| ====================================== |
| Secure Encrypted Virtualization (SEV) |
| ====================================== |
| |
| Overview |
| ======== |
| |
| Secure Encrypted Virtualization (SEV) is a feature found on AMD processors. |
| |
| SEV is an extension to the AMD-V architecture which supports running |
| virtual machines (VMs) under the control of a hypervisor. When enabled, |
| the memory contents of a VM will be transparently encrypted with a key |
| unique to that VM. |
| |
| The hypervisor can determine the SEV support through the CPUID |
| instruction. The CPUID function 0x8000001f reports information related |
| to SEV:: |
| |
| 0x8000001f[eax]: |
| Bit[1] indicates support for SEV |
| ... |
| [ecx]: |
| Bits[31:0] Number of encrypted guests supported simultaneously |
| |
| If support for SEV is present, MSR 0xc001_0010 (MSR_K8_SYSCFG) and MSR 0xc001_0015 |
| (MSR_K7_HWCR) can be used to determine if it can be enabled:: |
| |
| 0xc001_0010: |
| Bit[23] 1 = memory encryption can be enabled |
| 0 = memory encryption can not be enabled |
| |
| 0xc001_0015: |
| Bit[0] 1 = memory encryption can be enabled |
| 0 = memory encryption can not be enabled |
| |
| When SEV support is available, it can be enabled in a specific VM by |
| setting the SEV bit before executing VMRUN.:: |
| |
| VMCB[0x90]: |
| Bit[1] 1 = SEV is enabled |
| 0 = SEV is disabled |
| |
| SEV hardware uses ASIDs to associate a memory encryption key with a VM. |
| Hence, the ASID for the SEV-enabled guests must be from 1 to a maximum value |
| defined in the CPUID 0x8000001f[ecx] field. |
| |
| SEV Key Management |
| ================== |
| |
| The SEV guest key management is handled by a separate processor called the AMD |
| Secure Processor (AMD-SP). Firmware running inside the AMD-SP provides a secure |
| key management interface to perform common hypervisor activities such as |
| encrypting bootstrap code, snapshot, migrating and debugging the guest. For more |
| information, see the SEV Key Management spec [api-spec]_ |
| |
| The main ioctl to access SEV is KVM_MEM_ENCRYPT_OP. If the argument |
| to KVM_MEM_ENCRYPT_OP is NULL, the ioctl returns 0 if SEV is enabled |
| and ``ENOTTY` if it is disabled (on some older versions of Linux, |
| the ioctl runs normally even with a NULL argument, and therefore will |
| likely return ``EFAULT``). If non-NULL, the argument to KVM_MEM_ENCRYPT_OP |
| must be a struct kvm_sev_cmd:: |
| |
| struct kvm_sev_cmd { |
| __u32 id; |
| __u64 data; |
| __u32 error; |
| __u32 sev_fd; |
| }; |
| |
| |
| The ``id`` field contains the subcommand, and the ``data`` field points to |
| another struct containing arguments specific to command. The ``sev_fd`` |
| should point to a file descriptor that is opened on the ``/dev/sev`` |
| device, if needed (see individual commands). |
| |
| On output, ``error`` is zero on success, or an error code. Error codes |
| are defined in ``<linux/psp-dev.h>``. |
| |
| KVM implements the following commands to support common lifecycle events of SEV |
| guests, such as launching, running, snapshotting, migrating and decommissioning. |
| |
| 1. KVM_SEV_INIT |
| --------------- |
| |
| The KVM_SEV_INIT command is used by the hypervisor to initialize the SEV platform |
| context. In a typical workflow, this command should be the first command issued. |
| |
| Returns: 0 on success, -negative on error |
| |
| 2. KVM_SEV_LAUNCH_START |
| ----------------------- |
| |
| The KVM_SEV_LAUNCH_START command is used for creating the memory encryption |
| context. To create the encryption context, user must provide a guest policy, |
| the owner's public Diffie-Hellman (PDH) key and session information. |
| |
| Parameters: struct kvm_sev_launch_start (in/out) |
| |
| Returns: 0 on success, -negative on error |
| |
| :: |
| |
| struct kvm_sev_launch_start { |
| __u32 handle; /* if zero then firmware creates a new handle */ |
| __u32 policy; /* guest's policy */ |
| |
| __u64 dh_uaddr; /* userspace address pointing to the guest owner's PDH key */ |
| __u32 dh_len; |
| |
| __u64 session_addr; /* userspace address which points to the guest session information */ |
| __u32 session_len; |
| }; |
| |
| On success, the 'handle' field contains a new handle and on error, a negative value. |
| |
| KVM_SEV_LAUNCH_START requires the ``sev_fd`` field to be valid. |
| |
| For more details, see SEV spec Section 6.2. |
| |
| 3. KVM_SEV_LAUNCH_UPDATE_DATA |
| ----------------------------- |
| |
| The KVM_SEV_LAUNCH_UPDATE_DATA is used for encrypting a memory region. It also |
| calculates a measurement of the memory contents. The measurement is a signature |
| of the memory contents that can be sent to the guest owner as an attestation |
| that the memory was encrypted correctly by the firmware. |
| |
| Parameters (in): struct kvm_sev_launch_update_data |
| |
| Returns: 0 on success, -negative on error |
| |
| :: |
| |
| struct kvm_sev_launch_update { |
| __u64 uaddr; /* userspace address to be encrypted (must be 16-byte aligned) */ |
| __u32 len; /* length of the data to be encrypted (must be 16-byte aligned) */ |
| }; |
| |
| For more details, see SEV spec Section 6.3. |
| |
| 4. KVM_SEV_LAUNCH_MEASURE |
| ------------------------- |
| |
| The KVM_SEV_LAUNCH_MEASURE command is used to retrieve the measurement of the |
| data encrypted by the KVM_SEV_LAUNCH_UPDATE_DATA command. The guest owner may |
| wait to provide the guest with confidential information until it can verify the |
| measurement. Since the guest owner knows the initial contents of the guest at |
| boot, the measurement can be verified by comparing it to what the guest owner |
| expects. |
| |
| Parameters (in): struct kvm_sev_launch_measure |
| |
| Returns: 0 on success, -negative on error |
| |
| :: |
| |
| struct kvm_sev_launch_measure { |
| __u64 uaddr; /* where to copy the measurement */ |
| __u32 len; /* length of measurement blob */ |
| }; |
| |
| For more details on the measurement verification flow, see SEV spec Section 6.4. |
| |
| 5. KVM_SEV_LAUNCH_FINISH |
| ------------------------ |
| |
| After completion of the launch flow, the KVM_SEV_LAUNCH_FINISH command can be |
| issued to make the guest ready for the execution. |
| |
| Returns: 0 on success, -negative on error |
| |
| 6. KVM_SEV_GUEST_STATUS |
| ----------------------- |
| |
| The KVM_SEV_GUEST_STATUS command is used to retrieve status information about a |
| SEV-enabled guest. |
| |
| Parameters (out): struct kvm_sev_guest_status |
| |
| Returns: 0 on success, -negative on error |
| |
| :: |
| |
| struct kvm_sev_guest_status { |
| __u32 handle; /* guest handle */ |
| __u32 policy; /* guest policy */ |
| __u8 state; /* guest state (see enum below) */ |
| }; |
| |
| SEV guest state: |
| |
| :: |
| |
| enum { |
| SEV_STATE_INVALID = 0; |
| SEV_STATE_LAUNCHING, /* guest is currently being launched */ |
| SEV_STATE_SECRET, /* guest is being launched and ready to accept the ciphertext data */ |
| SEV_STATE_RUNNING, /* guest is fully launched and running */ |
| SEV_STATE_RECEIVING, /* guest is being migrated in from another SEV machine */ |
| SEV_STATE_SENDING /* guest is getting migrated out to another SEV machine */ |
| }; |
| |
| 7. KVM_SEV_DBG_DECRYPT |
| ---------------------- |
| |
| The KVM_SEV_DEBUG_DECRYPT command can be used by the hypervisor to request the |
| firmware to decrypt the data at the given memory region. |
| |
| Parameters (in): struct kvm_sev_dbg |
| |
| Returns: 0 on success, -negative on error |
| |
| :: |
| |
| struct kvm_sev_dbg { |
| __u64 src_uaddr; /* userspace address of data to decrypt */ |
| __u64 dst_uaddr; /* userspace address of destination */ |
| __u32 len; /* length of memory region to decrypt */ |
| }; |
| |
| The command returns an error if the guest policy does not allow debugging. |
| |
| 8. KVM_SEV_DBG_ENCRYPT |
| ---------------------- |
| |
| The KVM_SEV_DEBUG_ENCRYPT command can be used by the hypervisor to request the |
| firmware to encrypt the data at the given memory region. |
| |
| Parameters (in): struct kvm_sev_dbg |
| |
| Returns: 0 on success, -negative on error |
| |
| :: |
| |
| struct kvm_sev_dbg { |
| __u64 src_uaddr; /* userspace address of data to encrypt */ |
| __u64 dst_uaddr; /* userspace address of destination */ |
| __u32 len; /* length of memory region to encrypt */ |
| }; |
| |
| The command returns an error if the guest policy does not allow debugging. |
| |
| 9. KVM_SEV_LAUNCH_SECRET |
| ------------------------ |
| |
| The KVM_SEV_LAUNCH_SECRET command can be used by the hypervisor to inject secret |
| data after the measurement has been validated by the guest owner. |
| |
| Parameters (in): struct kvm_sev_launch_secret |
| |
| Returns: 0 on success, -negative on error |
| |
| :: |
| |
| struct kvm_sev_launch_secret { |
| __u64 hdr_uaddr; /* userspace address containing the packet header */ |
| __u32 hdr_len; |
| |
| __u64 guest_uaddr; /* the guest memory region where the secret should be injected */ |
| __u32 guest_len; |
| |
| __u64 trans_uaddr; /* the hypervisor memory region which contains the secret */ |
| __u32 trans_len; |
| }; |
| |
| References |
| ========== |
| |
| |
| See [white-paper]_, [api-spec]_, [amd-apm]_ and [kvm-forum]_ for more info. |
| |
| .. [white-paper] http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2013/12/AMD_Memory_Encryption_Whitepaper_v7-Public.pdf |
| .. [api-spec] https://support.amd.com/TechDocs/55766_SEV-KM_API_Specification.pdf |
| .. [amd-apm] https://support.amd.com/TechDocs/24593.pdf (section 15.34) |
| .. [kvm-forum] https://www.linux-kvm.org/images/7/74/02x08A-Thomas_Lendacky-AMDs_Virtualizatoin_Memory_Encryption_Technology.pdf |
