Re: [PATCH 16/28] KVM: x86/mmu: WARN if old _or_ new SPTE is REMOVED in non-atomic path

[Ben Gardon]

On Fri, Nov 19, 2021 at 8:51 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> WARN if the new_spte being set by __tdp_mmu_set_spte() is a REMOVED_SPTE,
> which is called out by the comment as being disallowed but not actually
> checked.  Keep the WARN on the old_spte as well, because overwriting a
> REMOVED_SPTE in the non-atomic path is also disallowed (as evidence by
> lack of splats with the existing WARN).
>
> Fixes: 08f07c800e9d ("KVM: x86/mmu: Flush TLBs after zap in TDP MMU PF handler")
> Cc: Ben Gardon <bgardon@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Ben Gardon <bgardon@xxxxxxxxxx>

> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 085f6b09e5f3..d9524b387221 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -638,13 +638,13 @@ static inline void __tdp_mmu_set_spte(struct kvm *kvm, struct tdp_iter *iter,
>         lockdep_assert_held_write(&kvm->mmu_lock);
>
>         /*
> -        * No thread should be using this function to set SPTEs to the
> +        * No thread should be using this function to set SPTEs to or from the
>          * temporary removed SPTE value.
>          * If operating under the MMU lock in read mode, tdp_mmu_set_spte_atomic
>          * should be used. If operating under the MMU lock in write mode, the
>          * use of the removed SPTE should not be necessary.
>          */
> -       WARN_ON(is_removed_spte(iter->old_spte));
> +       WARN_ON(is_removed_spte(iter->old_spte) || is_removed_spte(new_spte));
>
>         kvm_tdp_mmu_write_spte(iter->sptep, new_spte);
>
> --
> 2.34.0.rc2.393.gf8c9666880-goog
>