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Re: [Qemu-devel] RNG: Any reason QEMU doesn't default to `/dev/urandom`?

From: Markus Armbruster
Subject: Re: [Qemu-devel] RNG: Any reason QEMU doesn't default to `/dev/urandom`?
Date: Thu, 28 Jun 2018 14:15:14 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)

Kashyap Chamarthy <address@hidden> writes:

> QEMU still defaults to /dev/random as entropy source.  Any reason why
> not default to /dev/urandom?
> The other day Dan Berrangé explained elsewhere that /dev/urandom is
> recommended -- as it is non-blocking; and doesn't have the same
> limitations of /dev/random, which is a legacy interface.  (And other
> applications[*] are any way overriding the QEMU default to
> /dev/urandom.)
> `random(4)` says the following about the blocking nature of /dev/random:
>        The /dev/random device is a legacy interface which dates back to
>        a time where the cryptographic primitives used in the
>        implementation of /dev/urandom were not widely trusted.  It will
>        return random bytes only within the estimated number of bits of
>        fresh noise in the entropy pool, blocking if necessary.
>        /dev/random is suitable for applications  that  need high quality
>        randomness, and can afford indeterminate delays.
> And its "Usage" section says:
>        The  /dev/random  interface is considered a legacy interface, and
>        /dev/urandom is preferred and sufficient in all use cases, with
>        the exception of applications which require ran‐ domness during
>        early boot time; for these applications, getrandom(2) must be
>        used instead, because it will block until the entropy pool is
>        initialized.
>        If a seed file is saved across reboots as recommended below (all
>        major Linux distributions have done this since 2000 at least),
>        the output  is  cryptographically  secure  against attackers
>        without local root access as soon as it is reloaded in the boot
>        sequence, and perfectly adequate for network encryption session
>        keys.  Since reads from /dev/random may block, users will usually
>        want to open it in nonblocking mode (or perform a read with
>        timeout), and provide some sort of user notification if the
>        desired entropy is  not  immedi‐ ately available.
> [*] E.g. libguestfs:
> https://github.com/libguestfs/libguestfs/blob/master/lib/launch-direct.c#L592

There's also getrandom(2).

See random(7) for a comparison between getrandom(), /dev/urandom,

As you wrote, Linux's /dev/random blocks when the kernel entropy pool
has been depleted, while /dev/urandom doesn't.  There are systems where
both devices behave exactly the same, or only /dev/random exists.
Trying /dev/urandom first, and /dev/random as fallback is simple and
works okay across a wide range of hosts.  That said, getrandom(2) or
getentropy(3) are even nicer when available.

I can see two uses of /dev/random in QEMU outside tests:

* crypto/random-platform.c

    int qcrypto_random_init(Error **errp)
    #ifndef _WIN32
        /* TBD perhaps also add support for BSD getentropy / Linux
         * getrandom syscalls directly */
        fd = open("/dev/urandom", O_RDONLY);
        if (fd == -1 && errno == ENOENT) {
            fd = open("/dev/random", O_RDONLY);

        if (fd < 0) {
            error_setg(errp, "No /dev/urandom or /dev/random found");
            return -1;

        return 0;

  Looks good to me.  Resolving the TBD would be nice.

* backends/rng-random.c

    static void rng_random_init(Object *obj)
        RngRandom *s = RNG_RANDOM(obj);

        object_property_add_str(obj, "filename",

        s->filename = g_strdup("/dev/random");
        s->fd = -1;

   This is TYPE_RNG_RANDOM's instance_init() method.  Doesn't look so
   good, but it's "only" a default.

   What TYPE_RNG_RANDOM's intended use?  The manual suggests "backend
   for virtio-rng":

    @item -object rng-random,address@hidden,address@hidden/dev/random}

    Creates a random number generator backend which obtains entropy from
    a device on the host. The @option{id} parameter is a unique ID that
    will be used to reference this entropy backend from the @option{virtio-rng}
    device. The @option{filename} parameter specifies which file to obtain
    entropy from and if omitted defaults to @option{/dev/random}.

Regardless of other considerations, duplicating something as hairy as
getting high-quality random numbers from the host in a portable manner
is a Bad Idea.

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