linux-kernel - Re: [PATCH] dm crypt: Convert essiv from ahash to shash

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Date:   Mon, 16 Jul 2018 10:19:42 -0700
From:   Eric Biggers <ebiggers@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Arnd Bergmann <arnd@...db.de>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        dm-devel@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dm crypt: Convert essiv from ahash to shash

On Sun, Jul 15, 2018 at 08:59:12PM -0700, Kees Cook wrote:
> In preparing to remove all stack VLA usage from the kernel[1], this
> removes the discouraged use of AHASH_REQUEST_ON_STACK in favor of
> the smaller SHASH_DESC_ON_STACK by converting from ahash-wrapped-shash
> to direct shash. The stack allocation will be made a fixed size in a
> later patch to the crypto subsystem.
> 
> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com
> 
> Signed-off-by: Kees Cook <keescook@...omium.org>

Reviewed-by: Eric Biggers <ebiggers@...gle.com>

> ---
>  drivers/md/dm-crypt.c | 31 ++++++++++++++-----------------
>  1 file changed, 14 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
> index b61b069c33af..c4c922990090 100644
> --- a/drivers/md/dm-crypt.c
> +++ b/drivers/md/dm-crypt.c
> @@ -99,7 +99,7 @@ struct crypt_iv_operations {
>  };
>  
>  struct iv_essiv_private {
> -	struct crypto_ahash *hash_tfm;
> +	struct crypto_shash *hash_tfm;
>  	u8 *salt;
>  };
>  
> @@ -327,25 +327,22 @@ static int crypt_iv_plain64be_gen(struct crypt_config *cc, u8 *iv,
>  static int crypt_iv_essiv_init(struct crypt_config *cc)
>  {
>  	struct iv_essiv_private *essiv = &cc->iv_gen_private.essiv;
> -	AHASH_REQUEST_ON_STACK(req, essiv->hash_tfm);
> -	struct scatterlist sg;
> +	SHASH_DESC_ON_STACK(desc, essiv->hash_tfm);
>  	struct crypto_cipher *essiv_tfm;
>  	int err;
>  
> -	sg_init_one(&sg, cc->key, cc->key_size);
> -	ahash_request_set_tfm(req, essiv->hash_tfm);
> -	ahash_request_set_callback(req, CRYPTO_TFM_REQ_MAY_SLEEP, NULL, NULL);
> -	ahash_request_set_crypt(req, &sg, essiv->salt, cc->key_size);
> +	desc->tfm = essiv->hash_tfm;
> +	desc->flags = CRYPTO_TFM_REQ_MAY_SLEEP;
>  
> -	err = crypto_ahash_digest(req);
> -	ahash_request_zero(req);
> +	err = crypto_shash_digest(desc, cc->key, cc->key_size, essiv->salt);
> +	shash_desc_zero(desc);
>  	if (err)
>  		return err;
>  
>  	essiv_tfm = cc->iv_private;
>  
>  	err = crypto_cipher_setkey(essiv_tfm, essiv->salt,
> -			    crypto_ahash_digestsize(essiv->hash_tfm));
> +			    crypto_shash_digestsize(essiv->hash_tfm));
>  	if (err)
>  		return err;
>  
> @@ -356,7 +353,7 @@ static int crypt_iv_essiv_init(struct crypt_config *cc)
>  static int crypt_iv_essiv_wipe(struct crypt_config *cc)
>  {
>  	struct iv_essiv_private *essiv = &cc->iv_gen_private.essiv;
> -	unsigned salt_size = crypto_ahash_digestsize(essiv->hash_tfm);
> +	unsigned salt_size = crypto_shash_digestsize(essiv->hash_tfm);
>  	struct crypto_cipher *essiv_tfm;
>  	int r, err = 0;
>  
> @@ -408,7 +405,7 @@ static void crypt_iv_essiv_dtr(struct crypt_config *cc)
>  	struct crypto_cipher *essiv_tfm;
>  	struct iv_essiv_private *essiv = &cc->iv_gen_private.essiv;
>  
> -	crypto_free_ahash(essiv->hash_tfm);
> +	crypto_free_shash(essiv->hash_tfm);
>  	essiv->hash_tfm = NULL;
>  
>  	kzfree(essiv->salt);
> @@ -426,7 +423,7 @@ static int crypt_iv_essiv_ctr(struct crypt_config *cc, struct dm_target *ti,
>  			      const char *opts)
>  {
>  	struct crypto_cipher *essiv_tfm = NULL;
> -	struct crypto_ahash *hash_tfm = NULL;
> +	struct crypto_shash *hash_tfm = NULL;
>  	u8 *salt = NULL;
>  	int err;
>  
> @@ -436,14 +433,14 @@ static int crypt_iv_essiv_ctr(struct crypt_config *cc, struct dm_target *ti,
>  	}
>  
>  	/* Allocate hash algorithm */
> -	hash_tfm = crypto_alloc_ahash(opts, 0, CRYPTO_ALG_ASYNC);
> +	hash_tfm = crypto_alloc_shash(opts, 0, 0);
>  	if (IS_ERR(hash_tfm)) {
>  		ti->error = "Error initializing ESSIV hash";
>  		err = PTR_ERR(hash_tfm);
>  		goto bad;
>  	}
>  
> -	salt = kzalloc(crypto_ahash_digestsize(hash_tfm), GFP_KERNEL);
> +	salt = kzalloc(crypto_shash_digestsize(hash_tfm), GFP_KERNEL);
>  	if (!salt) {
>  		ti->error = "Error kmallocing salt storage in ESSIV";
>  		err = -ENOMEM;
> @@ -454,7 +451,7 @@ static int crypt_iv_essiv_ctr(struct crypt_config *cc, struct dm_target *ti,
>  	cc->iv_gen_private.essiv.hash_tfm = hash_tfm;
>  
>  	essiv_tfm = alloc_essiv_cipher(cc, ti, salt,
> -				       crypto_ahash_digestsize(hash_tfm));
> +				       crypto_shash_digestsize(hash_tfm));
>  	if (IS_ERR(essiv_tfm)) {
>  		crypt_iv_essiv_dtr(cc);
>  		return PTR_ERR(essiv_tfm);
> @@ -465,7 +462,7 @@ static int crypt_iv_essiv_ctr(struct crypt_config *cc, struct dm_target *ti,
>  
>  bad:
>  	if (hash_tfm && !IS_ERR(hash_tfm))
> -		crypto_free_ahash(hash_tfm);
> +		crypto_free_shash(hash_tfm);
>  	kfree(salt);
>  	return err;
>  }
> -- 
> 2.17.1
> 
> 
> -- 
> Kees Cook
> Pixel Security