Reviewed: https://review.opendev.org/616603 Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=de512f2c025429b72ade5a5ec38a6f1bde60af3c Submitter: Zuul Branch: master
commit de512f2c025429b72ade5a5ec38a6f1bde60af3c Author: Kashyap Chamarthy <[email protected]> Date: Thu Sep 20 11:16:40 2018 +0200 libvirt: Add a default VirtIO-RNG device to guests tl;dr: We're adding the default VirtIO-RNG device to ensure guests are not starved of entropy (and thus not hang) during boot time. Background ---------- From Nova Git history, commit b94550f419 ("libvirt: configuration element for a random number generator device") _did_ add a default RNG device (but with its entropy source to the undesirable '/dev/random'). However, the default RNG device was immediately removed in another commit (605677c -- "libvirt: remove explicit /dev/random rng default"), with this rationale: libvirt (or rather qemu) will default to /dev/random if no rng device path is specified [...] It's preferable for us to not duplicate this default to allow for a future where libvirt or the hypervisor needs to make more intelligent decisions about the default device to use. The above reasoning doesn't hold up, because: (a) libvirt does not make "policy" decisions, such as choosing an entropy source (or any other such). Therefore Nova, as a management application, should make the decision here. (b) More importantly, when QEMU exposes a VirtIO-RNG device to the guest, that device needs a source of entropy; and QEMU by default uses the legacy and problematic `/dev/random` as the source — instead of the preferred `/dev/urandom`. So QEMU's default for VirtIO-RNG devices is not sufficient, and Nova should not rely on it. (Discussion[+] on 'qemu-devel' list to consider changing QEMU's default.) * * * In this patch: - Make Nova configure a VirtIO-RNG device by default for guests. (Which will be using `/dev/urandom` as the default entropy source.) This will also work for Windows guests, when using VirtIO-Win drivers[*] on the Linux host. - The 'hw_rng_model' image metadata property is now rendered (temporarily) useless -- as it's not used anywhere outside the _add_rng_device() method. But we don't want to deprecate it yet, as we may extend it (see code comment for details); docucment that. [*] https://docs.pagure.org/docs-fedora/create-windows-vms-using-virtio.html [+] https://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg02724.html -- "[RFC] Virtio RNG: Consider changing the default entropy source to /dev/urandom?" Closes-Bug: #1789868 Change-Id: I28e66c9640c38d23b8c0dbd0b05f5260bfcf6d30 Signed-off-by: Kashyap Chamarthy <[email protected]> ** Changed in: nova Status: In Progress => Fix Released -- You received this bug notification because you are a member of Yahoo! Engineering Team, which is subscribed to OpenStack Compute (nova). https://bugs.launchpad.net/bugs/1789868 Title: RFE: Add a virtio-rng device to Nova instances by default Status in OpenStack Compute (nova): Fix Released Bug description: What do we want: Entropy! When do we want it: By default! Nova don't add a RNG to the guests by default; this seems a serious problem - the guests should get a good entropy source to ensure that everything during boot gets randomised, and also you sometimes get hangs where guests are starved of entropy (there are some newer guest kernels that seem more prone to this, but still it's a more general problem). Old arguments for why we didn't have a virtio-rng by default: a) We were using hosts /dev/random and it could block - we moved to urandom and that problem doesn't exist any more b) We didn't have windows drivers? We do now (IMHO this is fairly important - we really should have the RNG) To manage notifications about this bug go to: https://bugs.launchpad.net/nova/+bug/1789868/+subscriptions -- Mailing list: https://launchpad.net/~yahoo-eng-team Post to : [email protected] Unsubscribe : https://launchpad.net/~yahoo-eng-team More help : https://help.launchpad.net/ListHelp
