diff options
| -rw-r--r-- | docs/system/i386/amd-memory-encryption.rst | 54 |
1 files changed, 50 insertions, 4 deletions
diff --git a/docs/system/i386/amd-memory-encryption.rst b/docs/system/i386/amd-memory-encryption.rst index 215946f813..dcf4add0e7 100644 --- a/docs/system/i386/amd-memory-encryption.rst +++ b/docs/system/i386/amd-memory-encryption.rst @@ -47,7 +47,7 @@ The guest policy is passed as plaintext. A hypervisor may choose to read it, but should not modify it (any modification of the policy bits will result in bad measurement). The guest policy is a 4-byte data structure containing several flags that restricts what can be done on a running SEV guest. -See KM Spec section 3 and 6.2 for more details. +See SEV API Spec ([SEVAPI]_) section 3 and 6.2 for more details. The guest policy can be provided via the ``policy`` property:: @@ -92,7 +92,7 @@ expects. ``LAUNCH_FINISH`` finalizes the guest launch and destroys the cryptographic context. -See SEV KM API Spec ([SEVKM]_) 'Launching a guest' usage flow (Appendix A) for the +See SEV API Spec ([SEVAPI]_) 'Launching a guest' usage flow (Appendix A) for the complete flow chart. To launch a SEV guest:: @@ -118,6 +118,49 @@ a SEV-ES guest: - Requires in-kernel irqchip - the burden is placed on the hypervisor to manage booting APs. +Calculating expected guest launch measurement +--------------------------------------------- + +In order to verify the guest launch measurement, The Guest Owner must compute +it in the exact same way as it is calculated by the AMD-SP. SEV API Spec +([SEVAPI]_) section 6.5.1 describes the AMD-SP operations: + + GCTX.LD is finalized, producing the hash digest of all plaintext data + imported into the guest. + + The launch measurement is calculated as: + + HMAC(0x04 || API_MAJOR || API_MINOR || BUILD || GCTX.POLICY || GCTX.LD || MNONCE; GCTX.TIK) + + where "||" represents concatenation. + +The values of API_MAJOR, API_MINOR, BUILD, and GCTX.POLICY can be obtained +from the ``query-sev`` qmp command. + +The value of MNONCE is part of the response of ``query-sev-launch-measure``: it +is the last 16 bytes of the base64-decoded data field (see SEV API Spec +([SEVAPI]_) section 6.5.2 Table 52: LAUNCH_MEASURE Measurement Buffer). + +The value of GCTX.LD is +``SHA256(firmware_blob || kernel_hashes_blob || vmsas_blob)``, where: + +* ``firmware_blob`` is the content of the entire firmware flash file (for + example, ``OVMF.fd``). Note that you must build a stateless firmware file + which doesn't use an NVRAM store, because the NVRAM area is not measured, and + therefore it is not secure to use a firmware which uses state from an NVRAM + store. +* if kernel is used, and ``kernel-hashes=on``, then ``kernel_hashes_blob`` is + the content of PaddedSevHashTable (including the zero padding), which itself + includes the hashes of kernel, initrd, and cmdline that are passed to the + guest. The PaddedSevHashTable struct is defined in ``target/i386/sev.c``. +* if SEV-ES is enabled (``policy & 0x4 != 0``), ``vmsas_blob`` is the + concatenation of all VMSAs of the guest vcpus. Each VMSA is 4096 bytes long; + its content is defined inside Linux kernel code as ``struct vmcb_save_area``, + or in AMD APM Volume 2 ([APMVOL2]_) Table B-2: VMCB Layout, State Save Area. + +If kernel hashes are not used, or SEV-ES is disabled, use empty blobs for +``kernel_hashes_blob`` and ``vmsas_blob`` as needed. + Debugging --------- @@ -142,8 +185,11 @@ References `AMD Memory Encryption whitepaper <https://developer.amd.com/wordpress/media/2013/12/AMD_Memory_Encryption_Whitepaper_v7-Public.pdf>`_ -.. [SEVKM] `Secure Encrypted Virtualization Key Management - <http://developer.amd.com/wordpress/media/2017/11/55766_SEV-KM-API_Specification.pdf>`_ +.. [SEVAPI] `Secure Encrypted Virtualization API + <https://www.amd.com/system/files/TechDocs/55766_SEV-KM_API_Specification.pdf>`_ + +.. [APMVOL2] `AMD64 Architecture Programmer's Manual Volume 2: System Programming + <https://www.amd.com/system/files/TechDocs/24593.pdf>`_ KVM Forum slides: |