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

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Date:   Fri, 9 Oct 2020 18:07:38 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, "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 Thu, Oct 08, 2020 at 11:47:00AM +0200, Eric Dumazet wrote:
> 
> 
> On 10/8/20 10:30 AM, Hangbin Liu wrote:
> >>> @@ -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 ?
> > 
> > Would you like to help point where NEXTHDR_FRAGMENT was tested before IPv6
> > defragment?
> I think we have to ask the question : Should routers enforce the rule, or
> only end points ?

>From IPv6 Core Conformance test[1], it applied to both router and host(It will
marked specifically if a test only for router).

> 
> End points must handle NEXTHDR_FRAGMENT, in ipv6_frag_rcv()

Yes, I was also try put the check there, but it looks that would be too late
if module nf_defrag_ipv6 loaded

> >> Also ipv6_skip_exthdr() can return an error.
> > 
> > it returns -1 as error, If we tested it by (offset + 1 > skb->len), does
> > that count as an error handler?
> 
> If there is an error, then you want to send the ICMP, right ?

No, this is only for fragment header with no enough Upper-Layer header, which need
send ICMP Parameter Problem, Code 3 specifically. For other errors, I guess
the other code will take care of it.

So for -1 return, I just skipped it.
> 
> The (offset + 1) expression will become 0, and surely the test will be false,
> so you wont send the ICMP...

[1] v6LC.1.3.6: First Fragment Doesn’t Contain All Headers part A, B,
C and D at https://ipv6ready.org/docs/Core_Conformance_5_0_0.pdf

Thanks
Hangbin