XENMEM_exchange error handling issues

ISSUE DESCRIPTION

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.

IMPACT

A malicious guest administrator may be able to deny service by crashing the host or causing a deadlock.

VULNERABLE SYSTEMS

All Xen versions from at least 3.2 onwards are vulnerable. Older versions have not been inspected.