ocaml xenstored mishandles oversized message replies

ISSUE DESCRIPTION

The Ocaml xenstored implementation (“oxenstored”) cannot correctly handle a message reply larger than XENSTORE_PAYLOAD_SIZE when communicating with a client domain via the shared ring mechanism.
When this situation occurs the connection to the client domain will be shutdown and cannot be restarted leading to a denial of service to that domain.
Clients in the same domain as xenstored which are using the Unix domain socket mechanism are not vulnerable.

IMPACT

A malicious domain can create a directory containing a large number of entries in the hopes that a victim domain will attempt to list the contents of that directory. If this happens then the victim domain’s xenstore connection will be shutdown leading to a denial of service against that domain.
If the victim domain is a toolstack or control domain then this can lead to a denial of service against the whole system.

VULNERABLE SYSTEMS

All systems using oxenstored are potentially vulnerable.
oxenstored was added in Xen 4.1.0. From Xen 4.2.0 onward it is used by default if an ocaml toolstack was present at build time.
In its default configuration the C xenstored implementation is not vulnerable. By default this implementation imposes a quota on the maximum directory size which is less than XENSTORE_PAYLOAD_SIZE. If you have adjusted the quota using the --entry-size / -S option to a value larger than XENSTORE_PAYLOAD_SIZE (4096 bytes) then you may be vulnerable.
Systems where the toolstack and oxenstored live in the same domain will default to using Unix domain socket based communications and therefore are not vulnerable to the host wide denial of service by default. In such a configuration guest domains which do not list xenstore paths belonging to untrusted foreign domains will not be vulnerable to the DoS. (In the common case guests will not have permission to do so in any case.)