Date:   Mon, 15 Nov 2021 18:26:16 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc:     Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@intel.com>,
        Peter Gonda <pgonda@google.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,
        marcorr@google.com, sathyanarayanan.kuppuswamy@linux.intel.com
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support
Message-ID: <YZKmSDQJgCcR06nE@google.com>
References: <20210820155918.7518-1-brijesh.singh@amd.com>
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 <061ccd49-3b9f-d603-bafd-61a067c3f6fa@intel.com>
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 <YY7FAW5ti7YMeejj@google.com>
 <YZJTA1NyLCmVtGtY@work-vm>
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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 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.