KVM: Improve create VCPU parameter (CVE-2013-4587)

In multiple functions the vcpu_id is used as an offset into a bitfield.  Ag
malicious user could specify a vcpu_id greater than 255 in order to set or
clear bits in kernel memory.  This could be used to elevate priveges in the
kernel.  This patch verifies that the vcpu_id provided is less than 255.
The api documentation already specifies that the vcpu_id must be less than
max_vcpus, but this is currently not checked.

Reported-by: Andrew Honig <ahonig@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Andrew Honig <ahonig@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
int kvm_vm_ioctl_set_memory_region(struct kvm *kvm,
				   struct kvm_userspace_memory_region *mem)
{
	if (mem->slot >= KVM_USER_MEM_SLOTS)
		return -EINVAL;
	return kvm_set_memory_region(kvm, mem);
}

int kvm_vm_ioctl_set_memory_region(struct kvm *kvm,
				   struct kvm_userspace_memory_region *mem)
{
	if (mem->slot >= KVM_USER_MEM_SLOTS)
		return -EINVAL;
	return kvm_set_memory_region(kvm, mem);
}
