[PATCH v2] docs: Add measurement calculation details to amd-memory-encryption.txt

[Dov Murik <dovmurik@linux.ibm.com>]

Add a section explaining how the Guest Owner should calculate the
expected guest launch measurement for SEV and SEV-ES.

Also update the name and link to the SEV API Spec document.

Signed-off-by: Dov Murik <dovmurik@linux.ibm.com>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>

---

v2:
- Explain that firmware must be built without NVRAM store.
---
 docs/amd-memory-encryption.txt | 52 +++++++++++++++++++++++++++++++---
 1 file changed, 48 insertions(+), 4 deletions(-)

diff --git a/docs/amd-memory-encryption.txt b/docs/amd-memory-encryption.txt
index ffca382b5f..fcb712ee90 100644
--- a/docs/amd-memory-encryption.txt
+++ b/docs/amd-memory-encryption.txt
@@ -43,7 +43,7 @@ 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 [1] section 3 and 6.2 for more details.
 
 The guest policy can be provided via the 'policy' property (see below)
 
@@ -88,7 +88,7 @@ expects.
 LAUNCH_FINISH finalizes the guest launch and destroys the cryptographic
 context.
 
-See SEV KM API Spec [1] 'Launching a guest' usage flow (Appendix A) for the
+See SEV API Spec [1] 'Launching a guest' usage flow (Appendix A) for the
 complete flow chart.
 
 To launch a SEV guest
@@ -113,6 +113,47 @@ 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 [1]
+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 [1]
+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 [2] 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
 -----------
 Since the memory contents of a SEV guest are encrypted, hypervisor access to
@@ -134,8 +175,11 @@ References
 AMD Memory Encryption whitepaper:
 https://developer.amd.com/wordpress/media/2013/12/AMD_Memory_Encryption_Whitepaper_v7-Public.pdf
 
-Secure Encrypted Virtualization Key Management:
-[1] http://developer.amd.com/wordpress/media/2017/11/55766_SEV-KM-API_Specification.pdf
+Secure Encrypted Virtualization API:
+[1] https://www.amd.com/system/files/TechDocs/55766_SEV-KM_API_Specification.pdf
+
+AMD64 Architecture Programmer's Manual Volume 2: System Programming
+[2] https://www.amd.com/system/files/TechDocs/24593.pdf
 
 KVM Forum slides:
 http://www.linux-kvm.org/images/7/74/02x08A-Thomas_Lendacky-AMDs_Virtualizatoin_Memory_Encryption_Technology.pdf

base-commit: 212a33d3b0c65ae2583bb1d06cb140cd0890894c
-- 
2.25.1