linux-kernel - Re: [PATCH 1/2] random: avoid init'ing twice in credit race

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Message-ID: <Yn4xke0zvIU3bmsu@zx2c4.com>
Date:   Fri, 13 May 2022 12:23:13 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Dominik Brodowski <linux@...inikbrodowski.net>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] random: avoid init'ing twice in credit race

Hi Dominik,

On Fri, May 13, 2022 at 08:23:40AM +0200, Dominik Brodowski wrote:
> Am Mon, May 09, 2022 at 02:14:08PM +0200 schrieb Jason A. Donenfeld:
> > Since all changes of crng_init now go through credit_init_bits(), we can
> > fix a long standing race in which two concurrent callers of
> > credit_init_bits() have the new bit count >= some threshold, but are
> > doing so with crng_init as a lower threshold, checked outside of a lock,
> > resulting in crng_reseed() or similar being called twice.
> 
> Sidenote: crng_reseed() did manage quite fine if called twice in short
> order.

With regards to crng_finialize, it did, but not with regards to
prematurely emptying patches and all that. IOW, buggy but not that bad.

> 
> > In order to fix this, we can use the original cmpxchg value of the bit
> > count, and only change crng_init when the bit count transitions from
> > below a threshold to meeting the threshold.
> > 
> > Cc: Dominik Brodowski <linux@...inikbrodowski.net>
> > Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> 
> 	Reviewed-by: Dominik Brodowski <linux@...inikbrodowski.net>
> 
> Thanks,
> 	Dominik

Jason