Re: [RFC PATCH v2] md/dm-crypt - reuse eboiv skcipher for IV generation

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On Thu, 8 Aug 2019 at 14:53, Milan Broz <gmazyland@gmail.com> wrote:
>
> Hi,
>
> On 07/08/2019 07:50, Ard Biesheuvel wrote:
> > Instead of instantiating a separate cipher to perform the encryption
> > needed to produce the IV, reuse the skcipher used for the block data
> > and invoke it one additional time for each block to encrypt a zero
> > vector and use the output as the IV.
> >
> > For CBC mode, this is equivalent to using the bare block cipher, but
> > without the risk of ending up with a non-time invariant implementation
> > of AES when the skcipher itself is time variant (e.g., arm64 without
> > Crypto Extensions has a NEON based time invariant implementation of
> > cbc(aes) but no time invariant implementation of the core cipher other
> > than aes-ti, which is not enabled by default)
> >
> > This approach is a compromise between dm-crypt API flexibility and
> > reducing dependence on parts of the crypto API that should not usually
> > be exposed to other subsystems, such as the bare cipher API.
> >
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>
> For now I have just pair of images here to test, but seems checksums are ok.
>
> Tested-by: Milan Broz <gmazyland@gmail.com>
>
> I talked with Mike already, so it should go through DM tree now.
>

Thanks!


>
> > ---
> >  drivers/md/dm-crypt.c | 70 ++++++++++++++-----------------------------
> >  1 file changed, 22 insertions(+), 48 deletions(-)
> >
> > diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
> > index d5216bcc4649..48cd76c88d77 100644
> > --- a/drivers/md/dm-crypt.c
> > +++ b/drivers/md/dm-crypt.c
> > @@ -120,10 +120,6 @@ struct iv_tcw_private {
> >       u8 *whitening;
> >  };
> >
> > -struct iv_eboiv_private {
> > -     struct crypto_cipher *tfm;
> > -};
> > -
> >  /*
> >   * Crypt: maps a linear range of a block device
> >   * and encrypts / decrypts at the same time.
> > @@ -163,7 +159,6 @@ struct crypt_config {
> >               struct iv_benbi_private benbi;
> >               struct iv_lmk_private lmk;
> >               struct iv_tcw_private tcw;
> > -             struct iv_eboiv_private eboiv;
> >       } iv_gen_private;
> >       u64 iv_offset;
> >       unsigned int iv_size;
> > @@ -847,65 +842,47 @@ static int crypt_iv_random_gen(struct crypt_config *cc, u8 *iv,
> >       return 0;
> >  }
> >
> > -static void crypt_iv_eboiv_dtr(struct crypt_config *cc)
> > -{
> > -     struct iv_eboiv_private *eboiv = &cc->iv_gen_private.eboiv;
> > -
> > -     crypto_free_cipher(eboiv->tfm);
> > -     eboiv->tfm = NULL;
> > -}
> > -
> >  static int crypt_iv_eboiv_ctr(struct crypt_config *cc, struct dm_target *ti,
> >                           const char *opts)
> >  {
> > -     struct iv_eboiv_private *eboiv = &cc->iv_gen_private.eboiv;
> > -     struct crypto_cipher *tfm;
> > -
> > -     tfm = crypto_alloc_cipher(cc->cipher, 0, 0);
> > -     if (IS_ERR(tfm)) {
> > -             ti->error = "Error allocating crypto tfm for EBOIV";
> > -             return PTR_ERR(tfm);
> > +     if (test_bit(CRYPT_MODE_INTEGRITY_AEAD, &cc->cipher_flags)) {
> > +             ti->error = "AEAD transforms not supported for EBOIV";
> > +             return -EINVAL;
> >       }
> >
> > -     if (crypto_cipher_blocksize(tfm) != cc->iv_size) {
> > +     if (crypto_skcipher_blocksize(any_tfm(cc)) != cc->iv_size) {
> >               ti->error = "Block size of EBOIV cipher does "
> >                           "not match IV size of block cipher";
> > -             crypto_free_cipher(tfm);
> >               return -EINVAL;
> >       }
> >
> > -     eboiv->tfm = tfm;
> >       return 0;
> >  }
> >
> > -static int crypt_iv_eboiv_init(struct crypt_config *cc)
> > +static int crypt_iv_eboiv_gen(struct crypt_config *cc, u8 *iv,
> > +                         struct dm_crypt_request *dmreq)
> >  {
> > -     struct iv_eboiv_private *eboiv = &cc->iv_gen_private.eboiv;
> > +     u8 buf[MAX_CIPHER_BLOCKSIZE] __aligned(__alignof__(__le64));
> > +     struct skcipher_request *req;
> > +     struct scatterlist src, dst;
> > +     struct crypto_wait wait;
> >       int err;
> >
> > -     err = crypto_cipher_setkey(eboiv->tfm, cc->key, cc->key_size);
> > -     if (err)
> > -             return err;
> > -
> > -     return 0;
> > -}
> > -
> > -static int crypt_iv_eboiv_wipe(struct crypt_config *cc)
> > -{
> > -     /* Called after cc->key is set to random key in crypt_wipe() */
> > -     return crypt_iv_eboiv_init(cc);
> > -}
> > +     req = skcipher_request_alloc(any_tfm(cc), GFP_KERNEL | GFP_NOFS);
> > +     if (!req)
> > +             return -ENOMEM;
> >
> > -static int crypt_iv_eboiv_gen(struct crypt_config *cc, u8 *iv,
> > -                         struct dm_crypt_request *dmreq)
> > -{
> > -     struct iv_eboiv_private *eboiv = &cc->iv_gen_private.eboiv;
> > +     memset(buf, 0, cc->iv_size);
> > +     *(__le64 *)buf = cpu_to_le64(dmreq->iv_sector * cc->sector_size);
> >
> > -     memset(iv, 0, cc->iv_size);
> > -     *(__le64 *)iv = cpu_to_le64(dmreq->iv_sector * cc->sector_size);
> > -     crypto_cipher_encrypt_one(eboiv->tfm, iv, iv);
> > +     sg_init_one(&src, page_address(ZERO_PAGE(0)), cc->iv_size);
> > +     sg_init_one(&dst, iv, cc->iv_size);
> > +     skcipher_request_set_crypt(req, &src, &dst, cc->iv_size, buf);
> > +     skcipher_request_set_callback(req, 0, crypto_req_done, &wait);
> > +     err = crypto_wait_req(crypto_skcipher_encrypt(req), &wait);
> > +     skcipher_request_free(req);
> >
> > -     return 0;
> > +     return err;
> >  }
> >
> >  static const struct crypt_iv_operations crypt_iv_plain_ops = {
> > @@ -962,9 +939,6 @@ static struct crypt_iv_operations crypt_iv_random_ops = {
> >
> >  static struct crypt_iv_operations crypt_iv_eboiv_ops = {
> >       .ctr       = crypt_iv_eboiv_ctr,
> > -     .dtr       = crypt_iv_eboiv_dtr,
> > -     .init      = crypt_iv_eboiv_init,
> > -     .wipe      = crypt_iv_eboiv_wipe,
> >       .generator = crypt_iv_eboiv_gen
> >  };
> >
> >