Xenstore: Cooperating guests can create arbitrary numbers of nodes

ISSUE DESCRIPTION

Since the fix of XSA-322 any Xenstore node owned by a removed domain will be modified to be owned by Dom0. This will allow two malicious guests working together to create an arbitrary number of Xenstore nodes.
This is possible by domain A letting domain B write into domain A’s local Xenstore tree. Domain B can then create many nodes and reboot. The nodes created by domain B will now be owned by Dom0. By repeating this process over and over again an arbitrary number of nodes can be created, as Dom0’s number of nodes isn’t limited by Xenstore quota.

IMPACT

Two malicious guests working together can drive xenstored into an out of memory situation, resulting in a Denial of Service (DoS) of xenstored.
This inhibits creation of new guests and changing the configuration of already running guests.

VULNERABLE SYSTEMS

All versions of Xen with the fix for XSA-322 are in principle vulnerable.
Both Xenstore implementations (C and Ocaml) are vulnerable.