A dangling pointer access flaw was found in the way qemu handled hot-unplugging virtio devices. This flaw was introduced by virtio refactoring and exists in the virtio-pci implementation. When the virtio-blk-pci device is deleted, the virtio-blk-device is removed first (removal is done in post-order). Later, the virtio-blk-device is accessed again, but proxy->vdev->vq is no longer valid (a dangling pointer) and kvm_set_ioeventfd_pio fails.
A privileged guest user could use this flaw to crash the qemu process on the host system, causing a denial of service to it and any other running virtual machines.
Patches are available at http://thread.gmane.org/gmane.comp.emulators.qemu/234440
Acknowledgements:
This issue was discovered by Sibiao Luo of Red Hat.