linux-kernel - Re: [PATCH 1/5] crypto: chacha20 - Fix unaligned access when loading constants

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Date:   Wed, 22 Nov 2017 20:26:31 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "Theodore Ts'o" <tytso@....edu>,
        "Jason A . Donenfeld" <Jason@...c4.com>,
        Martin Willi <martin@...ongswan.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH 1/5] crypto: chacha20 - Fix unaligned access when loading constants

On 22 November 2017 at 19:51, Eric Biggers <ebiggers3@...il.com> wrote:
> From: Eric Biggers <ebiggers@...gle.com>
>
> The four 32-bit constants for the initial state of ChaCha20 were loaded
> from a char array which is not guaranteed to have the needed alignment.
>
> Fix it by just assigning the constants directly instead.
>
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>

I'm not thrilled about the open coded hex numbers but I don't care
enough to object.

Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>

> ---
>  crypto/chacha20_generic.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
>
> diff --git a/crypto/chacha20_generic.c b/crypto/chacha20_generic.c
> index 4a45fa4890c0..ec84e7837aac 100644
> --- a/crypto/chacha20_generic.c
> +++ b/crypto/chacha20_generic.c
> @@ -41,12 +41,10 @@ static void chacha20_docrypt(u32 *state, u8 *dst, const u8 *src,
>
>  void crypto_chacha20_init(u32 *state, struct chacha20_ctx *ctx, u8 *iv)
>  {
> -       static const char constant[16] = "expand 32-byte k";
> -
> -       state[0]  = le32_to_cpuvp(constant +  0);
> -       state[1]  = le32_to_cpuvp(constant +  4);
> -       state[2]  = le32_to_cpuvp(constant +  8);
> -       state[3]  = le32_to_cpuvp(constant + 12);
> +       state[0]  = 0x61707865; /* "expa" */
> +       state[1]  = 0x3320646e; /* "nd 3" */
> +       state[2]  = 0x79622d32; /* "2-by" */
> +       state[3]  = 0x6b206574; /* "te k" */
>         state[4]  = ctx->key[0];
>         state[5]  = ctx->key[1];
>         state[6]  = ctx->key[2];
> --
> 2.15.0.448.gf294e3d99a-goog
>