linux-kernel - Re: [PATCH net-next v5 02/20] zinc: introduce minimal cryptography library

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Date:   Tue, 25 Sep 2018 16:29:03 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Eric Biggers <ebiggers@...gle.com>,
        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>
Subject: Re: [PATCH net-next v5 02/20] zinc: introduce minimal cryptography library

Hey Arnd,

On Tue, Sep 25, 2018 at 9:18 AM Arnd Bergmann <arnd@...db.de> wrote:
> I think I misread your earlier sentence and thought you had said the
> exact opposite.
>
> For confirmation, I've downloaded your git tree and built it with my
> collection of compilers (gcc-4.6 through 8.1) and tried building it
> in various configurations. Nothing alarming stood out, the only
> thing that I think would might warrant some investigation is this one:
>
> lib/zinc/curve25519/curve25519-hacl64.h: In function 'curve25519_generic':
> lib/zinc/curve25519/curve25519-hacl64.h:785:1: warning: the frame size
> of 1536 bytes is larger than 500 bytes [-Wframe-larger-than=]
>
> Without KASAN, this takes 832 bytes, which is still more than it should
> use from a look at the source code.
>
> I first suspected some misoptimization around the get/put_unaligned_le64()
> calls, but playing around with it some more led me to this patch:

Excellent detective work. Thanks for spotting that before I had a time
to respond here. I'll also send that same fix to the HACL* team at
INRIA, so they can add it as a heuristic.

Jason