Description of problem:

The recent fix to validate the frontend's frame buffer description
neglected to limit the frame buffer size correctly. This lets a
malicious frontend make the backend attempt to map an arbitrary amount
of guest memory, which could be useful for a denial of service attack
against dom0.

Proposed upstream patch:

http://xenbits.xensource.com/xen-unstable.hg?rev/9044705960cb30cec385bdca7305bcf7db096721