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From:   Marc Orr <marcorr@google.com>
Date:   Fri, 12 Nov 2021 13:43:13 -0800
Message-ID: <CAA03e5Fw9cRnb=+eJmzEB+0QmdgaGZ7=fPTUYx7f55mGVXLRMA@mail.gmail.com>
Subject: Re: [PATCH Part2 v5 00/45] Add AMD Secure Nested Paging (SEV-SNP)
 Hypervisor Support
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Sean Christopherson <seanjc@google.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>
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On Fri, Nov 12, 2021 at 1:39 PM Andy Lutomirski <luto@kernel.org> wrote:
>
>
>
> On Fri, Nov 12, 2021, at 1:30 PM, Marc Orr wrote:
> > On Fri, Nov 12, 2021 at 12:38 PM Sean Christopherson <seanjc@google.com=
> wrote:
> >>
> >> On Fri, Nov 12, 2021, Borislav Petkov wrote:
> >> > On Fri, Nov 12, 2021 at 07:48:17PM +0000, Sean Christopherson wrote:
> >> > > Yes, but IMO inducing a fault in the guest because of _host_ bug i=
s wrong.
> >> >
> >> > What do you suggest instead?
> >>
> >> Let userspace decide what is mapped shared and what is mapped private.=
  The kernel
> >> and KVM provide the APIs/infrastructure to do the actual conversions i=
n a thread-safe
> >> fashion and also to enforce the current state, but userspace is the co=
ntrol plane.
> >>
> >> It would require non-trivial changes in userspace if there are multipl=
e processes
> >> accessing guest memory, e.g. Peter's networking daemon example, but it=
 _is_ fully
> >> solvable.  The exit to userspace means all three components (guest, ke=
rnel,
> >> and userspace) have full knowledge of what is shared and what is priva=
te.  There
> >> is zero ambiguity:
> >>
> >>   - if userspace accesses guest private memory, it gets SIGSEGV or wha=
tever.
> >>   - if kernel accesses guest private memory, it does BUG/panic/oops[*]
> >>   - if guest accesses memory with the incorrect C/SHARED-bit, it gets =
killed.
> >>
> >> This is the direction KVM TDX support is headed, though it's obviously=
 still a WIP.
> >>
> >> And ideally, to avoid implicit conversions at any level, hardware vend=
ors' ABIs
> >> define that:
> >>
> >>   a) All convertible memory, i.e. RAM, starts as private.
> >>   b) Conversions between private and shared must be done via explicit =
hypercall.
> >>
> >> Without (b), userspace and thus KVM have to treat guest accesses to th=
e incorrect
> >> type as implicit conversions.
> >>
> >> [*] Sadly, fully preventing kernel access to guest private is not poss=
ible with
> >>     TDX, especially if the direct map is left intact.  But maybe in th=
e future
> >>     TDX will signal a fault instead of poisoning memory and leaving a =
#MC mine.
> >
> > In this proposal, consider a guest driver instructing a device to DMA
> > write a 1 GB memory buffer. A well-behaved guest driver will ensure
> > that the entire 1 GB is marked shared. But what about a malicious or
> > buggy guest? Let's assume a bad guest driver instructs the device to
> > write guest private memory.
> >
> > So now, the virtual device, which might be implemented as some host
> > side process, needs to (1) check and lock all 4k constituent RMP
> > entries (so they're not converted to private while the DMA write is
> > taking palce), (2) write the 1 GB buffer, and (3) unlock all 4 k
> > constituent RMP entries? If I'm understanding this correctly, then the
> > synchronization will be prohibitively expensive.
>
> Let's consider a very very similar scenario: consider a guest driver sett=
ing up a 1 GB DMA buffer.  The virtual device, implemented as host process,=
 needs to (1) map (and thus lock *or* be prepared for faults) in 1GB / 4k p=
ages of guest memory (so they're not *freed* while the DMA write is taking =
place), (2) write the buffer, and (3) unlock all the pages.  Or it can lock=
 them at setup time and keep them locked for a long time if that's appropri=
ate.
>
> Sure, the locking is expensive, but it's nonnegotiable.  The RMP issue is=
 just a special case of the more general issue that the host MUST NOT ACCES=
S GUEST MEMORY AFTER IT'S FREED.

Good point.