Subject: Re: [PATCH] xt_TCPMSS: SYN packets are allowed to contain data

From: William Allen Simpson <[email-masked]>
Date: Tue, 19 Jan 2010 04:17:25 -0500

Simon Arlott wrote:
> The check for data only needs to apply where the packet length
> could be increased by adding the MSS option. (The MSS option
> itself applies to the sender's maximum receive size which is
> not relevant to any data in its own packet.)
> 
> This moves the check for (header size != packet size) to after
> attempting to modify an existing MSS option. Another check is
> needed before looking through the header to ensure it doesn't
> claim to be larger than the packet size.
> 
What's the path from tcp_v[4,6]_rcv() to these tests?

1) Header larger than the packet is already tested in about 5 places,
and my patch "tcp: harmonize tcp_vx_rcv header length assumptions"
tries to get them all down to just *one* test.

2) There certainly *can* be data on SYN.  That code is already in
2.6.33....

> The ERROR level printk() is also removed as it can be triggered
> by remote hosts and is not useful:
> [4941777.937417] xt_TCPMSS: bad length (40 bytes)
> [4941782.409724] xt_TCPMSS: bad length (40 bytes)
> [4941790.762332] xt_TCPMSS: bad length (40 bytes)
> 
> Signed-off-by: Simon Arlott <[email-masked]>
> ---
>  net/netfilter/xt_TCPMSS.c |   21 +++++++++++----------
>  1 files changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
> index eda64c1..76f92bf 100644
> --- a/net/netfilter/xt_TCPMSS.c
> +++ b/net/netfilter/xt_TCPMSS.c
> @@ -60,17 +60,9 @@ tcpmss_mangle_packet(struct sk_buff *skb,
>  	tcplen = skb->len - tcphoff;
>  	tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
>  
> -	/* Since it passed flags test in tcp match, we know it is is
> -	   not a fragment, and has data >= tcp header length.  SYN
> -	   packets should not contain data: if they did, then we risk
> -	   running over MTU, sending Frag Needed and breaking things
> -	   badly. --RR */
> -	if (tcplen != tcph->doff*4) {
> -		if (net_ratelimit())
> -			printk(KERN_ERR "xt_TCPMSS: bad length (%u bytes)\n",
> -			       skb->len);
> +	/* Header cannot be larger than the packet */
> +	if (tcplen < tcph->doff*4)
>  		return -1;
> -	}
>  
>  	if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
>  		if (dst_mtu(skb_dst(skb)) <= minlen) {
> @@ -115,6 +107,15 @@ tcpmss_mangle_packet(struct sk_buff *skb,
>  		}
>  	}
>  
> +	/* Since it passed flags test in tcp match, we know it is
> +	   not a fragment, and has data >= tcp header length.  SYN
> +	   packets should not contain data: if they did, then we risk
> +	   running over MTU, sending Frag Needed and breaking things
> +	   badly. --RR
> +	*/
> +	if (tcplen > tcph->doff*4)
> +		return -1;
> +
>  	/*
>  	 * MSS Option not found ?! add it..
>  	 */