Re: [PATCH] netfilter: nft_hash: Add hash offset value

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Mon, Sep 05, 2016 at 10:36:57AM +0200, Laura Garcia Liebana wrote:
> Add support to pass through an offset to the hash value. With this
> feature, the sysadmin is able to generate a hash with a given
> offset value.
> 
> Example:
> 
> 	meta mark set jhash ip saddr mod 2 seed 0xabcd sum 100
> 
> This option generates marks according to the source address from 100 to
> 101.
> 
> Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
> ---
>  include/uapi/linux/netfilter/nf_tables.h |  2 ++
>  net/netfilter/nft_hash.c                 | 13 +++++++++++--
>  2 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/netfilter/nf_tables.h b/include/uapi/linux/netfilter/nf_tables.h
> index 4dbeeed..8026684 100644
> --- a/include/uapi/linux/netfilter/nf_tables.h
> +++ b/include/uapi/linux/netfilter/nf_tables.h
> @@ -764,6 +764,7 @@ enum nft_meta_keys {
>   * @NFTA_HASH_LEN: source data length (NLA_U32)
>   * @NFTA_HASH_MODULUS: modulus value (NLA_U32)
>   * @NFTA_HASH_SEED: seed value (NLA_U32)
> + * @NFTA_HASH_SUM: Hash offset value (NLA_U32)
>   */
>  enum nft_hash_attributes {
>  	NFTA_HASH_UNSPEC,
> @@ -772,6 +773,7 @@ enum nft_hash_attributes {
>  	NFTA_HASH_LEN,
>  	NFTA_HASH_MODULUS,
>  	NFTA_HASH_SEED,
> +	NFTA_HASH_SUM,
>  	__NFTA_HASH_MAX,
>  };
>  #define NFTA_HASH_MAX	(__NFTA_HASH_MAX - 1)
> diff --git a/net/netfilter/nft_hash.c b/net/netfilter/nft_hash.c
> index b7e3b40..8ab04d9 100644
> --- a/net/netfilter/nft_hash.c
> +++ b/net/netfilter/nft_hash.c
> @@ -23,6 +23,7 @@ struct nft_hash {
>  	u8			len;
>  	u32			modulus;
>  	u32			seed;
> +	u32			sum;
>  };
>  
>  static void nft_hash_eval(const struct nft_expr *expr,
> @@ -35,7 +36,7 @@ static void nft_hash_eval(const struct nft_expr *expr,
>  
>  	h = reciprocal_scale(jhash(data, priv->len, priv->seed), priv->modulus);
>  
> -	regs->data[priv->dreg] = h;
> +	regs->data[priv->dreg] = priv->sum + h;
>  }
>  
>  const struct nla_policy nft_hash_policy[NFTA_HASH_MAX + 1] = {
> @@ -44,6 +45,7 @@ const struct nla_policy nft_hash_policy[NFTA_HASH_MAX + 1] = {
>  	[NFTA_HASH_LEN]			= { .type = NLA_U32 },
>  	[NFTA_HASH_MODULUS]		= { .type = NLA_U32 },
>  	[NFTA_HASH_SEED]		= { .type = NLA_U32 },
> +	[NFTA_HASH_SUM]			= { .type = NLA_U32 },
>  };
>  
>  static int nft_hash_init(const struct nft_ctx *ctx,
> @@ -60,6 +62,11 @@ static int nft_hash_init(const struct nft_ctx *ctx,
>  	    !tb[NFTA_HASH_MODULUS])
>  		return -EINVAL;
>  
> +	if (tb[NFTA_HASH_SUM])
> +		priv->sum = ntohl(nla_get_be32(tb[NFTA_HASH_SUM]));
> +	else
> +		priv->sum = 0;

There is a corner case that we should reject from the kernel, I think
this is:

        if (priv->sum + priv->modulus - 1 < priv->sum)
                return -EOVERFLOW;

We'll handle this from userspace anyway too, but I think it's easy to
reject this crazy this.