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 <kcham...@redhat.com> 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