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

[Kashyap Chamarthy]

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