Re: [PATCH net-next v2] netfilter: ebtables: avoid resetting limit rule state

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

Hi Linus,

On Sun, Dec 09, 2018 at 07:14:05AM +0100, Linus Lüssing wrote:
> So far any changes with ebtables will reset the state of limit rules,
> leading to spikes in traffic. This is especially noticeable if changes
> are done frequently, for instance via a daemon.
> 
> This patch fixes this by bailing out from (re)setting if the limit
> rule was initialized before.
> 
> When sending packets every 250ms for 600s, with a
> "--limit 1/sec --limit-burst 50" rule and a command like this
> in the background:
> 
> $ ebtables -N VOIDCHAIN
> $ while true; do ebtables -F VOIDCHAIN; sleep 30; done
> 
> The results are:
> 
> Before: ~1600 packets
> After: 650 packets
> 
> This also aligns the behavior to "xtables-nft-multi ebtables" which uses
> nft_limit instead of ebt_limit. In tests nft_limit did not suffer from
> this issue and rate limited to 650 just fine.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
> 
> ---
> 
> Changelog v2:
> 
> - Adjusted commit message (adjusted title, added test results with
>   nft_limit for comparison)
> - Excluded rate limiting variables from zeroing when passed to userspace
>   by increasing .usersize. This became necessary with 4.11 /
>   commit ec2318904965 ("xtables: extend matches and targets with .usersize")
> - Retested with 4.20-rc4 and current net-next/master (83af01ba1c2d)
> 
> v1 was:
> 
> "[net-next] bridge: ebtables: Avoid resetting limit rule state"
> -> https://lore.kernel.org/patchwork/patch/854802/
> ---
>  net/bridge/netfilter/ebt_limit.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/net/bridge/netfilter/ebt_limit.c b/net/bridge/netfilter/ebt_limit.c
> index 165b9d678cf1..2cf9861c3bce 100644
> --- a/net/bridge/netfilter/ebt_limit.c
> +++ b/net/bridge/netfilter/ebt_limit.c
> @@ -69,6 +69,10 @@ static int ebt_limit_mt_check(const struct xt_mtchk_param *par)
>  {
>  	struct ebt_limit_info *info = par->matchinfo;
>  
> +	/* Do not reset state on unrelated table changes */
> +	if (info->prev)
> +		return 0;

Hm, still I don't think we can follow this path, even if it works.

This means we trust userspace in what it sets for info->prev.

Using ebtables-nft (instead of ebtables-legacy) fixes this problem,
without this patch.

> +
>  	/* Check for overflow. */
>  	if (info->burst == 0 ||
>  	    user2credits(info->avg * info->burst) < user2credits(info->avg)) {
> @@ -105,7 +109,7 @@ static struct xt_match ebt_limit_mt_reg __read_mostly = {
>  	.match		= ebt_limit_mt,
>  	.checkentry	= ebt_limit_mt_check,
>  	.matchsize	= sizeof(struct ebt_limit_info),
> -	.usersize	= offsetof(struct ebt_limit_info, prev),
> +	.usersize	= sizeof(struct ebt_limit_info),
>  #ifdef CONFIG_COMPAT
>  	.compatsize	= sizeof(struct ebt_compat_limit_info),
>  #endif
> -- 
> 2.11.0
>