netdev - Re: [PATCH net 2/2] IPv6: reply ICMP error if the first fragment don't include all headers

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Message-ID: <91f5b71e-416d-ebf1-750b-3e1d5cf6b732@gmail.com>
Date:   Wed, 7 Oct 2020 11:35:41 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Hangbin Liu <liuhangbin@...il.com>, netdev@...r.kernel.org
Cc:     "David S . Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net 2/2] IPv6: reply ICMP error if the first fragment
 don't include all headers



On 10/7/20 5:55 AM, Hangbin Liu wrote:

>  		kfree_skb(skb);
> @@ -282,6 +285,21 @@ static struct sk_buff *ip6_rcv_core(struct sk_buff *skb, struct net_device *dev,
>  		}
>  	}
>  
> +	/* RFC 8200, Section 4.5 Fragment Header:
> +	 * If the first fragment does not include all headers through an
> +	 * Upper-Layer header, then that fragment should be discarded and
> +	 * an ICMP Parameter Problem, Code 3, message should be sent to
> +	 * the source of the fragment, with the Pointer field set to zero.
> +	 */
> +	nexthdr = hdr->nexthdr;
> +	offset = ipv6_skip_exthdr(skb, skb_transport_offset(skb), &nexthdr, &frag_off);
> +	if (frag_off == htons(IP6_MF) && !pskb_may_pull(skb, offset + 1)) {
> +		__IP6_INC_STATS(net, idev, IPSTATS_MIB_INHDRERRORS);
> +		icmpv6_param_prob(skb, ICMPV6_HDR_INCOMP, 0);
> +		rcu_read_unlock();
> +		return NULL;
> +	}
> +
>  	rcu_read_unlock();
>  
>  	/* Must drop socket now because of tproxy. */
> 

Ouch, this is quite a buggy patch.

I doubt we want to add yet another ipv6_skip_exthdr() call in IPv6 fast path.

Surely the presence of NEXTHDR_FRAGMENT is already tested elsewhere ?

Also, ipv6_skip_exthdr() does not pull anything in skb->head, it would be strange
to force a pull of hundreds of bytes just because you want to check if an extra byte is there,
if the packet could be forwarded as is, without additional memory allocations.

Testing skb->len should be more than enough at this stage.

Also ipv6_skip_exthdr() can return an error.