amd-memory-encryption.rst (8979B)
1 AMD Secure Encrypted Virtualization (SEV) 2 ========================================= 3 4 Secure Encrypted Virtualization (SEV) is a feature found on AMD processors. 5 6 SEV is an extension to the AMD-V architecture which supports running encrypted 7 virtual machines (VMs) under the control of KVM. Encrypted VMs have their pages 8 (code and data) secured such that only the guest itself has access to the 9 unencrypted version. Each encrypted VM is associated with a unique encryption 10 key; if its data is accessed by a different entity using a different key the 11 encrypted guests data will be incorrectly decrypted, leading to unintelligible 12 data. 13 14 Key management for this feature is handled by a separate processor known as the 15 AMD secure processor (AMD-SP), which is present in AMD SOCs. Firmware running 16 inside the AMD-SP provides commands to support a common VM lifecycle. This 17 includes commands for launching, snapshotting, migrating and debugging the 18 encrypted guest. These SEV commands can be issued via KVM_MEMORY_ENCRYPT_OP 19 ioctls. 20 21 Secure Encrypted Virtualization - Encrypted State (SEV-ES) builds on the SEV 22 support to additionally protect the guest register state. In order to allow a 23 hypervisor to perform functions on behalf of a guest, there is architectural 24 support for notifying a guest's operating system when certain types of VMEXITs 25 are about to occur. This allows the guest to selectively share information with 26 the hypervisor to satisfy the requested function. 27 28 Launching 29 --------- 30 31 Boot images (such as bios) must be encrypted before a guest can be booted. The 32 ``MEMORY_ENCRYPT_OP`` ioctl provides commands to encrypt the images: ``LAUNCH_START``, 33 ``LAUNCH_UPDATE_DATA``, ``LAUNCH_MEASURE`` and ``LAUNCH_FINISH``. These four commands 34 together generate a fresh memory encryption key for the VM, encrypt the boot 35 images and provide a measurement than can be used as an attestation of a 36 successful launch. 37 38 For a SEV-ES guest, the ``LAUNCH_UPDATE_VMSA`` command is also used to encrypt the 39 guest register state, or VM save area (VMSA), for all of the guest vCPUs. 40 41 ``LAUNCH_START`` is called first to create a cryptographic launch context within 42 the firmware. To create this context, guest owner must provide a guest policy, 43 its public Diffie-Hellman key (PDH) and session parameters. These inputs 44 should be treated as a binary blob and must be passed as-is to the SEV firmware. 45 46 The guest policy is passed as plaintext. A hypervisor may choose to read it, 47 but should not modify it (any modification of the policy bits will result 48 in bad measurement). The guest policy is a 4-byte data structure containing 49 several flags that restricts what can be done on a running SEV guest. 50 See SEV API Spec ([SEVAPI]_) section 3 and 6.2 for more details. 51 52 The guest policy can be provided via the ``policy`` property:: 53 54 # ${QEMU} \ 55 sev-guest,id=sev0,policy=0x1...\ 56 57 Setting the "SEV-ES required" policy bit (bit 2) will launch the guest as a 58 SEV-ES guest:: 59 60 # ${QEMU} \ 61 sev-guest,id=sev0,policy=0x5...\ 62 63 The guest owner provided DH certificate and session parameters will be used to 64 establish a cryptographic session with the guest owner to negotiate keys used 65 for the attestation. 66 67 The DH certificate and session blob can be provided via the ``dh-cert-file`` and 68 ``session-file`` properties:: 69 70 # ${QEMU} \ 71 sev-guest,id=sev0,dh-cert-file=<file1>,session-file=<file2> 72 73 ``LAUNCH_UPDATE_DATA`` encrypts the memory region using the cryptographic context 74 created via the ``LAUNCH_START`` command. If required, this command can be called 75 multiple times to encrypt different memory regions. The command also calculates 76 the measurement of the memory contents as it encrypts. 77 78 ``LAUNCH_UPDATE_VMSA`` encrypts all the vCPU VMSAs for a SEV-ES guest using the 79 cryptographic context created via the ``LAUNCH_START`` command. The command also 80 calculates the measurement of the VMSAs as it encrypts them. 81 82 ``LAUNCH_MEASURE`` can be used to retrieve the measurement of encrypted memory and, 83 for a SEV-ES guest, encrypted VMSAs. This measurement is a signature of the 84 memory contents and, for a SEV-ES guest, the VMSA contents, that can be sent 85 to the guest owner as an attestation that the memory and VMSAs were encrypted 86 correctly by the firmware. The guest owner may wait to provide the guest 87 confidential information until it can verify the attestation measurement. 88 Since the guest owner knows the initial contents of the guest at boot, the 89 attestation measurement can be verified by comparing it to what the guest owner 90 expects. 91 92 ``LAUNCH_FINISH`` finalizes the guest launch and destroys the cryptographic 93 context. 94 95 See SEV API Spec ([SEVAPI]_) 'Launching a guest' usage flow (Appendix A) for the 96 complete flow chart. 97 98 To launch a SEV guest:: 99 100 # ${QEMU} \ 101 -machine ...,confidential-guest-support=sev0 \ 102 -object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1 103 104 To launch a SEV-ES guest:: 105 106 # ${QEMU} \ 107 -machine ...,confidential-guest-support=sev0 \ 108 -object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1,policy=0x5 109 110 An SEV-ES guest has some restrictions as compared to a SEV guest. Because the 111 guest register state is encrypted and cannot be updated by the VMM/hypervisor, 112 a SEV-ES guest: 113 114 - Does not support SMM - SMM support requires updating the guest register 115 state. 116 - Does not support reboot - a system reset requires updating the guest register 117 state. 118 - Requires in-kernel irqchip - the burden is placed on the hypervisor to 119 manage booting APs. 120 121 Calculating expected guest launch measurement 122 --------------------------------------------- 123 124 In order to verify the guest launch measurement, The Guest Owner must compute 125 it in the exact same way as it is calculated by the AMD-SP. SEV API Spec 126 ([SEVAPI]_) section 6.5.1 describes the AMD-SP operations: 127 128 GCTX.LD is finalized, producing the hash digest of all plaintext data 129 imported into the guest. 130 131 The launch measurement is calculated as: 132 133 HMAC(0x04 || API_MAJOR || API_MINOR || BUILD || GCTX.POLICY || GCTX.LD || MNONCE; GCTX.TIK) 134 135 where "||" represents concatenation. 136 137 The values of API_MAJOR, API_MINOR, BUILD, and GCTX.POLICY can be obtained 138 from the ``query-sev`` qmp command. 139 140 The value of MNONCE is part of the response of ``query-sev-launch-measure``: it 141 is the last 16 bytes of the base64-decoded data field (see SEV API Spec 142 ([SEVAPI]_) section 6.5.2 Table 52: LAUNCH_MEASURE Measurement Buffer). 143 144 The value of GCTX.LD is 145 ``SHA256(firmware_blob || kernel_hashes_blob || vmsas_blob)``, where: 146 147 * ``firmware_blob`` is the content of the entire firmware flash file (for 148 example, ``OVMF.fd``). Note that you must build a stateless firmware file 149 which doesn't use an NVRAM store, because the NVRAM area is not measured, and 150 therefore it is not secure to use a firmware which uses state from an NVRAM 151 store. 152 * if kernel is used, and ``kernel-hashes=on``, then ``kernel_hashes_blob`` is 153 the content of PaddedSevHashTable (including the zero padding), which itself 154 includes the hashes of kernel, initrd, and cmdline that are passed to the 155 guest. The PaddedSevHashTable struct is defined in ``target/i386/sev.c``. 156 * if SEV-ES is enabled (``policy & 0x4 != 0``), ``vmsas_blob`` is the 157 concatenation of all VMSAs of the guest vcpus. Each VMSA is 4096 bytes long; 158 its content is defined inside Linux kernel code as ``struct vmcb_save_area``, 159 or in AMD APM Volume 2 ([APMVOL2]_) Table B-2: VMCB Layout, State Save Area. 160 161 If kernel hashes are not used, or SEV-ES is disabled, use empty blobs for 162 ``kernel_hashes_blob`` and ``vmsas_blob`` as needed. 163 164 Debugging 165 --------- 166 167 Since the memory contents of a SEV guest are encrypted, hypervisor access to 168 the guest memory will return cipher text. If the guest policy allows debugging, 169 then a hypervisor can use the DEBUG_DECRYPT and DEBUG_ENCRYPT commands to access 170 the guest memory region for debug purposes. This is not supported in QEMU yet. 171 172 Snapshot/Restore 173 ---------------- 174 175 TODO 176 177 Live Migration 178 --------------- 179 180 TODO 181 182 References 183 ---------- 184 185 `AMD Memory Encryption whitepaper 186 <https://developer.amd.com/wordpress/media/2013/12/AMD_Memory_Encryption_Whitepaper_v7-Public.pdf>`_ 187 188 .. [SEVAPI] `Secure Encrypted Virtualization API 189 <https://www.amd.com/system/files/TechDocs/55766_SEV-KM_API_Specification.pdf>`_ 190 191 .. [APMVOL2] `AMD64 Architecture Programmer's Manual Volume 2: System Programming 192 <https://www.amd.com/system/files/TechDocs/24593.pdf>`_ 193 194 KVM Forum slides: 195 196 * `AMD’s Virtualization Memory Encryption (2016) 197 <http://www.linux-kvm.org/images/7/74/02x08A-Thomas_Lendacky-AMDs_Virtualizatoin_Memory_Encryption_Technology.pdf>`_ 198 * `Extending Secure Encrypted Virtualization With SEV-ES (2018) 199 <https://www.linux-kvm.org/images/9/94/Extending-Secure-Encrypted-Virtualization-with-SEV-ES-Thomas-Lendacky-AMD.pdf>`_ 200 201 `AMD64 Architecture Programmer's Manual: 202 <http://support.amd.com/TechDocs/24593.pdf>`_ 203 204 * SME is section 7.10 205 * SEV is section 15.34 206 * SEV-ES is section 15.35