KVM: x86: Reload pit counters for all channels when restoring state

Currently if userspace restores the pit counters with a count of 0
on channels 1 or 2 and the guest attempts to read the count on those
channels, then KVM will perform a mod of 0 and crash.  This will ensure
that 0 values are converted to 65536 as per the spec.

This is CVE-2015-7513.

Signed-off-by: Andy Honig <ahonig@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
int kvm_set_shared_msr(unsigned slot, u64 value, u64 mask)
{
	unsigned int cpu = smp_processor_id();
	struct kvm_shared_msrs *smsr = per_cpu_ptr(shared_msrs, cpu);
	int err;

	if (((value ^ smsr->values[slot].curr) & mask) == 0)
		return 0;
	smsr->values[slot].curr = value;
	err = wrmsrl_safe(shared_msrs_global.msrs[slot], value);
	if (err)
		return 1;

	if (!smsr->registered) {
		smsr->urn.on_user_return = kvm_on_user_return;
		user_return_notifier_register(&smsr->urn);
		smsr->registered = true;
	}
	return 0;
}

int kvm_set_shared_msr(unsigned slot, u64 value, u64 mask)
{
	unsigned int cpu = smp_processor_id();
	struct kvm_shared_msrs *smsr = per_cpu_ptr(shared_msrs, cpu);
	int err;

	if (((value ^ smsr->values[slot].curr) & mask) == 0)
		return 0;
	smsr->values[slot].curr = value;
	err = wrmsrl_safe(shared_msrs_global.msrs[slot], value);
	if (err)
		return 1;

	if (!smsr->registered) {
		smsr->urn.on_user_return = kvm_on_user_return;
		user_return_notifier_register(&smsr->urn);
		smsr->registered = true;
	}
	return 0;
}
