linux-kernel - Re: [PATCH v3 0/8] Rework random blocking

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Date:   Fri, 27 Dec 2019 07:29:20 +0800
From:   Andy Lutomirski <luto@...capital.net>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Stephan Mueller <smueller@...onox.de>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        Lennart Poettering <mzxreary@...inter.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Alexander E. Patrakov" <patrakov@...il.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Willy Tarreau <w@....eu>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>
Subject: Re: [PATCH v3 0/8] Rework random blocking


>> On Dec 26, 2019, at 10:04 PM, Theodore Y. Ts'o <tytso@....edu> wrote:
>> 
>> On Thu, Dec 26, 2019 at 01:03:34PM +0100, Stephan Mueller wrote:
>> Agreed. I was just trying to outline that the removal of the blocking_pool is
>> a good thing. Even when we decide that random.c should receive a TRNG, we do
>> not need to re-add a blocking pool, but can easily use the existing ChaCha20
>> DRNG (most likely with its own instance).
> 
> Well, it depends on what you mean by "TRNG" --- the ChaCha20 DRNG only
> has a state of 256 bits.  So if you want to only depend on "true
> entropy" you can't extract more than 256 bits without violating that
> assumption, at least if you're using a very strict definition of TRNG.
> 
> By getting rid of the blocking pool, and making /dev/random work like
> getrandom with flags set to 0, we're effectively abandoning any kind
> of assertion that /dev/random is some kind of TRNG.  This is not
> insane; this is what the *BSD's have always done.
> 
> But once we do this, and /dev/random takes on the semantics of "block
> until the CRNG has been initialized, and then it won't block after
> that", if we change it so that it now has some different semantics,
> such as "one you extract a 256-bit key, the read from /dev/random will
> block until we can refill it, which might take seconds, minutes or
> hours", will be considered a regression, and we can't do that.

I don’t think Stephan was proposing that. He was proposing a way to implement a new interface that blocks.

> 
> Of course, we can hope that people will be using getrandom() and there
> will be very few new users of the /dev/random pathname.  But nothing
> is ever guaranteed..
> 
>                       - Ted