linux-kernel - Re: [PATCH v4 01/13] random: invalidate batched entropy after crng init

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Date:   Wed, 7 Jun 2017 19:58:35 -0400
From:   Theodore Ts'o <tytso@....edu>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Miller <davem@...emloft.net>,
        Eric Biggers <ebiggers3@...il.com>
Subject: Re: [PATCH v4 01/13] random: invalidate batched entropy after crng
 init

On Tue, Jun 06, 2017 at 07:47:52PM +0200, Jason A. Donenfeld wrote:
> It's possible that get_random_{u32,u64} is used before the crng has
> initialized, in which case, its output might not be cryptographically
> secure. For this problem, directly, this patch set is introducing the
> *_wait variety of functions, but even with that, there's a subtle issue:
> what happens to our batched entropy that was generated before
> initialization. Prior to this commit, it'd stick around, supplying bad
> numbers. After this commit, we force the entropy to be re-extracted
> after each phase of the crng has initialized.
> 
> In order to avoid a race condition with the position counter, we
> introduce a simple rwlock for this invalidation. Since it's only during
> this awkward transition period, after things are all set up, we stop
> using it, so that it doesn't have an impact on performance.
> 
> This should probably be backported to 4.11.
> 
> (Also: adding my copyright to the top. With the patch series from
> January, this patch, and then the ones that come after, I think there's
> a relevant amount of code in here to add my name to the top.)
> 
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

Thanks, applied.  This will be on the for_stable that I will be
sending to Linus sometime during 4.12-rcX.

					- Ted