pin count / page reference race in grant table code

ISSUE DESCRIPTION

Grant copying code made an implication that any grant pin would be accompanied by a suitable page reference. Other portions of code, however, did not match up with that assumption. When such a grant copy operation is being done on a grant of a dying domain, the assumption turns out wrong.

IMPACT

A malicious guest administrator can cause hypervisor memory corruption, most likely resulting in host crash and a Denial of Service. Privilege escalation and information leaks cannot be ruled out.

VULNERABLE SYSTEMS

Xen versions from 4.2 onwards are vulnerable. Xen versions 4.1 and earlier are not vulnerable.
Both x86 and ARM are vulnerable, and on x86 both PV and HVM guests can trigger the vulnerability.