linux-kernel - Re: [PATCH] random: use correct memory barriers for crng_node

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Message-ID: <20200917072644.GA5311@gondor.apana.org.au>
Date:   Thu, 17 Sep 2020 17:26:44 +1000
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     tytso@....edu, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] random: use correct memory barriers for crng_node_pool

Eric Biggers <ebiggers@...nel.org> wrote:
> From: Eric Biggers <ebiggers@...gle.com>
> 
> When a CPU selects which CRNG to use, it accesses crng_node_pool without
> a memory barrier.  That's wrong, because crng_node_pool can be set by
> another CPU concurrently.  Without a memory barrier, the crng_state that
> is used might not appear to be fully initialized.

The only architecture that requires a barrier for data dependency
is Alpha.  The correct primitive to ensure that barrier is present
is smp_barrier_depends, or you could just use READ_ONCE.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt