Missing alignment check in VCPUOP_register_vcpu_info

ISSUE DESCRIPTION

The hypercall VCPUOP_register_vcpu_info is used by a guest to register a shared region with the hypervisor. The region will be mapped into Xen address space so it can be directly accessed.
On Arm, the region is accessed with instructions which require a specific alignment. Unfortunately, there is no check that the address provided by the guest will be correctly aligned.
As a result, a malicious guest could cause a hypervisor crash by passing a misaligned address.

IMPACT

A malicious guest administrator may cause a hypervisor crash, resulting in a Denial of Service (DoS).

VULNERABLE SYSTEMS

All Xen versions are vulnerable.
Only Arm systems are vulnerable. x86 systems are not affected.