memop: limit guest specified extent order

memop: limit guest specified extent order

Allowing unbounded order values here causes almost unbounded loops
and/or partially incomplete requests, particularly in PoD code.

The added range checks in populate_physmap(), decrease_reservation(),
and the "in" one in memory_exchange() architecturally all could use
PADDR_BITS - PAGE_SHIFT, and are being artificially constrained to
MAX_ORDER.

This is XSA-31 / CVE-2012-5515.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>