KVM: MMU: Add validate_direct_spte() helper

Add a helper to verify that a direct shadow page is valid wrt the required
access permissions; drop the page if it is not valid.

Reviewed-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/mmu.c

@@ -1500,6 +1500,29 @@ static void drop_large_spte(struct kvm_vcpu *vcpu, u64 *sptep)
 	}
 }
 
+static void validate_direct_spte(struct kvm_vcpu *vcpu, u64 *sptep,
+				   unsigned direct_access)
+{
+	if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) {
+		struct kvm_mmu_page *child;
+
+		/*
+		 * For the direct sp, if the guest pte's dirty bit
+		 * changed form clean to dirty, it will corrupt the
+		 * sp's access: allow writable in the read-only sp,
+		 * so we should update the spte at this point to get
+		 * a new sp with the correct access.
+		 */
+		child = page_header(*sptep & PT64_BASE_ADDR_MASK);
+		if (child->role.access == direct_access)
+			return;
+
+		mmu_page_remove_parent_pte(child, sptep);
+		__set_spte(sptep, shadow_trap_nonpresent_pte);
+		kvm_flush_remote_tlbs(vcpu->kvm);
+	}
+}
+
 static void kvm_mmu_page_unlink_children(struct kvm *kvm,
 					 struct kvm_mmu_page *sp)
 {

arch/x86/kvm/paging_tmpl.h

@@ -338,30 +338,15 @@ static u64 *FNAME(fetch)(struct kvm_vcpu *vcpu, gva_t addr,
 			break;
 		}
 
-		if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)) {
-			struct kvm_mmu_page *child;
-
-			if (level != gw->level)
-				continue;
-
-			/*
-			 * For the direct sp, if the guest pte's dirty bit
-			 * changed form clean to dirty, it will corrupt the
-			 * sp's access: allow writable in the read-only sp,
-			 * so we should update the spte at this point to get
-			 * a new sp with the correct access.
-			 */
-			child = page_header(*sptep & PT64_BASE_ADDR_MASK);
-			if (child->role.access == direct_access)
-				continue;
-
-			mmu_page_remove_parent_pte(child, sptep);
-			__set_spte(sptep, shadow_trap_nonpresent_pte);
-			kvm_flush_remote_tlbs(vcpu->kvm);
-		}
+		if (is_shadow_present_pte(*sptep) && !is_large_pte(*sptep)
+		    && level == gw->level)
+			validate_direct_spte(vcpu, sptep, direct_access);
 
 		drop_large_spte(vcpu, sptep);
 
+		if (is_shadow_present_pte(*sptep))
+			continue;
+
 		if (level <= gw->level) {
 			direct = 1;
 			access = direct_access;