Re: [PATCH v3] docs/system/i386: Add measurement calculation details to amd-memory-encryption

[Dr. David Alan Gilbert]

* Dov Murik (dovmurik@linux.ibm.com) wrote:
> 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>
> 

Thanks; my guess is we're going to need to document the expected VMSA
values at some point.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> ---
> 
> v2:
> - Explain that firmware must be built without NVRAM store.
> 
> v3:
> - rstify
> ---
>  docs/system/i386/amd-memory-encryption.rst | 54 ++++++++++++++++++++--
>  1 file 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:
>  
> 
> base-commit: c13b8e9973635f34f3ce4356af27a311c993729c
> -- 
> 2.25.1
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK