KVM: fix unregistering coalesced mmio zone from wrong bus

If you register a kvm_coalesced_mmio_zone with '.pio = 0' but then
unregister it with '.pio = 1', KVM_UNREGISTER_COALESCED_MMIO will try to
unregister it from KVM_PIO_BUS rather than KVM_MMIO_BUS, which is a
no-op.  But it frees the kvm_coalesced_mmio_dev anyway, causing a
use-after-free.

Fix it by only unregistering and freeing the zone if the correct value
of 'pio' is provided.

Reported-by: syzbot+f87f60bb6f13f39b54e3@syzkaller.appspotmail.com
Fixes: 0804c849 ("kvm/x86 : add coalesced pio support")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

virt/kvm/coalesced_mmio.c

@@ -175,10 +175,14 @@ int kvm_vm_ioctl_unregister_coalesced_mmio(struct kvm *kvm,
 {
 	struct kvm_coalesced_mmio_dev *dev, *tmp;
 
+	if (zone->pio != 1 && zone->pio != 0)
+		return -EINVAL;
+
 	mutex_lock(&kvm->slots_lock);
 
 	list_for_each_entry_safe(dev, tmp, &kvm->coalesced_zones, list)
-		if (coalesced_mmio_in_range(dev, zone->addr, zone->size)) {
+		if (zone->pio == dev->zone.pio &&
+		    coalesced_mmio_in_range(dev, zone->addr, zone->size)) {
 			kvm_io_bus_unregister_dev(kvm,
 				zone->pio ? KVM_PIO_BUS : KVM_MMIO_BUS, &dev->dev);
 			kvm_iodevice_destructor(&dev->dev);