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