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

Marc Orr
2021-11-12T21:43:25Z

On Fri, Nov 12, 2021 at 1:39 PM Andy Lutomirski <luto@kernel.org> wrote:
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>
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> 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 is 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 in a thread-safe
> >> fashion and also to enforce the current state, but userspace is the control plane.
> >>
> >> It would require non-trivial changes in userspace if there are multiple 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, kernel,
> >> and userspace) have full knowledge of what is shared and what is private. There
> >> is zero ambiguity:
> >>
> >> - if userspace accesses guest private memory, it gets SIGSEGV or whatever.
> >> - 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 vendors' 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 the incorrect
> >> type as implicit conversions.
> >>
> >> [*] Sadly, fully preventing kernel access to guest private is not possible with
> >> TDX, especially if the direct map is left intact. But maybe in the 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 setting 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 pages 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 appropriate.
>
> 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 ACCESS GUEST MEMORY AFTER IT'S FREED.

Good point.