keys: Fix request_key() lack of Link perm check on found key

The request_key() syscall allows a process to gain access to the 'possessor'
permits of any key that grants it Search permission by virtue of request_key()
not checking whether a key it finds grants Link permission to the caller.

Signed-off-by: David Howells <dhowells@redhat.com>

security/keys/request_key.c

@@ -564,6 +564,16 @@ struct key *request_key_and_link(struct key_type *type,
 	key_ref = search_process_keyrings(&ctx);
 
 	if (!IS_ERR(key_ref)) {
+		if (dest_keyring) {
+			ret = key_task_permission(key_ref, current_cred(),
+						  KEY_NEED_LINK);
+			if (ret < 0) {
+				key_ref_put(key_ref);
+				key = ERR_PTR(ret);
+				goto error_free;
+			}
+		}
+
 		key = key_ref_to_ptr(key_ref);
 		if (dest_keyring) {
 			ret = key_link(dest_keyring, key);