Re: [PATCH 3/3] dm-crypt: Implement eboiv - encrypted byte-offset initialization vector.

[Milan Broz <gmazyland@gmail.com>]

Hi Herbert,

I have a question about the crypto_cipher API in dm-crypt:

We are apparently trying to deprecate cryto_cipher API (see the ESSIV patchset),
but I am not sure what API now should be used instead.

See the patch below - all we need is to one block encryption for IV.

This algorithm makes sense only for FDE (old compatible Bitlocker devices),
I really do not want this to be shared in some crypto module...

What API should I use here? Sync skcipher? Is the crypto_cipher API
really a problem in this case?

Thanks,
Milan


On 04/07/2019 15:10, Milan Broz wrote:
> This IV is used in some BitLocker devices with CBC encryption mode.
> 
> NOTE: maybe we need to use another crypto API if the bare cipher
>       API is going to be deprecated.
> 
> Signed-off-by: Milan Broz <gmazyland@gmail.com>
> ---
>  drivers/md/dm-crypt.c | 82 ++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 81 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
> index 96ead4492787..a5ffa1ac6a28 100644
> --- a/drivers/md/dm-crypt.c
> +++ b/drivers/md/dm-crypt.c
> @@ -120,6 +120,10 @@ 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.
> @@ -159,6 +163,7 @@ 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;
> @@ -290,6 +295,10 @@ static struct crypto_aead *any_tfm_aead(struct crypt_config *cc)
>   *       is calculated from initial key, sector number and mixed using CRC32.
>   *       Note that this encryption scheme is vulnerable to watermarking attacks
>   *       and should be used for old compatible containers access only.
> + *
> + * eboiv: Encrypted byte-offset IV (used in Bitlocker in CBC mode)
> + *        The IV is encrypted little-endian byte-offset (with the same key
> + *        and cipher as the volume).
>   */
>  
>  static int crypt_iv_plain_gen(struct crypt_config *cc, u8 *iv,
> @@ -838,6 +847,67 @@ 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 (crypto_cipher_blocksize(tfm) != 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)
> +{
> +	struct iv_eboiv_private *eboiv = &cc->iv_gen_private.eboiv;
> +	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);
> +}
> +
> +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(iv, 0, cc->iv_size);
> +	*(__le64 *)iv = cpu_to_le64(dmreq->iv_sector * cc->sector_size);
> +	crypto_cipher_encrypt_one(eboiv->tfm, iv, iv);
> +
> +	return 0;
> +}
> +
>  static const struct crypt_iv_operations crypt_iv_plain_ops = {
>  	.generator = crypt_iv_plain_gen
>  };
> @@ -890,6 +960,14 @@ static struct crypt_iv_operations crypt_iv_random_ops = {
>  	.generator = crypt_iv_random_gen
>  };
>  
> +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
> +};
> +
>  /*
>   * Integrity extensions
>   */
> @@ -2293,6 +2371,8 @@ static int crypt_ctr_ivmode(struct dm_target *ti, const char *ivmode)
>  		cc->iv_gen_ops = &crypt_iv_benbi_ops;
>  	else if (strcmp(ivmode, "null") == 0)
>  		cc->iv_gen_ops = &crypt_iv_null_ops;
> +	else if (strcmp(ivmode, "eboiv") == 0)
> +		cc->iv_gen_ops = &crypt_iv_eboiv_ops;
>  	else if (strcmp(ivmode, "lmk") == 0) {
>  		cc->iv_gen_ops = &crypt_iv_lmk_ops;
>  		/*
> @@ -3093,7 +3173,7 @@ static void crypt_io_hints(struct dm_target *ti, struct queue_limits *limits)
>  
>  static struct target_type crypt_target = {
>  	.name   = "crypt",
> -	.version = {1, 18, 1},
> +	.version = {1, 19, 0},
>  	.module = THIS_MODULE,
>  	.ctr    = crypt_ctr,
>  	.dtr    = crypt_dtr,
>