I have discovered a problem with the VirtualBox virtio-net network driver that leads to a lockup of the host machine's kernel and the need for a hard reset to make it working again. The bug had been reported to the VirtualBox bug tracker 8 days ago (https://www.virtualbox.org/ticket/11863), with the usual reaction from Oracle support (i.e. none). The bug can be reproduced easily as follows: - The host system is a 64-bit Linux (tested with Ubuntu 12.04 LTS and Kubuntu 13.04). Did not try 32 bit. - VirtualBox is the latest version 4.2.12 (using Oracle's Ubuntu repository). - Create a new VM, use e.g. Kubuntu live CD image (32 or 64 bit, makes no difference). No disk needed. - Network adapter is: Bridged, Adapter Type: virtio-net. Boot the system, ensure that network is working. - tracepath 8.8.8.8 Now, the virtual machine locks up and the host machine's kernel seems to have at least one core blocked. The host machine's console output is "BUG: soft lockup - CPU #2 stuck for 22s ...". Also, the network on the host machine does not work any more. For example, "ifconfig" just hangs. - To recover the host machine, it needs a hard reset. "sudo reboot", etc. will not work, since the kernel seems to hang. This bug is critical, since it makes the host machine's network unusable (particularly, if the host system is at a remote location), and it is very easy to trigger with just a simple, standard "tracepath" call inside a virtual machine. It is therefore trivial for a normal user in such a machine to trigger a denial of service. I did no further investigation of the problem yet, but if it is related to the path MTU discovery by tracepath, it might be possible to trigger it by a lot of other software as well. Best regards, Thomas