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

[Kashyap Chamarthy]

On Thu, Jun 28, 2018 at 02:15:14PM +0200, Markus Armbruster wrote:
> Kashyap Chamarthy <address@hidden> writes:

[...]

> There's also getrandom(2).
> 
> See random(7) for a comparison between getrandom(), /dev/urandom,
> /dev/random.
> 
> 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;
>         }
>     #else
>     [...]
>     #endif
> 
>         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",
>                                 rng_random_get_filename,
>                                 rng_random_set_filename,
>                                 NULL);
> 
>         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.

I see, thanks for the detailed responses, both.  This is not really a
high-priority item for management layers for now.  For now, (OpenStack)
Nova overrides the QEMU default.

-- 
/kashyap