linux-kernel - Re: [PATCH v11 44/45] virt: sevguest: Add support to get extended report

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Message-ID: <YiIc7aliqChnWThP@nazgul.tnic>
Date:   Fri, 4 Mar 2022 15:06:49 +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.shutemov@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        brijesh.ksingh@...il.com, tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v11 44/45] virt: sevguest: Add support to get extended
 report

On Thu, Mar 03, 2022 at 10:47:20AM -0600, Brijesh Singh wrote:
> I did not fail on !req.cert_len, because my read of the GHCB spec says that
> additional data (certificate blob) is optional. A user could call
> SNP_GET_EXT_REPORT without asking for the extended certificate. In this
> case, SNP_GET_EXT_REPORT == SNP_GET_REPORT.
> 
> Text from the GHCB spec section 4.1.8
> ---------------
> https://developer.amd.com/wp-content/resources/56421.pdf
> 
> The SNP Extended Guest Request NAE event is very similar to the SNP Guest
> Request NAE event. The difference is related to the additional data that can
> be returned based on the guest request. Any SNP Guest Request that does not
> support returning additional data must execute as if invoked as an SNP Guest
> Request.
> --------------

Sorry, it is still not clear to me how

"without asking for the extended certificate" == !req.certs_len

That's not explained in the help text either. And ->certs_len is part of
the input structure but nowhere does it say that when that thing is 0,
the request will be downgraded to a SNP_GET_REPORT.

How is the user of this supposed to know?

And regardless, you can still streamline the code as in the example I
gave so that it is clear which values are checked and for which does the
request get failed...

-- 
Regards/Gruss,
    Boris.

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