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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, May 10, 2019 at 12:12:41PM -0400, Michael S. Tsirkin wrote:
> 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?

The 3rd patch in this series addresses that.


Regards,
Daniel
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