Message-ID: <ffcf2585-feef-d86c-efbd-8a53f73437ad@kernel.org>
Date:   Fri, 12 Nov 2021 13:23:25 -0800
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Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support
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To:     Marc Orr <marcorr@google.com>,
        Sean Christopherson <seanjc@google.com>
Cc:     Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@intel.com>,
        Peter Gonda <pgonda@google.com>,
        Brijesh Singh <brijesh.singh@amd.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-coco@lists.linux.dev, linux-mm@kvack.org,
        linux-crypto@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <Thomas.Lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Sergio Lopez <slp@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dov Murik <dovmurik@linux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Michael Roth <Michael.Roth@amd.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Andi Kleen <ak@linux.intel.com>, tony.luck@intel.com,
        sathyanarayanan.kuppuswamy@linux.intel.com
References: <20210820155918.7518-1-brijesh.singh@amd.com>
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From:   Andy Lutomirski <luto@kernel.org>
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On 11/12/21 13:16, Marc Orr wrote:
>>> So cloud providers should have an interest to prevent such random stray
>>> accesses if they wanna have guests. :)
>>
>> Yes, but IMO inducing a fault in the guest because of _host_ bug is wrong.
> 
> I want to push back on "inducing a fault in the guest because of
> _host_ bug is wrong.". The guest is _required_ to be robust against
> the host maliciously (or accidentally) writing its memory. SNP
> security depends on the guest detecting such writes. Therefore, why is
> leveraging this system property that the guest will detect when its
> private memory has been written wrong?

 >
> Especially when its orders or
> magnitudes simpler than the alternative to have everything in the
> system -- kernel, user-space, and guest -- all coordinate to agree
> what's private and what's shared. Such a complex approach is likely to
> bring a lot of bugs, vulnerabilities, and limitations on future design
> into the picture.
> 

SEV-SNP, TDX, and any reasonable software solution all require that the 
host know which pages are private and which pages are shared.  Sure, the 
old SEV-ES Linux host implementation was very simple, but it's nasty and 
fundamentally can't support migration.