Re: [PATCH 5/5] kvm: x86: mmu: Verify that restored PTE has needed perms in fast page fault

Junaid Shahid
2017-01-25T20:05:04Z

Looks good. Thanks.

On Wednesday, January 25, 2017 12:49:06 PM Paolo Bonzini wrote:
>
> > - remove_acc_track = is_access_track_spte(spte);
> > -
> > - /* Verify that the fault can be handled in the fast path */
> > - if (!remove_acc_track && !remove_write_prot)
> > - break;
> > + new_spte = is_access_track_spte(spte)
> > + ? restore_acc_track_spte(spte)
> > + : spte;
>
> Just a tiny stylistic change to match the "new_spte |= PT_WRITABLE_MASK"
> in the block below:
>
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index aa3b0d14c019..2fd7586aad4d 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -3135,9 +3135,10 @@ static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
> break;
> }
>
> - new_spte = is_access_track_spte(spte)
> - ? restore_acc_track_spte(spte)
> - : spte;
> + new_spte = spte;
> +
> + if (is_access_track_spte(spte))
> + new_spte = restore_acc_track_spte(new_spte);
>
> /*
> * Currently, to simplify the code, write-protection can
>
> Paolo
>
> > /*
> > - * Do not fix write-permission on the large spte since we only
> > - * dirty the first page into the dirty-bitmap in
> > - * fast_pf_fix_direct_spte() that means other pages are missed
> > - * if its slot is dirty-logged.
> > - *
> > - * Instead, we let the slow page fault path create a normal spte
> > - * to fix the access.
> > - *
> > - * See the comments in kvm_arch_commit_memory_region().
> > + * Currently, to simplify the code, write-protection can
> > + * be removed in the fast path only if the SPTE was
> > + * write-protected for dirty-logging or access tracking.
> > */
> > - if (sp->role.level > PT_PAGE_TABLE_LEVEL && remove_write_prot)
> > + if ((error_code & PFERR_WRITE_MASK) &&
> > + spte_can_locklessly_be_made_writable(spte))
> > + {
> > + new_spte |= PT_WRITABLE_MASK;
> > +
> > + /*
> > + * Do not fix write-permission on the large spte since
> > + * we only dirty the first page into the dirty-bitmap in
> > + * fast_pf_fix_direct_spte() that means other pages are
> > + * missed if its slot is dirty-logged.
> > + *
> > + * Instead, we let the slow page fault path create a
> > + * normal spte to fix the access.
> > + *
> > + * See the comments in kvm_arch_commit_memory_region().
> > + */
> > + if (sp->role.level > PT_PAGE_TABLE_LEVEL)
> > + break;
> > + }
> > +
> > + /* Verify that the fault can be handled in the fast path */
> > + if (new_spte == spte ||
> > + !is_access_allowed(error_code, new_spte))
> > break;
> >
> > /*
> > @@ -3166,8 +3175,7 @@ static bool fast_page_fault(struct kvm_vcpu *vcpu, gva_t gva, int level,
> > */
> > fault_handled = fast_pf_fix_direct_spte(vcpu, sp,
> > iterator.sptep, spte,
> > - remove_write_prot,
> > - remove_acc_track);
> > + new_spte);
> > if (fault_handled)
> > break;
> >
> >