1.. SPDX-License-Identifier: GPL-2.0 2 3=================================================================== 4The Definitive SEV Guest API Documentation 5=================================================================== 6 71. General description 8====================== 9 10The SEV API is a set of ioctls that are used by the guest or hypervisor 11to get or set a certain aspect of the SEV virtual machine. The ioctls belong 12to the following classes: 13 14 - Hypervisor ioctls: These query and set global attributes which affect the 15 whole SEV firmware. These ioctl are used by platform provisioning tools. 16 17 - Guest ioctls: These query and set attributes of the SEV virtual machine. 18 192. API description 20================== 21 22This section describes ioctls that is used for querying the SEV guest report 23from the SEV firmware. For each ioctl, the following information is provided 24along with a description: 25 26 Technology: 27 which SEV technology provides this ioctl. SEV, SEV-ES, SEV-SNP or all. 28 29 Type: 30 hypervisor or guest. The ioctl can be used inside the guest or the 31 hypervisor. 32 33 Parameters: 34 what parameters are accepted by the ioctl. 35 36 Returns: 37 the return value. General error numbers (-ENOMEM, -EINVAL) 38 are not detailed, but errors with specific meanings are. 39 40The guest ioctl should be issued on a file descriptor of the /dev/sev-guest device. 41The ioctl accepts struct snp_user_guest_request. The input and output structure is 42specified through the req_data and resp_data field respectively. If the ioctl fails 43to execute due to a firmware error, then fw_err code will be set otherwise the 44fw_err will be set to 0x00000000000000ff. 45 46The firmware checks that the message sequence counter is one greater than 47the guests message sequence counter. If guest driver fails to increment message 48counter (e.g. counter overflow), then -EIO will be returned. 49 50:: 51 52 struct snp_guest_request_ioctl { 53 /* Message version number */ 54 __u32 msg_version; 55 56 /* Request and response structure address */ 57 __u64 req_data; 58 __u64 resp_data; 59 60 /* firmware error code on failure (see psp-sev.h) */ 61 __u64 fw_err; 62 }; 63 642.1 SNP_GET_REPORT 65------------------ 66 67:Technology: sev-snp 68:Type: guest ioctl 69:Parameters (in): struct snp_report_req 70:Returns (out): struct snp_report_resp on success, -negative on error 71 72The SNP_GET_REPORT ioctl can be used to query the attestation report from the 73SEV-SNP firmware. The ioctl uses the SNP_GUEST_REQUEST (MSG_REPORT_REQ) command 74provided by the SEV-SNP firmware to query the attestation report. 75 76On success, the snp_report_resp.data will contains the report. The report 77contain the format described in the SEV-SNP specification. See the SEV-SNP 78specification for further details. 79 802.2 SNP_GET_DERIVED_KEY 81----------------------- 82:Technology: sev-snp 83:Type: guest ioctl 84:Parameters (in): struct snp_derived_key_req 85:Returns (out): struct snp_derived_key_resp on success, -negative on error 86 87The SNP_GET_DERIVED_KEY ioctl can be used to get a key derive from a root key. 88The derived key can be used by the guest for any purpose, such as sealing keys 89or communicating with external entities. 90 91The ioctl uses the SNP_GUEST_REQUEST (MSG_KEY_REQ) command provided by the 92SEV-SNP firmware to derive the key. See SEV-SNP specification for further details 93on the various fields passed in the key derivation request. 94 95On success, the snp_derived_key_resp.data contains the derived key value. See 96the SEV-SNP specification for further details. 97 98 992.3 SNP_GET_EXT_REPORT 100---------------------- 101:Technology: sev-snp 102:Type: guest ioctl 103:Parameters (in/out): struct snp_ext_report_req 104:Returns (out): struct snp_report_resp on success, -negative on error 105 106The SNP_GET_EXT_REPORT ioctl is similar to the SNP_GET_REPORT. The difference is 107related to the additional certificate data that is returned with the report. 108The certificate data returned is being provided by the hypervisor through the 109SNP_SET_EXT_CONFIG. 110 111The ioctl uses the SNP_GUEST_REQUEST (MSG_REPORT_REQ) command provided by the SEV-SNP 112firmware to get the attestation report. 113 114On success, the snp_ext_report_resp.data will contain the attestation report 115and snp_ext_report_req.certs_address will contain the certificate blob. If the 116length of the blob is smaller than expected then snp_ext_report_req.certs_len will 117be updated with the expected value. 118 119See GHCB specification for further detail on how to parse the certificate blob. 120 1213. SEV-SNP CPUID Enforcement 122============================ 123 124SEV-SNP guests can access a special page that contains a table of CPUID values 125that have been validated by the PSP as part of the SNP_LAUNCH_UPDATE firmware 126command. It provides the following assurances regarding the validity of CPUID 127values: 128 129 - Its address is obtained via bootloader/firmware (via CC blob), and those 130 binaries will be measured as part of the SEV-SNP attestation report. 131 - Its initial state will be encrypted/pvalidated, so attempts to modify 132 it during run-time will result in garbage being written, or #VC exceptions 133 being generated due to changes in validation state if the hypervisor tries 134 to swap the backing page. 135 - Attempts to bypass PSP checks by the hypervisor by using a normal page, or 136 a non-CPUID encrypted page will change the measurement provided by the 137 SEV-SNP attestation report. 138 - The CPUID page contents are *not* measured, but attempts to modify the 139 expected contents of a CPUID page as part of guest initialization will be 140 gated by the PSP CPUID enforcement policy checks performed on the page 141 during SNP_LAUNCH_UPDATE, and noticeable later if the guest owner 142 implements their own checks of the CPUID values. 143 144It is important to note that this last assurance is only useful if the kernel 145has taken care to make use of the SEV-SNP CPUID throughout all stages of boot. 146Otherwise, guest owner attestation provides no assurance that the kernel wasn't 147fed incorrect values at some point during boot. 148 149 150Reference 151--------- 152 153SEV-SNP and GHCB specification: developer.amd.com/sev 154 155The driver is based on SEV-SNP firmware spec 0.9 and GHCB spec version 2.0. 156