Subject: Re: [PATCH] netfilter: nf_conntrack: reserve two bytes for nf_ct_ext->len

From: Patrick McHardy <[email-masked]>
Date: Thu, 27 Mar 2014 15:45:05 +0000

On Thu, Mar 27, 2014 at 07:32:34PM +0400, Andrey Vagin wrote:
> "len" contains sizeof(nf_ct_ext) and size of extensions. In a worst
> case it can contain all extensions. Bellow you can find sizes for all
> types of extensions. Their sum is definitely bigger than 256.
> 
> nf_ct_ext_types[0]->len = 24
> nf_ct_ext_types[1]->len = 32
> nf_ct_ext_types[2]->len = 24
> nf_ct_ext_types[3]->len = 32
> nf_ct_ext_types[4]->len = 152
> nf_ct_ext_types[5]->len = 2
> nf_ct_ext_types[6]->len = 16
> nf_ct_ext_types[7]->len = 8
> 
> I have seen "len" up to 280 and my host has crashes w/o this patch.

Very nice catch. I suppose we also need to either increase the size of
offset[] or rearrange the extension so 4 (ECACHE) comes last.

> 
> Cc: Pablo Neira Ayuso <[email-masked]>
> Cc: Patrick McHardy <[email-masked]>
> Cc: Jozsef Kadlecsik <[email-masked]>
> Cc: "David S. Miller" <[email-masked]>
> Signed-off-by: Andrey Vagin <[email-masked]>
> ---
>  include/net/netfilter/nf_conntrack_extend.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/net/netfilter/nf_conntrack_extend.h b/include/net/netfilter/nf_conntrack_extend.h
> index 956b175..36fd6bf 100644
> --- a/include/net/netfilter/nf_conntrack_extend.h
> +++ b/include/net/netfilter/nf_conntrack_extend.h
> @@ -48,7 +48,7 @@ enum nf_ct_ext_id {
>  struct nf_ct_ext {
>  	struct rcu_head rcu;
>  	u8 offset[NF_CT_EXT_NUM];
> -	u8 len;
> +	u16 len;
>  	char data[0];
>  };
>  
> -- 
> 1.8.5.3
> 
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