Re: [PATCH v8 28/40] KVM: SEV: Add documentation for SEV-SNP CPUID Enforcement

Borislav Petkov
2022-01-07T13:22:32Z

On Fri, Dec 10, 2021 at 09:43:20AM -0600, Brijesh Singh wrote:
> From: Michael Roth <michael.roth@amd.com>
>
> Update the documentation with SEV-SNP CPUID enforcement.
>
> Signed-off-by: Michael Roth <michael.roth@amd.com>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> .../virt/kvm/amd-memory-encryption.rst | 28 +++++++++++++++++++
> 1 file changed, 28 insertions(+)
>
> diff --git a/Documentation/virt/kvm/amd-memory-encryption.rst b/Documentation/virt/kvm/amd-memory-encryption.rst
> index 5c081c8c7164..aa8292fa579a 100644
> --- a/Documentation/virt/kvm/amd-memory-encryption.rst
> +++ b/Documentation/virt/kvm/amd-memory-encryption.rst
> @@ -427,6 +427,34 @@ issued by the hypervisor to make the guest ready for execution.
>
> Returns: 0 on success, -negative on error
>
> +SEV-SNP CPUID Enforcement
> +=========================
> +
> +SEV-SNP guests can access a special page that contains a table of CPUID values
> +that have been validated by the PSP as part of SNP_LAUNCH_UPDATE firmware
^
the

> +command. It provides the following assurances regarding the validity of CPUID
> +values:
> +
> + - Its address is obtained via bootloader/firmware (via CC blob), whose
> + binares will be measured as part of the SEV-SNP attestation report.

Unknown word [binares] in Documentation.
Suggestions: ['binaries', 'Linares', 'bi nares', 'bi-nares', 'bin ares', 'bin-ares', 'nares']

Also:

s/whose binaries/and those binaries/

> + - Its initial state will be encrypted/pvalidated, so attempts to modify
> + it during run-time will be result in garbage being written, or #VC

s/be //

> + exceptions being generated due to changes in validation state if the
> + hypervisor tries to swap the backing page.
> + - Attempts to bypass PSP checks by hypervisor by using a normal page, or a
^
the

> + non-CPUID encrypted page will change the measurement provided by the
> + SEV-SNP attestation report.
> + - The CPUID page contents are *not* measured, but attempts to modify the
> + expected contents of a CPUID page as part of guest initialization will be
> + gated by the PSP CPUID enforcement policy checks performed on the page
> + during SNP_LAUNCH_UPDATE, and noticeable later if the guest owner
> + implements their own checks of the CPUID values.
> +
> +It is important to note that this last assurance is only useful if the kernel
> +has taken care to make use of the SEV-SNP CPUID throughout all stages of boot.
> +Otherwise guest owner attestation provides no assurance that the kernel wasn't
^
,

> +fed incorrect values at some point during boot.
> +
> References
> ==========
>
> --
> 2.25.1
>

--
Regards/Gruss,
Boris.