KVM: arm64: Reject shared table walks in the hyp code

Exclusive table walks are the only supported table walk in the hyp, as
there is no construct like RCU available in the hypervisor code. Reject
any attempt to do a shared table walk by returning an error and allowing
the caller to clean up the mess.
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221118182222.3932898-4-oliver.upton@linux.dev

arch/arm64/include/asm/kvm_pgtable.h

@@ -229,7 +229,18 @@ static inline kvm_pte_t *kvm_dereference_pteref(struct kvm_pgtable_walker *walke
 	return pteref;
 }
 
-static inline void kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker) {}
+static inline int kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
+{
+	/*
+	 * Due to the lack of RCU (or a similar protection scheme), only
+	 * non-shared table walkers are allowed in the hypervisor.
+	 */
+	if (walker->flags & KVM_PGTABLE_WALK_SHARED)
+		return -EPERM;
+
+	return 0;
+}
+
 static inline void kvm_pgtable_walk_end(struct kvm_pgtable_walker *walker) {}
 
 static inline bool kvm_pgtable_walk_lock_held(void)
@@ -247,10 +258,12 @@ static inline kvm_pte_t *kvm_dereference_pteref(struct kvm_pgtable_walker *walke
 	return rcu_dereference_check(pteref, !(walker->flags & KVM_PGTABLE_WALK_SHARED));
 }
 
-static inline void kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
+static inline int kvm_pgtable_walk_begin(struct kvm_pgtable_walker *walker)
 {
 	if (walker->flags & KVM_PGTABLE_WALK_SHARED)
 		rcu_read_lock();
+
+	return 0;
 }
 
 static inline void kvm_pgtable_walk_end(struct kvm_pgtable_walker *walker)

arch/arm64/kvm/hyp/pgtable.c

@@ -289,7 +289,10 @@ int kvm_pgtable_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
 	};
 	int r;
 
-	kvm_pgtable_walk_begin(walker);
+	r = kvm_pgtable_walk_begin(walker);
+	if (r)
+		return r;
+
 	r = _kvm_pgtable_walk(pgt, &walk_data);
 	kvm_pgtable_walk_end(walker);