Lock order reversal between page allocation and grant table locks

ISSUE DESCRIPTION

The locks page_alloc_lock and grant_table.lock are not always taken in the same order. This opens the possibility of deadlock.

IMPACT

A malicious guest administrator can deny service to the entire host.

VULNERABLE SYSTEMS

Xen versions going back to at least Xen 3.2 are vulnerable.
To exploit the vulnerability, the attacker must have control of more than one vcpu, either by controlling a malicious multi-vcpu guest, or by controlling more than one guest.