KVM: SVM: Exit to userspace on ENOMEM/EFAULT GHCB errors

Exit to userspace if setup_vmgexit_scratch() fails due to OOM or because
copying data from guest (userspace) memory failed/faulted.  The OOM
scenario is clearcut, it's userspace's decision as to whether it should
terminate the guest, free memory, etc...

As for -EFAULT, arguably, any guest issue is a violation of the guest's
contract with userspace, and thus userspace needs to decide how to
proceed.  E.g. userspace defines what is RAM vs. MMIO and communicates
that directly to the guest, KVM is not involved in deciding what is/isn't
RAM nor in communicating that information to the guest.  If the scratch
GPA doesn't resolve to a memslot, then the guest is not honoring the
memory configuration as defined by userspace.

And if userspace unmaps an hva for whatever reason, then exiting to
userspace with -EFAULT is absolutely the right thing to do.  KVM's ABI
currently sucks and doesn't provide enough information to act on the
-EFAULT, but that will hopefully be remedied in the future as there are
multiple use cases, e.g. uffd and virtiofs truncation, that shouldn't
require any work in KVM beyond returning -EFAULT with a small amount of
metadata.

KVM could define its ABI such that failure to access the scratch area is
reflected into the guest, i.e. establish a contract with userspace, but
that's undesirable as it limits KVM's options in the future, e.g. in the
potential uffd case any failure on a uaccess needs to kick out to
userspace.  KVM does have several cases where it reflects these errors
into the guest, e.g. kvm_pv_clock_pairing() and Hyper-V emulation, but
KVM would preferably "fix" those instead of propagating the falsehood
that any memory failure is the guest's fault.

Lastly, returning a boolean as an "error" for that a helper that isn't
named accordingly never works out well.

Fixes: ad5b3532

 ("KVM: SVM: Do not terminate SEV-ES guests on GHCB validation failure")
Cc: Alper Gun <alpergun@google.com>
Cc: Peter Gonda <pgonda@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220225205209.3881130-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>