Subject: Re: [PATCH v2] netfilter: TCPMSS: fix dropped packets when MSS option is unaligned

From: Pablo Neira Ayuso <[email-masked]>
Date: Fri, 29 May 2026 00:52:52 +0200

Hi,

On Thu, May 28, 2026 at 11:34:11PM +0100, Kacper Kokot wrote:
> Padding TCP options with NOPs is optional, so it is legal to send an
> MSS option that is not aligned to a word boundary and therefore not
> aligned for checksum calculation. The current TCPMSS target is not
> robust to this: when the MSS option is unaligned it produces an
> invalid checksum, and the packet is dropped.

Yes, but how many stacks do this?

> This has not been observed in any real environment.

... then why is this a fix?

> Senders place the MSS at the beginning of the options block, where
> it is naturally aligned, but the spec allows unaligned options and
> the kernel shouldn't silently drop legal packets.

This is questionably a "clean packet".

And "the kernel is not silently dropping anything, it is policy that
would drop it", and there are mechanisms to track what rule is
dropping this packet.

> When the changed word is not aligned, the modified bytes straddle two
> checksum words, and using the standard incremental update helper
> (which assumes alignment) produces an invalid checksum:
> 
>     | w1     | w2     |
> OLD |  a  b  |  c  d  |
> NEW |  a  b' |  c' d  |
> 
> Since b' and c' sit across w1 and w2, we could compute the incremental
> checksum in two operations by recalculating w1 and then w2:
> 
>     C' = C - w1 + w1' - w2 + w2'
> 
> But working it out:
> 
>     C' = C - w1 - w2 + w1' + w2'
>        = C - (a * 2^8 + b)  - (c * 2^8 + d)
>            + (a * 2^8 + b') + (c' * 2^8 + d)
>        = C + 2^8 * (a - a + c' - c) + (b' - b + d - d)
>        = C + 2^8 * (c' - c) + (b' - b)
>        = C - (2^8 * c + b) + (2^8 * c' + b')
> 
> So an unaligned incremental checksum can be done in a single operation
> by byteswapping the changed bytes before passing them to the helper.
> This patch implements that trick for unaligned MSS option updates.

If this is a fix as the title specify, there no Fixes: tag?

To me, this qualifies as an enhancement, if anything.

Maybe more correct subject is:

  "netfilter: TCPMSS: handle packets with unaligned MSS option"

since this mangling unaligned MSS has never been supported this far.

Thanks

> Signed-off-by: Kacper Kokot <[email-masked]>
> ---
> I decided to go with the get_unaligned_be16 suggestion because
> it's idiomatic and it produces shorter assembly on x86-64
> (6 instructions vs 9). SYN processing is a cold path so
> I didn't look into it further.
> 
> v2:
>  - Use get_unaligned_be16 (Fernando's suggestion)
>  - Fix alignment check expression (David)
>  - Mention it's a theoretical bug in the commit message
>  - Drop cc stable, the bug is only theoretical
>
>  net/netfilter/xt_TCPMSS.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/net/netfilter/xt_TCPMSS.c b/net/netfilter/xt_TCPMSS.c
> index 80e1634bc51f..32c87a520361 100644
> --- a/net/netfilter/xt_TCPMSS.c
> +++ b/net/netfilter/xt_TCPMSS.c
> @ -117,8 +117,9 @@ tcpmss_mangle_packet(struct sk_buff *skb,
>  	for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
>  		if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
>  			u_int16_t oldmss;
> +			u16 csum_oldmss, csum_newmss;
>  
> -			oldmss = (opt[i+2] << 8) | opt[i+3];
> +			oldmss = get_unaligned_be16(&opt[i+2]);
>  
>  			/* Never increase MSS, even when setting it, as
>  			 * doing so results in problems for hosts that rely
> @@ -130,8 +131,19 @@ tcpmss_mangle_packet(struct sk_buff *skb,
>  			opt[i+2] = (newmss & 0xff00) >> 8;
>  			opt[i+3] = newmss & 0x00ff;
>  
> +			csum_oldmss = htons(oldmss);
> +			csum_newmss = htons(newmss);
> +
> +			/* MSS may be unaligned; fix up the incremental checksum
> +			 * to avoid an invalid checksum and a dropped packet.
> +			 */
> +			if (((char *)&opt[i + 2] - (char *)tcph) & 0x1) {
> +				csum_oldmss = swab16(csum_oldmss);
> +				csum_newmss = swab16(csum_newmss);
> +			}
> +
>  			inet_proto_csum_replace2(&tcph->check, skb,
> -						 htons(oldmss), htons(newmss),
> +						 csum_oldmss, csum_newmss,
>  						 false);
>  			return 0;
>  		}
> -- 
> 2.43.0
>