======================================
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_MEMORY_ENCRYPT_OP.  If the argument
to KVM_MEMORY_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_MEMORY_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
