netdev - Re: [PATCH net-next v5 09/20] zinc: Poly1305 ARM and ARM64 implementations

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Message-ID: <CAHmME9r_qbbYo=sxOvUauXDetJ3uA2A=WdpvjzQwMgqYF3=h1A@mail.gmail.com>
Date:   Wed, 19 Sep 2018 02:17:10 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Samuel Neves <sneves@....uc.pt>,
        Andrew Lutomirski <luto@...nel.org>,
        Jean-Philippe Aumasson <jeanphilippe.aumasson@...il.com>,
        Andy Polyakov <appro@...nssl.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH net-next v5 09/20] zinc: Poly1305 ARM and ARM64 implementations

Hi Eric,

On Wed, Sep 19, 2018 at 12:55 AM Eric Biggers <ebiggers@...nel.org> wrote:
> This will compute the wrong digest if called with simd_context=HAVE_FULL_SIMD
> and then later with simd_context=HAVE_NO_SIMD, since poly1305_blocks_neon()
> converts the accumulator from base 32 to base 26, whereas poly1305_blocks_arm()
> assumes it is still in base 32.  Is that intentional?  I'm sure this is a rare
> case, but my understanding is that the existing crypto API doesn't preclude
> calling successive steps in different contexts.  And I'm concerned that it could
> be relevant in some cases, e.g. especially if people are importing a hash state
> that was exported earlier.  Handling it by silently computing the wrong digest
> is not a great idea...

Indeed you're right; Samuel and I were just discussing that recently.
I'd rather handle this correctly even if the contexts change, so I'll
see if I can fix this up properly for that unlikely case in the next
revision.

Jason