Re: [PATCH 02/16] KVM: x86: clamp host mapping level to max_level in kvm_mmu_max_mapping_level

[Sean Christopherson]

On Sat, Aug 07, 2021, Paolo Bonzini wrote:
> This patch started as a way to make kvm_mmu_hugepage_adjust a bit simpler,
> in preparation for switching it to struct kvm_page_fault, but it does
> fix a microscopic bug in zapping collapsible PTEs.

I think this also fixes a bug where userspace backs guest memory with a 1gb hugepage
but only assigns a subset of the page to the guest.  1gb pages would be disallowed
by the memslot, but not 2mb.  kvm_mmu_max_mapping_level() would fall through to the
host_pfn_mapping_level() logic, see the 1gb huge, and map the whole thing into the
guest.  I can't imagine any userspace would actually do something like that, but the
failure mode is serious enough that it warrants a Fixes: + Cc: stable@.