linux-kernel - Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor Support

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Message-Id: <40c1794a-104e-4bcd-add5-2096aefc23e1@www.fastmail.com>
Date:   Mon, 15 Nov 2021 21:00:23 -0800
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Marc Orr" <marcorr@...gle.com>,
        "Sean Christopherson" <seanjc@...gle.com>
Cc:     "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        "Borislav Petkov" <bp@...en8.de>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Peter Gonda" <pgonda@...gle.com>,
        "Brijesh Singh" <brijesh.singh@....com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "kvm list" <kvm@...r.kernel.org>, linux-coco@...ts.linux.dev,
        linux-mm@...ck.org,
        "Linux Crypto Mailing List" <linux-crypto@...r.kernel.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Joerg Roedel" <jroedel@...e.de>,
        "Tom Lendacky" <Thomas.Lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Ard Biesheuvel" <ardb@...nel.org>,
        "Paolo Bonzini" <pbonzini@...hat.com>,
        "Vitaly Kuznetsov" <vkuznets@...hat.com>,
        "Wanpeng Li" <wanpengli@...cent.com>,
        "Jim Mattson" <jmattson@...gle.com>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Sergio Lopez" <slp@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Srinivas Pandruvada" <srinivas.pandruvada@...ux.intel.com>,
        "David Rientjes" <rientjes@...gle.com>,
        "Dov Murik" <dovmurik@...ux.ibm.com>,
        "Tobin Feldman-Fitzthum" <tobin@....com>,
        "Michael Roth" <Michael.Roth@....com>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        "Andi Kleen" <ak@...ux.intel.com>,
        "Tony Luck" <tony.luck@...el.com>,
        "Sathyanarayanan Kuppuswamy" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP) Hypervisor
 Support



On Mon, Nov 15, 2021, at 10:41 AM, Marc Orr wrote:
> On Mon, Nov 15, 2021 at 10:26 AM Sean Christopherson <seanjc@...gle.com> wrote:
>>
>> On Mon, Nov 15, 2021, Dr. David Alan Gilbert wrote:
>> > * Sean Christopherson (seanjc@...gle.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 accesses
>> > > > should not happen if not previously agreed upon by both.
>> > >
>> > > Key word "should".
>> > >
>> > > > Because, as explained on IRC, if host touches a private guest page,
>> > > > whatever the host does to that page, the next time the guest runs, 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 random 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 host a
>> > bad GPA to DMA into would trigger this wouldn't it?
>>
>> No, because as Andy pointed out, host userspace must already guard against 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=>HVA lookup, it needs to incorporate the "shared"
>> state of the GPA.  If the host goes and DMAs into the completely wrong HVA=>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.
>
> "If the host goes and DMAs into the completely wrong HVA=>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’t understand the point you’re trying to make. If the host _kernel_has a bug that allows a guest to trigger invalid host memory 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’s exploited.