Re: [PATCH v3 11/28] KVM: x86/mmu: WARN if old _or_ new SPTE is REMOVED in non-atomic path

[Mingwei Zhang]

On Sat, Feb 26, 2022, Sean Christopherson 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>

Reviewed-by: Mingwei Zhang <mizhang@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 8fbf3364f116..1dcdf1a4fcc1 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -640,13 +640,13 @@ 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);
>  
> -- 
> 2.35.1.574.g5d30c73bfb-goog
>