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How to lock up a VirtualBox host machine with a guest using tracepath over virtio-net network interface
From: Thomas Dreibholz <dreibh () simula no>
Date: Fri, 21 Jun 2013 16:33:35 +0200


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 
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, 


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