libvchan failure handling malicious ring indexes

ISSUE DESCRIPTION

libvchan (a library for inter-domain communication) does not correctly handle unusual or malicious contents in the xenstore ring. A malicious guest can exploit this to cause a libvchan-using facility to read or write past the end of the ring.

IMPACT

libvchan-using facilities are vulnerable to denial of service and perhaps privilege escalation.
There are no such services provided in the upstream Xen Project codebase.

VULNERABLE SYSTEMS

All versions of libvchan are vulnerable. Only installations which use libvchan for communication involving untrusted domains are vulnerable.
libvirt, xapi, xend, libxl and xl do not use libvchan. If your installation contains other Xen-related software components it is possible that they use libvchan and might be vulnerable.
Xen versions 4.1 and earlier do not contain libvchan.