linux-kernel - Re: [RFC][PATCH 0/6] /dev/random

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Date:	Mon, 25 Apr 2016 10:02:08 +0200
From:	Stephan Mueller <smueller@...onox.de>
To:	Nikos Mavrogiannopoulos <nmav@...tls.org>
Cc:	Ted Tso <tytso@....edu>, Herbert Xu <herbert@...dor.apana.org.au>,
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Sandy Harris <sandyinchina@...il.com>
Subject: Re: [RFC][PATCH 0/6] /dev/random - a new approach

Am Montag, 25. April 2016, 09:55:14 schrieb Nikos Mavrogiannopoulos:

Hi Nikos,

> On Thu, Apr 21, 2016 at 5:16 PM, Stephan Mueller <smueller@...onox.de> 
wrote:
> >> > ... DRBG is “minimally” seeded with 112^6 bits of entropy.
> >> > This is commonly achieved even before user space is initiated.
> >> 
> >> Unfortunately one of the issues of the /dev/urandom interface is the
> >> fact that it may start providing random numbers even before the
> >> seeding is complete. From the above quote, I understand that this
> >> issue is not addressed by the new interface. That's a serious
> >> limitation (of the current and inherited by the new implementation),
> >> since most/all newly deployed systems from "cloud" images generate
> >> keys using /dev/urandom (for sshd for example) on boot, and it is
> >> unknown to these applications whether they operate with uninitialized
> >> seed.
> > 
> > One more item to consider: If you do not want to change to use
> > getrandom(2), the LRNG provides you with another means.
> 
> The main problem is not about willing to switch to getrandom() or not,
> but finding any system where getrandom() exists. Today due to libc not
> having the call, we can only use /dev/urandom and applications would
> most likely continue to do so long time after getrandom() is
> introduced to libc.

Implement the syscall yourself with syscall(). If you get ENOSYS back, revert 
to your old logic of seeding from /dev/urandom.

If you know you are on kernels >= 3.14, you could use the following steps in 
your library:

- poll /proc/sys/kernel/random/entropy_avail in spaces of, say, one second and 
block your seeding process until that value becomes non-zero

- if you unblock, seed from /dev/urandom and you have the guarantee of having 
a /dev/urandom seeded with 128 bits.
> 
> regards,
> Nikos


Ciao
Stephan