Re: [PATCH] netfilter: Can't fail and free after table replacement

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

On Wed, Apr 02, 2014 at 05:35:13PM +0200, Thomas Graf wrote:
> All xtables variants suffer from the defect that the copy_to_user()
> to copy the counters to user memory may fail after the table has
> already been exchanged and thus exposed. Return an error at this
> point will result in freeing the already exposed table. Any
> subsequent packet processing will result in a kernel panic.
> 
> We can't copy the counters before exposing the new tables as we
> want provide the counter state after the old table has been
> unhooked. Therefore convert this into a silent error.
> 
> Cc: Florian Westphal <fw@strlen.de>
> Signed-off-by: Thomas Graf <tgraf@suug.ch>
> ---
>  net/bridge/netfilter/ebtables.c | 4 +---
>  net/ipv4/netfilter/arp_tables.c | 5 +++--
>  net/ipv4/netfilter/ip_tables.c  | 5 +++--
>  net/ipv6/netfilter/ip6_tables.c | 5 +++--
>  4 files changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c
> index 0e474b1..7a3dc98 100644
> --- a/net/bridge/netfilter/ebtables.c
> +++ b/net/bridge/netfilter/ebtables.c
> @@ -1044,10 +1044,8 @@ static int do_replace_finish(struct net *net, struct ebt_replace *repl,
>  	if (repl->num_counters &&
>  	   copy_to_user(repl->counters, counterstmp,
>  	   repl->num_counters * sizeof(struct ebt_counter))) {
> -		ret = -EFAULT;
> +		/* Silent error, can't fail, new table is already in place */
>  	}
> -	else
> -		ret = 0;
>  

This seems good to me.

Perhaps we can spot a warning like in rtnetlink to inform the user
that counters are not reliable anymore?