xen: xenbus_dev_frontend: Verify body of XS_TRANSACTION_END

By guaranteeing that the argument of XS_TRANSACTION_END is valid we can
assume that the transaction has been closed when we get an XS_ERROR
response from xenstore (Note that we already verify that it's a valid
transaction id).
Signed-off-by: Simon Gaiser <simon@invisiblethingslab.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

drivers/xen/xenbus/xenbus_dev_frontend.c

@@ -429,6 +429,10 @@ static int xenbus_write_transaction(unsigned msg_type,
 {
 	int rc;
 	struct xenbus_transaction_holder *trans = NULL;
+	struct {
+		struct xsd_sockmsg hdr;
+		char body[];
+	} *msg = (void *)u->u.buffer;
 
 	if (msg_type == XS_TRANSACTION_START) {
 		trans = kzalloc(sizeof(*trans), GFP_KERNEL);
@@ -437,11 +441,15 @@ static int xenbus_write_transaction(unsigned msg_type,
 			goto out;
 		}
 		list_add(&trans->list, &u->transactions);
-	} else if (u->u.msg.tx_id != 0 &&
-		   !xenbus_get_transaction(u, u->u.msg.tx_id))
+	} else if (msg->hdr.tx_id != 0 &&
+		   !xenbus_get_transaction(u, msg->hdr.tx_id))
 		return xenbus_command_reply(u, XS_ERROR, "ENOENT");
+	else if (msg_type == XS_TRANSACTION_END &&
+		 !(msg->hdr.len == 2 &&
+		   (!strcmp(msg->body, "T") || !strcmp(msg->body, "F"))))
+		return xenbus_command_reply(u, XS_ERROR, "EINVAL");
 
-	rc = xenbus_dev_request_and_reply(&u->u.msg, u);
+	rc = xenbus_dev_request_and_reply(&msg->hdr, u);
 	if (rc && trans) {
 		list_del(&trans->list);
 		kfree(trans);