At 10:38 PM +0300 8/2/10, Yaron Sheffer wrote: >the interesting thread on seeding and reseeding /dev/random did not mention >that many of the most problematic systems in this respect are virtual >machines. Such machines (when used for "cloud computing") are not only >servers, so have few sources of true and hard-to-observe entropy. Often the >are cloned from snapshots of a single virtual machine, i.e. many VMs start >life with one common RNG state, that doesn't even know that it's a clone. > >In addition to the mitigations that were discussed on the list, such machines >could benefit from seeding /dev/random (or periodically reseeding it) from the >*host machine's* RNG. This is one thing that's guaranteed to be different >between VM instances. So my question to the list: is this useful? Is this >doable with popular systems (e.g. Linux running on VMWare or VirtualBox)? Is >this actually being done?
It is certainly doable: put a "file" on the host whose contents are random and change every second. On the VM, read that file on wakeup or boot and mix it into /dev/random. This guarantees a different value for each wakeup/boot, but not that every cloned machine that starts will have a unique state (because they might start within the same refresh. If you need that, you probably want to automatically mix a microsecond-accurate time at the same time. --Paul Hoffman, Director --VPN Consortium --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to majord...@metzdowd.com
