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 41device. The ioctl accepts struct snp_user_guest_request. The input and 42output structure is specified through the req_data and resp_data field 43respectively. If the ioctl fails to execute due to a firmware error, then 44the fw_error code will be set, otherwise fw_error will be set to -1. 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 /* bits[63:32]: VMM error code, bits[31:0] firmware error code (see psp-sev.h) */ 61 union { 62 __u64 exitinfo2; 63 struct { 64 __u32 fw_error; 65 __u32 vmm_error; 66 }; 67 }; 68 }; 69 70The host ioctls are issued to a file descriptor of the /dev/sev device. 71The ioctl accepts the command ID/input structure documented below. 72 73:: 74 75 struct sev_issue_cmd { 76 /* Command ID */ 77 __u32 cmd; 78 79 /* Command request structure */ 80 __u64 data; 81 82 /* Firmware error code on failure (see psp-sev.h) */ 83 __u32 error; 84 }; 85 86 872.1 SNP_GET_REPORT 88------------------ 89 90:Technology: sev-snp 91:Type: guest ioctl 92:Parameters (in): struct snp_report_req 93:Returns (out): struct snp_report_resp on success, -negative on error 94 95The SNP_GET_REPORT ioctl can be used to query the attestation report from the 96SEV-SNP firmware. The ioctl uses the SNP_GUEST_REQUEST (MSG_REPORT_REQ) command 97provided by the SEV-SNP firmware to query the attestation report. 98 99On success, the snp_report_resp.data will contains the report. The report 100contain the format described in the SEV-SNP specification. See the SEV-SNP 101specification for further details. 102 1032.2 SNP_GET_DERIVED_KEY 104----------------------- 105:Technology: sev-snp 106:Type: guest ioctl 107:Parameters (in): struct snp_derived_key_req 108:Returns (out): struct snp_derived_key_resp on success, -negative on error 109 110The SNP_GET_DERIVED_KEY ioctl can be used to get a key derive from a root key. 111The derived key can be used by the guest for any purpose, such as sealing keys 112or communicating with external entities. 113 114The ioctl uses the SNP_GUEST_REQUEST (MSG_KEY_REQ) command provided by the 115SEV-SNP firmware to derive the key. See SEV-SNP specification for further details 116on the various fields passed in the key derivation request. 117 118On success, the snp_derived_key_resp.data contains the derived key value. See 119the SEV-SNP specification for further details. 120 121 1222.3 SNP_GET_EXT_REPORT 123---------------------- 124:Technology: sev-snp 125:Type: guest ioctl 126:Parameters (in/out): struct snp_ext_report_req 127:Returns (out): struct snp_report_resp on success, -negative on error 128 129The SNP_GET_EXT_REPORT ioctl is similar to the SNP_GET_REPORT. The difference is 130related to the additional certificate data that is returned with the report. 131The certificate data returned is being provided by the hypervisor through the 132SNP_SET_EXT_CONFIG. 133 134The ioctl uses the SNP_GUEST_REQUEST (MSG_REPORT_REQ) command provided by the SEV-SNP 135firmware to get the attestation report. 136 137On success, the snp_ext_report_resp.data will contain the attestation report 138and snp_ext_report_req.certs_address will contain the certificate blob. If the 139length of the blob is smaller than expected then snp_ext_report_req.certs_len will 140be updated with the expected value. 141 142See GHCB specification for further detail on how to parse the certificate blob. 143 1442.4 SNP_PLATFORM_STATUS 145----------------------- 146:Technology: sev-snp 147:Type: hypervisor ioctl cmd 148:Parameters (out): struct sev_user_data_snp_status 149:Returns (out): 0 on success, -negative on error 150 151The SNP_PLATFORM_STATUS command is used to query the SNP platform status. The 152status includes API major, minor version and more. See the SEV-SNP 153specification for further details. 154 1552.5 SNP_COMMIT 156-------------- 157:Technology: sev-snp 158:Type: hypervisor ioctl cmd 159:Returns (out): 0 on success, -negative on error 160 161SNP_COMMIT is used to commit the currently installed firmware using the 162SEV-SNP firmware SNP_COMMIT command. This prevents roll-back to a previously 163committed firmware version. This will also update the reported TCB to match 164that of the currently installed firmware. 165 1662.6 SNP_SET_CONFIG 167------------------ 168:Technology: sev-snp 169:Type: hypervisor ioctl cmd 170:Parameters (in): struct sev_user_data_snp_config 171:Returns (out): 0 on success, -negative on error 172 173SNP_SET_CONFIG is used to set the system-wide configuration such as 174reported TCB version in the attestation report. The command is similar 175to SNP_CONFIG command defined in the SEV-SNP spec. The current values of 176the firmware parameters affected by this command can be queried via 177SNP_PLATFORM_STATUS. 178 1793. SEV-SNP CPUID Enforcement 180============================ 181 182SEV-SNP guests can access a special page that contains a table of CPUID values 183that have been validated by the PSP as part of the SNP_LAUNCH_UPDATE firmware 184command. It provides the following assurances regarding the validity of CPUID 185values: 186 187 - Its address is obtained via bootloader/firmware (via CC blob), and those 188 binaries will be measured as part of the SEV-SNP attestation report. 189 - Its initial state will be encrypted/pvalidated, so attempts to modify 190 it during run-time will result in garbage being written, or #VC exceptions 191 being generated due to changes in validation state if the hypervisor tries 192 to swap the backing page. 193 - Attempts to bypass PSP checks by the hypervisor by using a normal page, or 194 a non-CPUID encrypted page will change the measurement provided by the 195 SEV-SNP attestation report. 196 - The CPUID page contents are *not* measured, but attempts to modify the 197 expected contents of a CPUID page as part of guest initialization will be 198 gated by the PSP CPUID enforcement policy checks performed on the page 199 during SNP_LAUNCH_UPDATE, and noticeable later if the guest owner 200 implements their own checks of the CPUID values. 201 202It is important to note that this last assurance is only useful if the kernel 203has taken care to make use of the SEV-SNP CPUID throughout all stages of boot. 204Otherwise, guest owner attestation provides no assurance that the kernel wasn't 205fed incorrect values at some point during boot. 206 207 208Reference 209--------- 210 211SEV-SNP and GHCB specification: developer.amd.com/sev 212 213The driver is based on SEV-SNP firmware spec 0.9 and GHCB spec version 2.0. 214