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

Marc Orr
2021-11-12T21:30:39Z

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.