The Definitive SEV Guest API Documentation¶

1. General description¶

The SEV API is a set of ioctls that are used by the guest or hypervisor to get or set a certain aspect of the SEV virtual machine. The ioctls belong to the following classes:

Hypervisor ioctls: These query and set global attributes which affect the whole SEV firmware. These ioctl are used by platform provisioning tools.

Guest ioctls: These query and set attributes of the SEV virtual machine.

2. API description¶

This section describes ioctls that is used for querying the SEV guest report from the SEV firmware. For each ioctl, the following information is provided along with a description:

Technology:
which SEV technology provides this ioctl. SEV, SEV-ES, SEV-SNP or all.

Type:
hypervisor or guest. The ioctl can be used inside the guest or the hypervisor.

Parameters:
what parameters are accepted by the ioctl.

Returns:
the return value. General error numbers (-ENOMEM, -EINVAL) are not detailed, but errors with specific meanings are.

The guest ioctl should be issued on a file descriptor of the /dev/sev-guest device. The ioctl accepts struct snp_user_guest_request. The input and output structure is specified through the req_data and resp_data field respectively. If the ioctl fails to execute due to a firmware error, then the fw_error code will be set, otherwise fw_error will be set to -1.

The firmware checks that the message sequence counter is one greater than the guests message sequence counter. If guest driver fails to increment message counter (e.g. counter overflow), then -EIO will be returned.

struct snp_guest_request_ioctl {
        /* Message version number */
        __u32 msg_version;

        /* Request and response structure address */
        __u64 req_data;
        __u64 resp_data;

        /* bits[63:32]: VMM error code, bits[31:0] firmware error code (see psp-sev.h) */
        union {
                __u64 exitinfo2;
                struct {
                        __u32 fw_error;
                        __u32 vmm_error;
                };
        };
};

2.1 SNP_GET_REPORT¶

Technology:: sev-snp
Type:: guest ioctl
Parameters (in):: struct snp_report_req
Returns (out):: struct snp_report_resp on success, -negative on error

The SNP_GET_REPORT ioctl can be used to query the attestation report from the SEV-SNP firmware. The ioctl uses the SNP_GUEST_REQUEST (MSG_REPORT_REQ) command provided by the SEV-SNP firmware to query the attestation report.

On success, the snp_report_resp.data will contains the report. The report contain the format described in the SEV-SNP specification. See the SEV-SNP specification for further details.

2.2 SNP_GET_DERIVED_KEY¶

Technology:: sev-snp
Type:: guest ioctl
Parameters (in):: struct snp_derived_key_req
Returns (out):: struct snp_derived_key_resp on success, -negative on error

The SNP_GET_DERIVED_KEY ioctl can be used to get a key derive from a root key. The derived key can be used by the guest for any purpose, such as sealing keys or communicating with external entities.

The ioctl uses the SNP_GUEST_REQUEST (MSG_KEY_REQ) command provided by the SEV-SNP firmware to derive the key. See SEV-SNP specification for further details on the various fields passed in the key derivation request.

On success, the snp_derived_key_resp.data contains the derived key value. See the SEV-SNP specification for further details.

2.3 SNP_GET_EXT_REPORT¶

Technology:: sev-snp
Type:: guest ioctl
Parameters (in/out):: struct snp_ext_report_req
Returns (out):: struct snp_report_resp on success, -negative on error

The SNP_GET_EXT_REPORT ioctl is similar to the SNP_GET_REPORT. The difference is related to the additional certificate data that is returned with the report. The certificate data returned is being provided by the hypervisor through the SNP_SET_EXT_CONFIG.

The ioctl uses the SNP_GUEST_REQUEST (MSG_REPORT_REQ) command provided by the SEV-SNP firmware to get the attestation report.

On success, the snp_ext_report_resp.data will contain the attestation report and snp_ext_report_req.certs_address will contain the certificate blob. If the length of the blob is smaller than expected then snp_ext_report_req.certs_len will be updated with the expected value.

See GHCB specification for further detail on how to parse the certificate blob.

3. SEV-SNP CPUID Enforcement¶

SEV-SNP guests can access a special page that contains a table of CPUID values that have been validated by the PSP as part of the SNP_LAUNCH_UPDATE firmware command. It provides the following assurances regarding the validity of CPUID values:

Its address is obtained via bootloader/firmware (via CC blob), and those binaries will be measured as part of the SEV-SNP attestation report.

Its initial state will be encrypted/pvalidated, so attempts to modify it during run-time will result in garbage being written, or #VC exceptions being generated due to changes in validation state if the hypervisor tries to swap the backing page.

Attempts to bypass PSP checks by the hypervisor by using a normal page, or a non-CPUID encrypted page will change the measurement provided by the SEV-SNP attestation report.

The CPUID page contents are not measured, but attempts to modify the expected contents of a CPUID page as part of guest initialization will be gated by the PSP CPUID enforcement policy checks performed on the page during SNP_LAUNCH_UPDATE, and noticeable later if the guest owner implements their own checks of the CPUID values.

It is important to note that this last assurance is only useful if the kernel has taken care to make use of the SEV-SNP CPUID throughout all stages of boot. Otherwise, guest owner attestation provides no assurance that the kernel wasn’t fed incorrect values at some point during boot.

Reference¶

SEV-SNP and GHCB specification: developer.amd.com/sev

The driver is based on SEV-SNP firmware spec 0.9 and GHCB spec version 2.0.