From: Eric Biggers <ebiggers@google.com>
Subject: Re: [PATCH] poly1305: generic C can be faster on chips
 with slow unaligned access
Date: Mon, 7 Nov 2016 11:25:05 -0800
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Cc: Herbert Xu <herbert@gondor.apana.org.au>,
 Martin Willi <martin@strongswan.org>, LKML <linux-kernel@vger.kernel.org>,
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To: "Jason A. Donenfeld" <Jason@zx2c4.com>
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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@google.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