Re: [PATCH] x86/kvm/mmu: Remove unnecessary ‘NULL’ values from sptep

[Huang, Kai]

On Fri, 2023-09-01 at 09:48 -0700, Sean Christopherson wrote:
> On Sun, Sep 03, 2023, Li zeming wrote:
> > sptep is assigned first, so it does not need to initialize the assignment.
> > 
> > Signed-off-by: Li zeming <zeming@xxxxxxxxxxxx>
> > ---
> >  arch/x86/kvm/mmu/mmu.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index ec169f5c7dce..95f745aec4aa 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3457,7 +3457,7 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> >  	struct kvm_mmu_page *sp;
> >  	int ret = RET_PF_INVALID;
> >  	u64 spte = 0ull;
> > -	u64 *sptep = NULL;
> > +	u64 *sptep;
> >  	uint retry_count = 0;
> >  
> >  	if (!page_fault_can_be_fast(fault))
> 
> Hmm, this is safe, but there's some ugliness lurking.  Theoretically, it's possible
> for spte to be left untouched by the walkers.  That _shouldn't_ happen, as it means
> there's a bug somewhere in KVM.  But if that did happen, on the second or later
> iteration, it's (again, theoretically) possible to consume a stale spte.
> 
> 		if (tdp_mmu_enabled)
> 			sptep = kvm_tdp_mmu_fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
> 		else
> 			sptep = fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
> 
> 		if (!is_shadow_present_pte(spte)) <=== could consume stale data
> 			break;
> 
> If we're going to tidy up sptep, I think we should also give spte similar treatment
> and harden KVM in the process, e.g.
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 6325bb3e8c2b..ae2f87bbbf0a 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3430,8 +3430,8 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  {
>         struct kvm_mmu_page *sp;
>         int ret = RET_PF_INVALID;
> -       u64 spte = 0ull;
> -       u64 *sptep = NULL;
> +       u64 spte;
> +       u64 *sptep;
>         uint retry_count = 0;
>  
>         if (!page_fault_can_be_fast(fault))
> @@ -3447,6 +3447,14 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>                 else
>                         sptep = fast_pf_get_last_sptep(vcpu, fault->addr, &spte);
>  
> +               /*
> +                * It's entirely possible for the mapping to have been zapped
> +                * by a different task, but the root page is should always be
> +                * available as the vCPU holds a reference to its root(s).
> +                */
> +               if (WARN_ON_ONCE(!sptep))
> +                       spte = REMOVED_SPTE;

If I recall correctly, REMOVED_SPTE is only used by TDP MMU code.  Should we use
0 (or initial SPTE value for case like TDX) instead of REMOVED_SPTE?

And I agree this code is more error proof (although theoretically for now).
  
> +
>                 if (!is_shadow_present_pte(spte))
>                         break;
>  
>