netdev - Re: [PATCHv5 net 2/2] IPv6: reply ICMP error if the first fragment don't include all headers

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Message-ID: <20201027095712.GP2531@dhcp-12-153.nay.redhat.com>
Date:   Tue, 27 Oct 2020 17:57:12 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     "Georg Kohmann (geokohma)" <geokohma@...co.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCHv5 net 2/2] IPv6: reply ICMP error if the first fragment
 don't include all headers

On Tue, Oct 27, 2020 at 07:57:06AM +0000, Georg Kohmann (geokohma) wrote:
> > +	/* 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 (offset >= 0) {
> > +		/* Check some common protocols' header */
> > +		if (nexthdr == IPPROTO_TCP)
> > +			offset += sizeof(struct tcphdr);
> > +		else if (nexthdr == IPPROTO_UDP)
> > +			offset += sizeof(struct udphdr);
> > +		else if (nexthdr == IPPROTO_ICMPV6)
> > +			offset += sizeof(struct icmp6hdr);
> > +		else
> > +			offset += 1;
> > +
> > +		if (frag_off == htons(ip6_mf) && offset > skb->len) {
> 
> This do not catch atomic fragments (fragmented packet with only one fragment). frag_off also contains two reserved bits (both 0) that might change in the future.

Thanks, I also didn't aware this scenario.

> I suggest you only check that the offset is 0:
> frag_off & htons(IP6_OFFSET)

This will match all other fragment packets. RFC request we reply ICMP for the
first fragment packet, Do you mean

if (!frag_off & htons(IP6_OFFSET) && offset > skb->len)

Thanks
Hangbin