linux-kernel - Re: [RFC PATCH v2 04/12] crypto: chacha

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Date:   Fri, 19 Oct 2018 11:28:05 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        "Jason A . Donenfeld" <Jason@...c4.com>,
        Greg Kaiser <gkaiser@...gle.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Samuel Neves <samuel.c.p.neves@...il.com>,
        Michael Halcrow <mhalcrow@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fscrypt@...r.kernel.org,
        Tomer Ashur <tomer.ashur@...t.kuleuven.be>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Paul Crowley <paulcrowley@...gle.com>
Subject: Re: [RFC PATCH v2 04/12] crypto: chacha - add XChaCha12 support

Hi Ard,

On Fri, Oct 19, 2018 at 10:34:41PM +0800, Ard Biesheuvel wrote:
> > diff --git a/include/crypto/chacha.h b/include/crypto/chacha.h
> > index ae79e9983c72f..3d261f5cd156d 100644
> > --- a/include/crypto/chacha.h
> > +++ b/include/crypto/chacha.h
> > @@ -5,6 +5,11 @@
> >   * XChaCha extends ChaCha's nonce to 192 bits, while provably retaining ChaCha's
> >   * security.  Here they share the same key size, tfm context, and setkey
> >   * function; only their IV size and encrypt/decrypt function differ.
> > + *
> > + * The ChaCha paper specifies 20, 12, and 8-round variants.  In general, it is
> > + * recommended to use the 20-round variant ChaCha20.  However, the other
> > + * variants can be needed in some performance-sensitive scenarios.  The generic
> > + * ChaCha code currently allows only the 20 and 12-round variants.
> >   */
> >
> >  #ifndef _CRYPTO_CHACHA_H
> > @@ -39,6 +44,8 @@ void crypto_chacha_init(u32 *state, struct chacha_ctx *ctx, u8 *iv);
> >
> >  int crypto_chacha20_setkey(struct crypto_skcipher *tfm, const u8 *key,
> >                            unsigned int keysize);
> > +int crypto_chacha12_setkey(struct crypto_skcipher *tfm, const u8 *key,
> > +                          unsigned int keysize);
> >
> >  int crypto_chacha_crypt(struct skcipher_request *req);
> >  int crypto_xchacha_crypt(struct skcipher_request *req);
> > diff --git a/lib/chacha.c b/lib/chacha.c
> > index 0a2c2e5b7b84d..c4d69a83fcd2d 100644
> > --- a/lib/chacha.c
> > +++ b/lib/chacha.c
> > @@ -21,7 +21,7 @@ static void chacha_permute(u32 *x, int nrounds)
> >         int i;
> >
> >         /* whitelist the allowed round counts */
> > -       BUG_ON(nrounds != 20);
> > +       BUG_ON(nrounds != 20 && nrounds != 12);
> >
> 
> I didn't spot this until this patch, but BUG_ON() may bring down the
> kernel, and so it should really only be used as a last resort. (i.e.,
> if this is called from non-process context things may explode rather
> painfully)
> 
> I didn't look at the entire file [which is a bit cumbersome while
> reviewing incremental changes like this] and so I don't really have
> another suggestion right now, but please try to come up with something
> better if you can.
> 

I'll change it to WARN_ON_ONCE(), I guess.  I do still want it to be very noisy
if something fishy is going on with the round count.

- Eric