In the Ocaml xenstored implementation, the internal representation of
the tree has special cases for the root node, because this node has no
parent.

Unfortunately, permissions were not checked for certain operations on
the root node.

Unprivileged guests can get and modify permissions, list, and delete
the root node.  Deleting the whole xenstore tree is a hostwide denial
of service.  Depending on the circumstances, the vulnerability can
also be leveraged into an ability to gain write access to any part of
xenstore.