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

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Message-ID: <Ydg+lNhVmQbJ+Lxb@zn.tnic>
Date:   Fri, 7 Jan 2022 14:22:28 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v8 28/40] KVM: SEV: Add documentation for SEV-SNP CPUID
 Enforcement

On Fri, Dec 10, 2021 at 09:43:20AM -0600, Brijesh Singh wrote:
> From: Michael Roth <michael.roth@....com>
> 
> Update the documentation with SEV-SNP CPUID enforcement.
> 
> Signed-off-by: Michael Roth <michael.roth@....com>
> Signed-off-by: Brijesh Singh <brijesh.singh@....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.

https://people.kernel.org/tglx/notes-about-netiquette