Xenstore: Guests can create arbitrary number of nodes via transactions

ISSUE DESCRIPTION

In case a node has been created in a transaction and it is later deleted in the same transaction, the transaction will be terminated with an error.
As this error is encountered only when handling the deleted node at transaction finalization, the transaction will have been performed partially and without updating the accounting information. This will enable a malicious guest to create arbitrary number of nodes.

IMPACT

A malicious guest can cause memory shortage in xenstored, resulting in a Denial of Service (DoS) of xenstored.
This will inhibit creating new guests and changing the configuration of already running guests.

VULNERABLE SYSTEMS

All systems running Xen version 4.9 and newer are affected.
Only systems running the C variant of Xenstore (xenstored or xenstore- stubdom) are vulnerable.
Systems using the Ocaml variant of Xenstore (oxenstored) are not vulnerable.