netdev - Re: [PATCH] net: phy: mscc: avoid skcipher API for single block AES encryption

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Date:   Wed, 24 Jun 2020 21:01:23 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     netdev@...r.kernel.org,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Antoine Tenart <antoine.tenart@...tlin.com>,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH] net: phy: mscc: avoid skcipher API for single block AES encryption

On Wed, 24 Jun 2020 at 18:32, Eric Biggers <ebiggers@...nel.org> wrote:
>
> On Wed, Jun 24, 2020 at 03:34:27PM +0200, Ard Biesheuvel wrote:
> > The skcipher API dynamically instantiates the transformation object on
> > request that implements the requested algorithm optimally on the given
> > platform. This notion of optimality only matters for cases like bulk
> > network or disk encryption, where performance can be a bottleneck, or
> > in cases where the algorithm itself is not known at compile time.
> >
> > In the mscc macsec case, we are dealing with AES encryption of a single
> > block, and so neither concern applies, and we are better off using the
> > AES library interface, which is lightweight and safe for this kind of
> > use.
> >
> > Note that the scatterlist API does not permit references to buffers that
> > are located on the stack, so the existing code is incorrect in any case,
> > but avoiding the skcipher and scatterlist APIs altogether is the most
> > straight-forward approach to fixing this.
> >
> > Cc: Antoine Tenart <antoine.tenart@...tlin.com>
> > Cc: Andrew Lunn <andrew@...n.ch>
> > Cc: Florian Fainelli <f.fainelli@...il.com>
> > Cc: Heiner Kallweit <hkallweit1@...il.com>
> > Cc: "David S. Miller" <davem@...emloft.net>
> > Cc: Jakub Kicinski <kuba@...nel.org>
> > Cc: <stable@...r.kernel.org>
> > Fixes: 28c5107aa904e ("net: phy: mscc: macsec support")
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
> >  drivers/net/phy/Kconfig            |  3 +-
> >  drivers/net/phy/mscc/mscc_macsec.c | 40 +++++---------------
> >  2 files changed, 10 insertions(+), 33 deletions(-)
> >
> > diff --git a/drivers/net/phy/Kconfig b/drivers/net/phy/Kconfig
> > index f25702386d83..e9c05848ec52 100644
> > --- a/drivers/net/phy/Kconfig
> > +++ b/drivers/net/phy/Kconfig
> > @@ -480,8 +480,7 @@ config MICROCHIP_T1_PHY
> >  config MICROSEMI_PHY
> >       tristate "Microsemi PHYs"
> >       depends on MACSEC || MACSEC=n
> > -     select CRYPTO_AES
> > -     select CRYPTO_ECB
> > +     select CRYPTO_LIB_AES
> >       help
> >         Currently supports VSC8514, VSC8530, VSC8531, VSC8540 and VSC8541 PHYs
>
> Shouldn't it be 'select CRYPTO_LIB_AES if MACSEC', since
> mscc_macsec.c is only compiled if MACSEC?
>

Good point, I'll change that.

> >
> > diff --git a/drivers/net/phy/mscc/mscc_macsec.c b/drivers/net/phy/mscc/mscc_macsec.c
> > index b4d3dc4068e2..d53ca884b5c9 100644
> > --- a/drivers/net/phy/mscc/mscc_macsec.c
> > +++ b/drivers/net/phy/mscc/mscc_macsec.c
> > @@ -10,7 +10,7 @@
> >  #include <linux/phy.h>
> >  #include <dt-bindings/net/mscc-phy-vsc8531.h>
> >
> > -#include <crypto/skcipher.h>
> > +#include <crypto/aes.h>
> >
> >  #include <net/macsec.h>
> >
> > @@ -500,39 +500,17 @@ static u32 vsc8584_macsec_flow_context_id(struct macsec_flow *flow)
> >  static int vsc8584_macsec_derive_key(const u8 key[MACSEC_KEYID_LEN],
> >                                    u16 key_len, u8 hkey[16])
> >  {
> > -     struct crypto_skcipher *tfm = crypto_alloc_skcipher("ecb(aes)", 0, 0);
> > -     struct skcipher_request *req = NULL;
> > -     struct scatterlist src, dst;
> > -     DECLARE_CRYPTO_WAIT(wait);
> > -     u32 input[4] = {0};
> > +     const u8 input[AES_BLOCK_SIZE] = {0};
> > +     struct crypto_aes_ctx ctx;
> >       int ret;
> >
> > -     if (IS_ERR(tfm))
> > -             return PTR_ERR(tfm);
> > -
> > -     req = skcipher_request_alloc(tfm, GFP_KERNEL);
> > -     if (!req) {
> > -             ret = -ENOMEM;
> > -             goto out;
> > -     }
> > -
> > -     skcipher_request_set_callback(req, CRYPTO_TFM_REQ_MAY_BACKLOG |
> > -                                   CRYPTO_TFM_REQ_MAY_SLEEP, crypto_req_done,
> > -                                   &wait);
> > -     ret = crypto_skcipher_setkey(tfm, key, key_len);
> > -     if (ret < 0)
> > -             goto out;
> > -
> > -     sg_init_one(&src, input, 16);
> > -     sg_init_one(&dst, hkey, 16);
> > -     skcipher_request_set_crypt(req, &src, &dst, 16, NULL);
> > -
> > -     ret = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
> > +     ret = aes_expandkey(&ctx, key, key_len);
> > +     if (ret)
> > +             return ret;
> >
> > -out:
> > -     skcipher_request_free(req);
> > -     crypto_free_skcipher(tfm);
> > -     return ret;
> > +     aes_encrypt(&ctx, hkey, input);
> > +     memzero_explicit(&ctx, sizeof(ctx));
> > +     return 0;
> >  }
> >
>
> Otherwise this looks good.  You can add:
>
>         Reviewed-by: Eric Biggers <ebiggers@...gle.com>
>

Thanks