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docs/system/i386: Add measurement calculation details to amd-memory-encryption 

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 links to the SEV API Spec document.

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

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20220217110059.2320497-1-dovmurik@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Dov Murik 2022-02-17 11:00:59 +00:00 committed by Paolo Bonzini
parent 7bd16378bb
commit 776a6a32b4
1 changed files with 50 additions and 4 deletions
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54
docs/system/i386/amd-memory-encryption.rst
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@ -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: