linux-kernel - Re: [PATCH] poly1305: generic C can be faster on chips with slow unaligned access

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Message-ID: <20161107192505.GB34388@google.com>
Date:   Mon, 7 Nov 2016 11:25:05 -0800
From:   Eric Biggers <ebiggers@...gle.com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     David Miller <davem@...emloft.net>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        linux-crypto@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Martin Willi <martin@...ongswan.org>,
        WireGuard mailing list <wireguard@...ts.zx2c4.com>,
        René van Dorst <opensource@...rst.com>
Subject: Re: [PATCH] poly1305: generic C can be faster on chips with slow
 unaligned access

On Mon, Nov 07, 2016 at 08:02:35PM +0100, Jason A. Donenfeld wrote:
> On Mon, Nov 7, 2016 at 7:26 PM, Eric Biggers <ebiggers@...gle.com> wrote:
> >
> > I was not referring to any users in particular, only what users could do.  As an
> > example, if you did crypto_shash_update() with 32, 15, then 17 bytes, and the
> > underlying algorithm is poly1305-generic, the last block would end up
> > misaligned.  This doesn't appear possible with your pseudocode because it only
> > passes in multiples of the block size until the very end.  However I don't see
> > it claimed anywhere that shash API users have to do that.
> 
> Actually it appears that crypto/poly1305_generic.c already buffers
> incoming blocks to a buffer that definitely looks aligned, to prevent
> this condition!
> 

No it does *not* buffer all incoming blocks, which is why the source pointer can
fall out of alignment.  Yes, I actually tested this.  In fact this situation is
even hit, in both possible places, in the self-tests.

Eric