encrypted-keys: fix rcu and sparse messages

Enabling CONFIG_PROVE_RCU and CONFIG_SPARSE_RCU_POINTER resulted in
"suspicious rcu_dereference_check() usage!" and "incompatible types
in comparison expression (different address spaces)" messages.

Access the masterkey directly when holding the rwsem.

Changelog v1:
- Use either rcu_read_lock()/rcu_derefence_key()/rcu_read_unlock()
or remove the unnecessary rcu_derefence() - David Howells
Reported-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

security/keys/encrypted-keys/encrypted.c

@@ -314,7 +314,7 @@ static struct key *request_user_key(const char *master_desc, u8 **master_key,
 		goto error;
 
 	down_read(&ukey->sem);
-	upayload = rcu_dereference(ukey->payload.data);
+	upayload = ukey->payload.data;
 	*master_key = upayload->data;
 	*master_keylen = upayload->datalen;
 error:

security/keys/encrypted-keys/masterkey_trusted.c

@@ -39,7 +39,7 @@ struct key *request_trusted_key(const char *trusted_desc,
 		goto error;
 
 	down_read(&tkey->sem);
-	tpayload = rcu_dereference(tkey->payload.data);
+	tpayload = tkey->payload.data;
 	*master_key = tpayload->key;
 	*master_keylen = tpayload->key_len;
 error: