linux-kernel - Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory

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Message-Id: <a979ba51-8693-43a0-bebb-b8b1938ff74c@www.fastmail.com>
Date:   Thu, 02 Sep 2021 13:42:28 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Dave Hansen" <dave.hansen@...el.com>,
        "Sean Christopherson" <seanjc@...gle.com>
Cc:     "Joerg Roedel" <jroedel@...e.de>,
        "Yu Zhang" <yu.c.zhang@...ux.intel.com>,
        "David Hildenbrand" <david@...hat.com>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        "Vitaly Kuznetsov" <vkuznets@...hat.com>,
        "Wanpeng Li" <wanpengli@...cent.com>,
        "Jim Mattson" <jmattson@...gle.com>,
        "Joerg Roedel" <joro@...tes.org>, "kvm list" <kvm@...r.kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Borislav Petkov" <bp@...en8.de>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        "Andi Kleen" <ak@...ux.intel.com>,
        "David Rientjes" <rientjes@...gle.com>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Tom Lendacky" <thomas.lendacky@....com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Ingo Molnar" <mingo@...hat.com>,
        "Varad Gautam" <varad.gautam@...e.com>,
        "Dario Faggioli" <dfaggioli@...e.com>,
        "the arch/x86 maintainers" <x86@...nel.org>, linux-mm@...ck.org,
        linux-coco@...ts.linux.dev,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Sathyanarayanan Kuppuswamy" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private memory



On Thu, Sep 2, 2021, at 12:07 PM, Dave Hansen wrote:
> On 9/2/21 11:57 AM, Sean Christopherson wrote:
> > On Thu, Sep 02, 2021, Andy Lutomirski wrote:
> >> On 9/2/21 2:27 AM, Joerg Roedel wrote:
> >>> On Wed, Sep 01, 2021 at 09:07:59AM -0700, Andy Lutomirski wrote:
> >>>> In principle, you could actually initialize a TDX guest with all of its
> >>>> memory shared and all of it mapped in the host IOMMU.
> >>> Not sure how this works in TDX, but in SEV code fetches are always
> >>> treated as encrypted. So this approach would not work with SEV, not to
> >>> speak about attestation, which will not work with this approach either
> >>> :)
> >>>
> >> Oof.
> > TDX is kinda similar.  _All_ accesses are private if paging is disabled because
> > the shared bit is either bit 48 or bit 51 in the GPA, i.e. can't be reached if
> > paging is disabled.  The vCPU is hardcoded to start in unpaged protected mode,
> > so at least some amount of guest memory needs to be private.
> 
> That's a rule we should definitely add to our page table checker.  Just
> like how we can look for W+X, we should also look for Shared+X.
> 

The only case I can thing of where the TDX vs SEV rule matters is for some mildly crazy user who wants to run user code out of an unencrypted DAX device (or virtio-fs, I guess).  We can save that for another year :)