Use of v2 grant tables may cause crash on ARM

ISSUE DESCRIPTION

ARM never properly implemented grant table v2, either in the hypervisor or in Linux.
Unfortunately, an ARM guest can still request v2 grant tables; they will simply not be properly set up, resulting in subsequent grant-related hypercalls hitting BUG() checks.

IMPACT

An unprivileged guest can cause a BUG() check in the hypervisor, resulting in a denial-of-service.

VULNERABLE SYSTEMS

Only ARM systems are vulnerable. All supported versions of Xen are vulnerable.