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Date:   Mon, 15 Nov 2021 21:00:23 -0800
From:   "Andy Lutomirski" <luto@kernel.org>
To:     "Marc Orr" <marcorr@google.com>,
        "Sean Christopherson" <seanjc@google.com>
Cc:     "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        "Borislav Petkov" <bp@alien8.de>,
        "Dave Hansen" <dave.hansen@intel.com>,
        "Peter Gonda" <pgonda@google.com>,
        "Brijesh Singh" <brijesh.singh@amd.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        "kvm list" <kvm@vger.kernel.org>, linux-coco@lists.linux.dev,
        linux-mm@kvack.org,
        "Linux Crypto Mailing List" <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 (Intel)" <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" <tony.luck@intel.com>,
        "Sathyanarayanan Kuppuswamy" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor
 Support
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On Mon, Nov 15, 2021, at 10:41 AM, Marc Orr wrote:
> On Mon, Nov 15, 2021 at 10:26 AM Sean Christopherson <seanjc@google.co=
m> wrote:
>>
>> On Mon, Nov 15, 2021, Dr. David Alan Gilbert wrote:
>> > * Sean Christopherson (seanjc@google.com) wrote:
>> > > On Fri, Nov 12, 2021, Borislav Petkov wrote:
>> > > > On Fri, Nov 12, 2021 at 09:59:46AM -0800, Dave Hansen wrote:
>> > > > > Or, is there some mechanism that prevent guest-private memory=
 from being
>> > > > > accessed in random host kernel code?
>> > >
>> > > Or random host userspace code...
>> > >
>> > > > So I'm currently under the impression that random host->guest a=
ccesses
>> > > > should not happen if not previously agreed upon by both.
>> > >
>> > > Key word "should".
>> > >
>> > > > Because, as explained on IRC, if host touches a private guest p=
age,
>> > > > whatever the host does to that page, the next time the guest ru=
ns, it'll
>> > > > get a #VC where it will see that that page doesn't belong to it=
 anymore
>> > > > and then, out of paranoia, it will simply terminate to protect =
itself.
>> > > >
>> > > > So cloud providers should have an interest to prevent such rand=
om stray
>> > > > accesses if they wanna have guests. :)
>> > >
>> > > Yes, but IMO inducing a fault in the guest because of _host_ bug =
is wrong.
>> >
>> > Would it necessarily have been a host bug?  A guest telling the hos=
t a
>> > bad GPA to DMA into would trigger this wouldn't it?
>>
>> No, because as Andy pointed out, host userspace must already guard ag=
ainst a bad
>> GPA, i.e. this is just a variant of the guest telling the host to DMA=
 to a GPA
>> that is completely bogus.  The shared vs. private behavior just means=
 that when
>> host userspace is doing a GPA=3D>HVA lookup, it needs to incorporate =
the "shared"
>> state of the GPA.  If the host goes and DMAs into the completely wron=
g HVA=3D>PFN,
>> then that is a host bug; that the bug happened to be exploited by a b=
uggy/malicious
>> guest doesn't change the fact that the host messed up.
>
> "If the host goes and DMAs into the completely wrong HVA=3D>PFN, then
> that is a host bug; that the bug happened to be exploited by a
> buggy/malicious guest doesn't change the fact that the host messed
> up."
> ^^^
> Again, I'm flabbergasted that you are arguing that it's OK for a guest
> to exploit a host bug to take down host-side processes or the host
> itself, either of which could bring down all other VMs on the machine.
>
> I'm going to repeat -- this is not OK! Period.

I don=E2=80=99t understand the point you=E2=80=99re trying to make. If t=
he host _kernel_has a bug that allows a guest to trigger invalid host me=
mory access, this is bad. We want to know about it and fix it, abcs the =
security folks want to minimize the chance that such a bug exists.

If host _userspace_ such a bug, the kernel should not crash if it=E2=80=99=
s exploited.