xen-kernel -- XENMEM_exchange error handling issues

The Xen Project reports:

Error handling in the operation may involve handing back pages to
the domain. This operation may fail when in parallel the domain gets
torn down. So far this failure unconditionally resulted in the host
being brought down due to an internal error being assumed. This is
CVE-2015-8339.
Furthermore error handling so far wrongly included the release of a
lock. That lock, however, was either not acquired or already released
on all paths leading to the error handling sequence. This is
CVE-2015-8340.
A malicious guest administrator may be able to deny service by
crashing the host or causing a deadlock.