data that should be included in the attestation report. See the SEV-SNP spec for further details.

The `SNP_LAUNCH_FINISH` uses the following parameters, which can be configured by the corresponding parameters documented in the QAPI schema for the ‘sev-snp-guest’ object.

key | type | default | meaning
id-block | string | none | base64 encoded ID block
id-auth | string | none | base64 encoded authentication information
author-key-enabled | bool | 0 | auth block contains author key
host_data | string | none | host provided data

To launch a SEV-SNP guest (additional parameters are documented in the QAPI schema for the ‘sev-snp-guest’ object):

    # ${QEMU} \
      -machine ...,confidential-guest-support=sev0 \
      -object sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1

## Debugging

Since the memory contents of a SEV guest are encrypted, hypervisor access to the guest memory will return cipher text. If the guest policy allows debugging, then a hypervisor can use the DEBUG_DECRYPT and DEBUG_ENCRYPT commands to access the guest memory region for debug purposes. This is not supported in QEMU yet.

## Snapshot/Restore

TODO

## Live Migration

TODO

## References

AMD Memory Encryption whitepaper

[SEVAPI] (1,2,3,4)

Secure Encrypted Virtualization API

[APMVOL2]

AMD64 Architecture Programmer’s Manual Volume 2: System Programming

KVM Forum slides:

- AMD’s Virtualization Memory Encryption (2016)

- Extending Secure Encrypted Virtualization With SEV-ES (2018)

AMD64 Architecture Programmer’s Manual:

- SME is section 7.10

- SEV is section 15.34

- SEV-ES is section 15.35