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

[Laszlo Ersek]

On 09/21/18 17:43, 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
> 
> 

The libvirt domain documentation <https://libvirt.org/formatdomain.html>
also says,

"When no file name is specified, the hypervisor default is used. For
QEMU, the default is /dev/random. However, the recommended source of
entropy is /dev/urandom (as it doesn't have the limitations of
/dev/random)."

Thanks
Laszlo