afs: Provide an RCU-capable key lookup

Provide an RCU-capable key lookup function.  We don't want to call
afs_request_key() in RCU-mode pathwalk as request_key() might sleep, even if
we don't ask it to construct anything as it might find a key that is currently
undergoing construction.
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/internal.h

@@ -1217,6 +1217,7 @@ extern void afs_cache_permit(struct afs_vnode *, struct key *, unsigned int,
 			     struct afs_status_cb *);
 extern void afs_zap_permits(struct rcu_head *);
 extern struct key *afs_request_key(struct afs_cell *);
+extern struct key *afs_request_key_rcu(struct afs_cell *);
 extern int afs_check_permit(struct afs_vnode *, struct key *, afs_access_t *);
 extern int afs_permission(struct inode *, int);
 extern void __exit afs_clean_up_permit_cache(void);

fs/afs/security.c

@@ -27,8 +27,37 @@ struct key *afs_request_key(struct afs_cell *cell)
 	_enter("{%x}", key_serial(cell->anonymous_key));
 
 	_debug("key %s", cell->anonymous_key->description);
-	key = request_key(&key_type_rxrpc, cell->anonymous_key->description,
-			  NULL);
+	key = request_key_net(&key_type_rxrpc, cell->anonymous_key->description,
+			      cell->net->net, NULL);
+	if (IS_ERR(key)) {
+		if (PTR_ERR(key) != -ENOKEY) {
+			_leave(" = %ld", PTR_ERR(key));
+			return key;
+		}
+
+		/* act as anonymous user */
+		_leave(" = {%x} [anon]", key_serial(cell->anonymous_key));
+		return key_get(cell->anonymous_key);
+	} else {
+		/* act as authorised user */
+		_leave(" = {%x} [auth]", key_serial(key));
+		return key;
+	}
+}
+
+/*
+ * Get a key when pathwalk is in rcuwalk mode.
+ */
+struct key *afs_request_key_rcu(struct afs_cell *cell)
+{
+	struct key *key;
+
+	_enter("{%x}", key_serial(cell->anonymous_key));
+
+	_debug("key %s", cell->anonymous_key->description);
+	key = request_key_net_rcu(&key_type_rxrpc,
+				  cell->anonymous_key->description,
+				  cell->net->net);
 	if (IS_ERR(key)) {
 		if (PTR_ERR(key) != -ENOKEY) {
 			_leave(" = %ld", PTR_ERR(key));

include/linux/key.h

@@ -324,7 +324,7 @@ static inline struct key *request_key(struct key_type *type,
 }
 
 #ifdef CONFIG_NET
-/*
+/**
  * request_key_net - Request a key for a net namespace and wait for construction
  * @type: Type of key.
  * @description: The searchable description of the key.
@@ -341,6 +341,18 @@ static inline struct key *request_key(struct key_type *type,
  */
 #define request_key_net(type, description, net, callout_info) \
 	request_key_tag(type, description, net->key_domain, callout_info);
+
+/**
+ * request_key_net_rcu - Request a key for a net namespace under RCU conditions
+ * @type: Type of key.
+ * @description: The searchable description of the key.
+ * @net: The network namespace that is the key's domain of operation.
+ *
+ * As for request_key_rcu() except that only keys that operate the specified
+ * network namespace are used.
+ */
+#define request_key_net_rcu(type, description, net) \
+	request_key_rcu(type, description, net->key_domain);
 #endif /* CONFIG_NET */
 
 extern int wait_for_key_construction(struct key *key, bool intr);