linux-kernel - Re: [PATCH 1/1] tcp: configurable source port perturb table size

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Date:   Tue, 15 Nov 2022 17:32:08 -0800
From:   Kuniyuki Iwashima <kuniyu@...zon.com>
To:     <glex.spb@...il.com>
CC:     <davem@...emloft.net>, <dsahern@...nel.org>, <edumazet@...gle.com>,
        <kuba@...nel.org>, <linux-kernel@...r.kernel.org>,
        <netdev@...r.kernel.org>, <pabeni@...hat.com>,
        <paul@...pouillou.net>, <yoshfuji@...ux-ipv6.org>,
        Kuniyuki Iwashima <kuniyu@...zon.com>
Subject: Re: [PATCH 1/1] tcp: configurable source port perturb table size

From:   Gleb Mazovetskiy <glex.spb@...il.com>
Date:   Mon, 14 Nov 2022 22:56:16 +0000
> On embedded systems with little memory and no relevant
> security concerns, it is beneficial to reduce the size
> of the table.
> 
> Reducing the size from 2^16 to 2^8 saves 255 KiB
> of kernel RAM.
> 
> Makes the table size configurable as an expert option.
> 
> The size was previously increased from 2^8 to 2^16
> in commit 4c2c8f03a5ab ("tcp: increase source port perturb table to
> 2^16").
> 
> Signed-off-by: Gleb Mazovetskiy <glex.spb@...il.com>

Looks good.

Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>


> ---
>  net/ipv4/Kconfig           | 10 ++++++++++
>  net/ipv4/inet_hashtables.c | 10 +++++-----
>  2 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/net/ipv4/Kconfig b/net/ipv4/Kconfig
> index e983bb0c5012..2dfb12230f08 100644
> --- a/net/ipv4/Kconfig
> +++ b/net/ipv4/Kconfig
> @@ -402,6 +402,16 @@ config INET_IPCOMP
>  
>  	  If unsure, say Y.
>  
> +config INET_TABLE_PERTURB_ORDER
> +	int "INET: Source port perturbation table size (as power of 2)" if EXPERT
> +	default 16
> +	help
> +	  Source port perturbation table size (as power of 2) for
> +	  RFC 6056 3.3.4.  Algorithm 4: Double-Hash Port Selection Algorithm.
> +
> +	  The default is almost always what you want.
> +	  Only change this if you know what you are doing.
> +
>  config INET_XFRM_TUNNEL
>  	tristate
>  	select INET_TUNNEL
> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> index d3dc28156622..033bf3c2538f 100644
> --- a/net/ipv4/inet_hashtables.c
> +++ b/net/ipv4/inet_hashtables.c
> @@ -906,13 +906,13 @@ EXPORT_SYMBOL_GPL(inet_bhash2_update_saddr);
>   * Note that we use 32bit integers (vs RFC 'short integers')
>   * because 2^16 is not a multiple of num_ephemeral and this
>   * property might be used by clever attacker.
> + *
>   * RFC claims using TABLE_LENGTH=10 buckets gives an improvement, though
> - * attacks were since demonstrated, thus we use 65536 instead to really
> - * give more isolation and privacy, at the expense of 256kB of kernel
> - * memory.
> + * attacks were since demonstrated, thus we use 65536 by default instead
> + * to really give more isolation and privacy, at the expense of 256kB
> + * of kernel memory.
>   */
> -#define INET_TABLE_PERTURB_SHIFT 16
> -#define INET_TABLE_PERTURB_SIZE (1 << INET_TABLE_PERTURB_SHIFT)
> +#define INET_TABLE_PERTURB_SIZE (1 << CONFIG_INET_TABLE_PERTURB_ORDER)
>  static u32 *table_perturb;
>  
>  int __inet_hash_connect(struct inet_timewait_death_row *death_row,
> -- 
> 2.37.2