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From:   Marc Orr <marcorr@google.com>
Date:   Sat, 13 Nov 2021 23:54:03 -0800
Message-ID: <CAA03e5Hhmji-uhv4eh4cgyu0XBf9=C5r8MtGtWcB480eaVGvSg@mail.gmail.com>
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support
To:     Sean Christopherson <seanjc@google.com>
Cc:     Peter Gonda <pgonda@google.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@intel.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>,
        Andy Lutomirski <luto@kernel.org>,
        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
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On Sat, Nov 13, 2021 at 10:35 AM Sean Christopherson <seanjc@google.com> wrote:
>
> On Fri, Nov 12, 2021, Marc Orr wrote:
> > > > > If *it* is the host kernel, then you probably shouldn't do that -
> > > > > otherwise you just killed the host kernel on which all those guests are
> > > > > running.
> > > >
> > > > I agree, it seems better to terminate the single guest with an issue.
> > > > Rather than killing the host (and therefore all guests). So I'd
> > > > suggest even in this case we do the 'convert to shared' approach or
> > > > just outright terminate the guest.
> > > >
> > > > Are there already examples in KVM of a KVM bug in servicing a VM's
> > > > request results in a BUG/panic/oops? That seems not ideal ever.
> > >
> > > Plenty of examples.  kvm_spurious_fault() is the obvious one.  Any NULL pointer
> > > deref will lead to a BUG, etc...  And it's not just KVM, e.g. it's possible, if
> > > unlikely, for the core kernel to run into guest private memory (e.g. if the kernel
> > > botches an RMP change), and if that happens there's no guarantee that the kernel
> > > can recover.
> > >
> > > I fully agree that ideally KVM would have a better sense of self-preservation,
> > > but IMO that's an orthogonal discussion.
> >
> > I don't think we should treat the possibility of crashing the host
> > with live VMs nonchalantly. It's a big deal. Doing so has big
> > implications on the probability that any cloud vendor wil bee able to
> > deploy this code to production. And aren't cloud vendors one of the
> > main use cases for all of this confidential compute stuff? I'm
> > honestly surprised that so many people are OK with crashing the host.
>
> I'm not treating it nonchalantly, merely acknowledging that (a) some flavors of kernel
> bugs (or hardware issues!) are inherently fatal to the system, and (b) crashing the
> host may be preferable to continuing on in certain cases, e.g. if continuing on has a
> high probablity of corrupting guest data.

I disagree. Crashing the host -- and _ALL_ of its VMs (including
non-confidential VMs) -- is not preferable to crashing a single SNP
VM. Especially when that SNP VM is guaranteed to detect the memory
corruption and react accordingly.