Re: [Qemu-devel] [RFC] Virtio RNG: Consider changing the default entropy source to /dev/urandom?

[Nikos Mavrogiannopoulos]

In terms of RHEL what is preferred is (1) use a crypto lib, and (2) if
that's not possible use getrandom(). That is summarized in this
article:

https://www.redhat.com/en/blog/understanding-red-hat-enterprise-linux-random-number-generator-interface

On Thu, May 2, 2019 at 8:02 PM Kashyap Chamarthy <address@hidden> wrote:
>
> [Reviving this old thread as I don't think we came to a conclusion on
> this.]
>
> On Fri, Sep 21, 2018 at 05:43:23PM +0200, Kashyap Chamarthy wrote:
> > Hi folks,
> >
> > As Markus pointed out in this 'qemu-devel' thread[1],
> > backends/rng-random.c uses '/dev/random' in TYPE_RNG_RANDOM's
> > instance_init() method:
> >
> >     [...]
> >     static void rng_random_init(Object *obj)
> >     {
> >         RngRandom *s = RNG_RANDOM(obj);
> >
> >         object_property_add_str(obj, "filename",
> >                                 rng_random_get_filename,
> >                                 rng_random_set_filename,
> >                                 NULL);
> >
> >         s->filename = g_strdup("/dev/random");
> >         s->fd = -1;
> >     }
> >     [...]
> >
> > And I've looked at hw/virtio/virtio-rng.c:
> >
> >     [...]
> >     static void virtio_rng_device_realize(DeviceState *dev, Error **errp)
> >     {
> >     [...]
> >
> >         if (vrng->conf.rng == NULL) {
> >             vrng->conf.default_backend = 
> > RNG_RANDOM(object_new(TYPE_RNG_RANDOM));
> >     [...]
> >
> > From the above, I'm assuming QEMU uses `/dev/random` as the _default_
> > entropy source for a 'virtio-rng-pci' device.  If my assumption is
> > correct, any reason why not to change the default entropy source for
> > 'virtio-rng-pci' devices to `/dev/urandom` (which is the preferred[2]
> > source of entropy)?
> >
> > And I understand (thanks: Eric Blake for correcting my confusion) that
> > there are two cases to distinguish:
> >
> > (a) When QEMU needs a random number, the entropy source it chooses.
> >     IIUC, the answer is: QEMU defers to GnuTLS by default, which uses
> >     getrandom(2), which in turn uses '/dev/urandom' as its entropy
> >     source; if getrandom(2) isn't available, GnuTLS uses `/dev/urandom`
> >     anyway.  (Thanks: Nikos for clarifying this.)
> >
> >     If QEMU is built with GnuTLS _disabled_, which I'm not sure if any
> >     Linux distribution does, then it uses libgcrypt, which in turn uses
> >     the undesired and legacy `/dev/random` as the default entropy
> >     source.
> >
> > (b) When QEMU exposes a Virtio RNG device to the guest, that device
> >     needs a source of entropy, and IIUC, that source needs to be
> >     "non-blocking" (i.e. `/dev/urandom`).  However, currently QEMU
> >     defaults to the problematic `/dev/random`.
> >
> > I'd like to get some more clarity on case (b).
> >
> >
> > [1] https://lists.nongnu.org/archive/html/qemu-devel/2018-06/msg08335.html
> >     -- RNG: Any reason QEMU doesn't default to `/dev/urandom`
> >
> > [2] http://man7.org/linux/man-pages/man4/urandom.4.html
> >
> >
> > --
> > /kashyap
> >
>
> --
> /kashyap