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 --------- 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 KM Spec 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=,session-file= ``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 KM API Spec ([SEVKM]_) '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. 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 `_ .. [SEVKM] `Secure Encrypted Virtualization Key Management `_ 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