[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