KVM: arm64: Use an opaque type for pteps

Use an opaque type for pteps and require visitors explicitly dereference
the pointer before using. Protecting page table memory with RCU requires
that KVM dereferences RCU-annotated pointers before using. However, RCU
is not available for use in the nVHE hypervisor and the opaque type can
be conditionally annotated with RCU for the stage-2 MMU.

Call the type a 'pteref' to avoid a naming collision with raw pteps. No
functional change intended.
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221107215644.1895162-7-oliver.upton@linux.dev

arch/arm64/include/asm/kvm_pgtable.h

@@ -37,6 +37,13 @@ static inline u64 kvm_get_parange(u64 mmfr0)
 
 typedef u64 kvm_pte_t;
 
+typedef kvm_pte_t *kvm_pteref_t;
+
+static inline kvm_pte_t *kvm_dereference_pteref(kvm_pteref_t pteref, bool shared)
+{
+	return pteref;
+}
+
 #define KVM_PTE_VALID			BIT(0)
 
 #define KVM_PTE_ADDR_MASK		GENMASK(47, PAGE_SHIFT)
@@ -175,7 +182,7 @@ typedef bool (*kvm_pgtable_force_pte_cb_t)(u64 addr, u64 end,
 struct kvm_pgtable {
 	u32					ia_bits;
 	u32					start_level;
-	kvm_pte_t				*pgd;
+	kvm_pteref_t				pgd;
 	struct kvm_pgtable_mm_ops		*mm_ops;
 
 	/* Stage-2 only */

arch/arm64/kvm/hyp/pgtable.c

@@ -175,13 +175,14 @@ static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data *data,
 }
 
 static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data,
-			      struct kvm_pgtable_mm_ops *mm_ops, kvm_pte_t *pgtable, u32 level);
+			      struct kvm_pgtable_mm_ops *mm_ops, kvm_pteref_t pgtable, u32 level);
 
 static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
 				      struct kvm_pgtable_mm_ops *mm_ops,
-				      kvm_pte_t *ptep, u32 level)
+				      kvm_pteref_t pteref, u32 level)
 {
 	enum kvm_pgtable_walk_flags flags = data->walker->flags;
+	kvm_pte_t *ptep = kvm_dereference_pteref(pteref, false);
 	struct kvm_pgtable_visit_ctx ctx = {
 		.ptep	= ptep,
 		.old	= READ_ONCE(*ptep),
@@ -193,7 +194,7 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
 		.flags	= flags,
 	};
 	int ret = 0;
-	kvm_pte_t *childp;
+	kvm_pteref_t childp;
 	bool table = kvm_pte_table(ctx.old, level);
 
 	if (table && (ctx.flags & KVM_PGTABLE_WALK_TABLE_PRE))
@@ -214,7 +215,7 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
 		goto out;
 	}
 
-	childp = kvm_pte_follow(ctx.old, mm_ops);
+	childp = (kvm_pteref_t)kvm_pte_follow(ctx.old, mm_ops);
 	ret = __kvm_pgtable_walk(data, mm_ops, childp, level + 1);
 	if (ret)
 		goto out;
@@ -227,7 +228,7 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data,
 }
 
 static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data,
-			      struct kvm_pgtable_mm_ops *mm_ops, kvm_pte_t *pgtable, u32 level)
+			      struct kvm_pgtable_mm_ops *mm_ops, kvm_pteref_t pgtable, u32 level)
 {
 	u32 idx;
 	int ret = 0;
@@ -236,12 +237,12 @@ static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data,
 		return -EINVAL;
 
 	for (idx = kvm_pgtable_idx(data, level); idx < PTRS_PER_PTE; ++idx) {
-		kvm_pte_t *ptep = &pgtable[idx];
+		kvm_pteref_t pteref = &pgtable[idx];
 
 		if (data->addr >= data->end)
 			break;
 
-		ret = __kvm_pgtable_visit(data, mm_ops, ptep, level);
+		ret = __kvm_pgtable_visit(data, mm_ops, pteref, level);
 		if (ret)
 			break;
 	}
@@ -262,9 +263,9 @@ static int _kvm_pgtable_walk(struct kvm_pgtable *pgt, struct kvm_pgtable_walk_da
 		return -EINVAL;
 
 	for (idx = kvm_pgd_page_idx(pgt, data->addr); data->addr < data->end; ++idx) {
-		kvm_pte_t *ptep = &pgt->pgd[idx * PTRS_PER_PTE];
+		kvm_pteref_t pteref = &pgt->pgd[idx * PTRS_PER_PTE];
 
-		ret = __kvm_pgtable_walk(data, pgt->mm_ops, ptep, pgt->start_level);
+		ret = __kvm_pgtable_walk(data, pgt->mm_ops, pteref, pgt->start_level);
 		if (ret)
 			break;
 	}
@@ -507,7 +508,7 @@ int kvm_pgtable_hyp_init(struct kvm_pgtable *pgt, u32 va_bits,
 {
 	u64 levels = ARM64_HW_PGTABLE_LEVELS(va_bits);
 
-	pgt->pgd = (kvm_pte_t *)mm_ops->zalloc_page(NULL);
+	pgt->pgd = (kvm_pteref_t)mm_ops->zalloc_page(NULL);
 	if (!pgt->pgd)
 		return -ENOMEM;
 
@@ -544,7 +545,7 @@ void kvm_pgtable_hyp_destroy(struct kvm_pgtable *pgt)
 	};
 
 	WARN_ON(kvm_pgtable_walk(pgt, 0, BIT(pgt->ia_bits), &walker));
-	pgt->mm_ops->put_page(pgt->pgd);
+	pgt->mm_ops->put_page(kvm_dereference_pteref(pgt->pgd, false));
 	pgt->pgd = NULL;
 }
 
@@ -1157,7 +1158,7 @@ int __kvm_pgtable_stage2_init(struct kvm_pgtable *pgt, struct kvm_s2_mmu *mmu,
 	u32 start_level = VTCR_EL2_TGRAN_SL0_BASE - sl0;
 
 	pgd_sz = kvm_pgd_pages(ia_bits, start_level) * PAGE_SIZE;
-	pgt->pgd = mm_ops->zalloc_pages_exact(pgd_sz);
+	pgt->pgd = (kvm_pteref_t)mm_ops->zalloc_pages_exact(pgd_sz);
 	if (!pgt->pgd)
 		return -ENOMEM;
 
@@ -1200,7 +1201,7 @@ void kvm_pgtable_stage2_destroy(struct kvm_pgtable *pgt)
 
 	WARN_ON(kvm_pgtable_walk(pgt, 0, BIT(pgt->ia_bits), &walker));
 	pgd_sz = kvm_pgd_pages(pgt->ia_bits, pgt->start_level) * PAGE_SIZE;
-	pgt->mm_ops->free_pages_exact(pgt->pgd, pgd_sz);
+	pgt->mm_ops->free_pages_exact(kvm_dereference_pteref(pgt->pgd, false), pgd_sz);
 	pgt->pgd = NULL;
 }
 

arch/arm64/kvm/mmu.c

@@ -640,7 +640,7 @@ static struct kvm_pgtable_mm_ops kvm_user_mm_ops = {
 static int get_user_mapping_size(struct kvm *kvm, u64 addr)
 {
 	struct kvm_pgtable pgt = {
-		.pgd		= (kvm_pte_t *)kvm->mm->pgd,
+		.pgd		= (kvm_pteref_t)kvm->mm->pgd,
 		.ia_bits	= VA_BITS,
 		.start_level	= (KVM_PGTABLE_MAX_LEVELS -
 				   CONFIG_PGTABLE_LEVELS),