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

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Date:   Wed, 1 Sep 2021 09:49:24 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     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>,
        Joerg Roedel <jroedel@...e.de>,
        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>,
        Dave Hansen <dave.hansen@...el.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>
Subject: Re: [RFC] KVM: mm: fd-based approach for supporting KVM guest private
 memory

On 01.09.21 06:58, Andy Lutomirski wrote:
> On Tue, Aug 31, 2021, at 12:07 PM, David Hildenbrand wrote:
>> On 28.08.21 00:18, Sean Christopherson wrote:
>>> On Thu, Aug 26, 2021, David Hildenbrand wrote:
>>>> You'll end up with a VMA that corresponds to the whole file in a single
>>>> process only, and that cannot vanish, not even in parts.
>>>
>>> How would userspace tell the kernel to free parts of memory that it doesn't want
>>> assigned to the guest, e.g. to free memory that the guest has converted to
>>> not-private?
>>
>> I'd guess one possibility could be fallocate(FALLOC_FL_PUNCH_HOLE).
>>
>> Questions are: when would it actually be allowed to perform such a
>> destructive operation? Do we have to protect from that? How would KVM
>> protect from user space replacing private pages by shared pages in any
>> of the models we discuss?
>>
> 
> What do you mean?  If userspace maliciously replaces a shared page by a private page, then the guest crashes.

Assume we have private pages in a fd and fallocate(FALLOC_FL_PUNCH_HOLE) 
random pages the guest is still using. If we "only" crash the guest, 
everything is fine.

> 
> (The actual meaning here is a bit different on SNP-ES vs TDX.  In SNP-ES, a given GPA can be shared, private, or nonexistent.  A guest accesses it with a special bit set in the guest page tables to indicate whether it expects shared or private, and the CPU will produce an appropriate error if the bit doesn't match the page.

Rings a bell, thanks for reminding me.

  In TDX, there is actually an entirely separate shared vs private 
address space, and, in theory, a given "GPA" can exist as shared and as 
private at once.  The full guest n-bit GPA plus the shared/private bit 
is logically an N+1 bit address, and it's possible to map all of it at 
once, half shared, and half private.  In practice, the defined 
guest->host APIs don't really support that usage.

Thanks, that explains a lot.

-- 
Thanks,

David / dhildenb