netdev - Re: [PATCH v1 2/2] ipv6: always accept routing headers with 0 segments left

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Date: Tue, 25 Jun 2024 14:38:59 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <mathis.marion@...abs.com>
CC: <alex.aring@...il.com>, <davem@...emloft.net>, <dsahern@...nel.org>,
	<edumazet@...gle.com>, <jerome.pouiller@...abs.com>, <kuba@...nel.org>,
	<kylian.balan@...abs.com>, <linux-kernel@...r.kernel.org>,
	<netdev@...r.kernel.org>, <pabeni@...hat.com>, <kuniyu@...zon.com>
Subject: Re: [PATCH v1 2/2] ipv6: always accept routing headers with 0 segments left

From: Mathis Marion <Mathis.Marion@...abs.com>
Date: Mon, 24 Jun 2024 16:15:33 +0200
> From: Mathis Marion <mathis.marion@...abs.com>
> 
> Routing headers of type 3 and 4 would be rejected even if segments left
> was 0, in the case that they were disabled through system configuration.
> 
> RFC 8200 section 4.4 specifies:
> 
>       If Segments Left is zero, the node must ignore the Routing header
>       and proceed to process the next header in the packet, whose type
>       is identified by the Next Header field in the Routing header.

I think this part is only applied to an unrecognized Routing Type,
so only applied when the network stack does not know the type.

   https://www.rfc-editor.org/rfc/rfc8200.html#section-4.4

   If, while processing a received packet, a node encounters a Routing
   header with an unrecognized Routing Type value, the required behavior
   of the node depends on the value of the Segments Left field, as
   follows:

      If Segments Left is zero, the node must ignore the Routing header
      and proceed to process the next header in the packet, whose type
      is identified by the Next Header field in the Routing header.

That's why RPL with segment length 0 was accepted before 8610c7c6e3bd.

But now the kernel recognizes RPL and it's intentionally disabled
by default with net.ipv6.conf.$DEV.rpl_seg_enabled since introduced.

And SRv6 has been rejected since 1ababeba4a21f for the same reason.


> 
> Signed-off-by: Mathis Marion <mathis.marion@...abs.com>
> ---
>  net/ipv6/exthdrs.c | 17 ++++++-----------
>  1 file changed, 6 insertions(+), 11 deletions(-)
> 
> diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
> index 083dbbafb166..913160b0fe13 100644
> --- a/net/ipv6/exthdrs.c
> +++ b/net/ipv6/exthdrs.c
> @@ -662,17 +662,6 @@ static int ipv6_rthdr_rcv(struct sk_buff *skb)
>  		return -1;
>  	}
>  
> -	switch (hdr->type) {
> -	case IPV6_SRCRT_TYPE_4:
> -		/* segment routing */
> -		return ipv6_srh_rcv(skb);
> -	case IPV6_SRCRT_TYPE_3:
> -		/* rpl segment routing */
> -		return ipv6_rpl_srh_rcv(skb);
> -	default:
> -		break;
> -	}
> -
>  looped_back:
>  	if (hdr->segments_left == 0) {
>  		switch (hdr->type) {
> @@ -708,6 +697,12 @@ static int ipv6_rthdr_rcv(struct sk_buff *skb)
>  		}
>  		break;
>  #endif
> +	case IPV6_SRCRT_TYPE_3:
> +		/* rpl segment routing */
> +		return ipv6_rpl_srh_rcv(skb);
> +	case IPV6_SRCRT_TYPE_4:
> +		/* segment routing */
> +		return ipv6_srh_rcv(skb);
>  	default:
>  		goto unknown_rh;
>  	}
> -- 
> 2.43.0