Re: [Qemu-devel] [PATCH v3 1/3] VirtIO-RNG: Update default entropy source to `/dev/urandom`

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, May 10, 2019 at 03:42:01PM +0200, Laurent Vivier wrote:
> From: Kashyap Chamarthy <kchamart@redhat.com>
> 
> When QEMU exposes a VirtIO-RNG device to the guest, that device needs a
> source of entropy, and that source needs to be "non-blocking", like
> `/dev/urandom`.  However, currently QEMU defaults to the problematic
> `/dev/random`, which is "blocking" (as in, it waits until sufficient
> entropy is available).
> 
> Why prefer `/dev/urandom` over `/dev/random`?
> ---------------------------------------------
> 
> The man pages of urandom(4) and random(4) state:
> 
>     "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."
> 
> Further, the "Usage" section of the said man pages state:
> 
>     "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 randomness during early boot
>     time; for these applications, getrandom(2) must be used instead,
>     because it will block until the entropy pool is initialized.

So how about just using getrandom then?

> 
>     "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 immediately
>     available."
> 
> And refer to random(7) for a comparison of `/dev/random` and
> `/dev/urandom`.
> 
>     - - -
> 
> Given the above, change the entropy source for VirtIO-RNG device to
> `/dev/urandom`.
> 
> Related discussion in these[1][2] past threads.
> 
> [1] https://lists.nongnu.org/archive/html/qemu-devel/2018-06/msg08335.html
>     -- "RNG: Any reason QEMU doesn't default to `/dev/urandom`?"
> [2] https://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg02724.html
>     -- "[RFC] Virtio RNG: Consider changing the default entropy source to
>        /dev/urandom"
> 
> Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
> Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> ---
>  backends/rng-random.c | 2 +-
>  qemu-options.hx       | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/backends/rng-random.c b/backends/rng-random.c
> index e2a49b0571d7..eff36ef14084 100644
> --- a/backends/rng-random.c
> +++ b/backends/rng-random.c
> @@ -112,7 +112,7 @@ static void rng_random_init(Object *obj)
>                              rng_random_set_filename,
>                              NULL);
>  
> -    s->filename = g_strdup("/dev/random");
> +    s->filename = g_strdup("/dev/urandom");
>      s->fd = -1;
>  }
>  
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 0191ef8b1eb7..4df0ea3aed5c 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4286,7 +4286,7 @@ 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}.
> +entropy from and if omitted defaults to @option{/dev/urandom}.
>  
>  @item -object rng-egd,id=@var{id},chardev=@var{chardevid}
>  
> -- 
> 2.20.1