Re: [PATCH 16/18] KVM: x86: Take mem attributes into account when faulting memory

[Sean Christopherson <seanjc@google.com>]

On Thu, Aug 22, 2024, Nicolas Saenz Julienne wrote:
> On Sun Jun 9, 2024 at 3:49 PM UTC, Nicolas Saenz Julienne wrote:
> > Take into account access restrictions memory attributes when faulting
> > guest memory. Prohibited memory accesses will cause an user-space fault
> > exit.
> >
> > Additionally, bypass a warning in the !tdp case. Access restrictions in
> > guest page tables might not necessarily match the host pte's when memory
> > attributes are in use.
> >
> > Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>
> 
> I now realize that only taking into account memory attributes during
> faults isn't good enough for VSM. We should check the attributes anytime
> KVM takes GPAs as input for any action initiated by the guest. If the
> memory attributes are incompatible with such action, it should be
> stopped. Failure to do so opens side channels that unprivileged VTLs can
> abuse to infer information about privileged VTL. Some examples I came up
> with:
> - Guest page walks: VTL0 could install malicious directory entries that
>   point to GPAs only visible to VTL1. KVM will happily continue the
>   walk. Among other things, this could be use to infer VTL1's GVA->GPA
>   mappings.
> - PV interfaces like the Hyper-V TSC page or VP assist page, could be
>   used to modify portions of VTL1 memory.
> - Hyper-V hypercalls that take GPAs as input/output can be abused in a
>   myriad of ways. Including ones that exit into user-space.
> 
> We would be protected against all these if we implemented the memory
> access restrictions through the memory slots API. As is, it has the
> drawback of having to quiesce the whole VM for any non-trivial slot
> modification (i.e. VSM's memory protections). But if we found a way to
> speed up the slot updates we could rely on that, and avoid having to
> teach kvm_read/write_guest() and friends to deal with memattrs. Note
> that we would still need to use memory attributes to request for faults
> to exit onto user-space on those select GPAs. Any opinions or
> suggestions?
> 
> Note that, for now, I'll stick with the memory attributes approach to
> see what the full solution looks like.

FWIW, I suspect we'll be better off honoring memory attributes.  It's not just
the KVM side that has issues with memslot updates, my understanding is userspace
has also built up "slow" code with respect to memslot updates, in part because
it's such a slow path in KVM.