linux-kernel - Re: [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM

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Date:   Wed, 22 Feb 2023 13:53:17 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Mike Rapoport <rppt@...nel.org>,
        Chao Peng <chao.p.peng@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Arnd Bergmann <arnd@...db.de>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Miaohe Lin <linmiaohe@...wei.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shuah Khan <shuah@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, aarcange@...hat.com, ddutile@...hat.com,
        dhildenb@...hat.com, Quentin Perret <qperret@...gle.com>,
        tabba@...gle.com, Michael Roth <michael.roth@....com>,
        mhocko@...e.com, wei.w.wang@...el.com
Subject: Re: [PATCH v10 0/9] KVM: mm: fd-based approach for supporting KVM

On Thu, Feb 16, 2023, David Hildenbrand wrote:
> On 16.02.23 06:13, Mike Rapoport wrote:
> > Hi,
> > 
> > On Fri, Dec 02, 2022 at 02:13:38PM +0800, Chao Peng wrote:
> > > This patch series implements KVM guest private memory for confidential
> > > computing scenarios like Intel TDX[1]. If a TDX host accesses
> > > TDX-protected guest memory, machine check can happen which can further
> > > crash the running host system, this is terrible for multi-tenant
> > > configurations. The host accesses include those from KVM userspace like
> > > QEMU. This series addresses KVM userspace induced crash by introducing
> > > new mm and KVM interfaces so KVM userspace can still manage guest memory
> > > via a fd-based approach, but it can never access the guest memory
> > > content.
> > 
> > Sorry for jumping late.
> > 
> > Unless I'm missing something, hibernation will also cause an machine check
> > when there is TDX-protected memory in the system. When the hibernation
> > creates memory snapshot it essentially walks all physical pages and saves
> > their contents, so for TDX memory this will trigger machine check, right?

For hibernation specifically, I think that should be handled elsewhere as hibernation
is simply incompatible with TDX, SNP, pKVM, etc. without paravirtualizing the
guest, as none of those technologies support auto-export a la s390.  I suspect
the right approach is to disallow hibernation if KVM is running any protected guests.

> I recall bringing that up in the past (also memory access due to kdump,
> /prov/kcore) and was told that the main focus for now is preventing
> unprivileged users from crashing the system, that is, not mapping such
> memory into user space (e.g., QEMU). In the long run, we'll want to handle
> such pages also properly in the other events where the kernel might access
> them.

Ya, unless someone strongly objects, the plan is to essentially treat "attacks"
from privileged users as out of to scope for initial support, and then iterate
as needed to fix/enable more features.

FWIW, read accesses, e.g. kdump, should be ok for TDX and SNP as they both play
nice with "bad" reads.  pKVM is a different beast though as I believe any access
to guest private memory will fault.  But my understanding is that this series
would be a big step forward for pKVM, which currently doesn't have any safeguards.