Xenstore: Guests can cause Xenstore to not free temporary memory

ISSUE DESCRIPTION

When working on a request of a guest, xenstored might need to allocate quite large amounts of memory temporarily. This memory is freed only after the request has been finished completely.
A request is regarded to be finished only after the guest has read the response message of the request from the ring page. Thus a guest not reading the response can cause xenstored to not free the temporary memory.
This can result in memory shortages causing Denial of Service (DoS) of xenstored.

IMPACT

A malicious guest can cause DoS of xenstored, resulting in the inability to create new guests or to change the configuration of already running guests.

VULNERABLE SYSTEMS

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