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

From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To: Hangbin Liu <liuhangbin@gmail.com>
Cc: "Georg Kohmann (geokohma)" <geokohma@cisco.com>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	Jakub Kicinski <kuba@kernel.org>,
	"David S . Miller" <davem@davemloft.net>,
	Eric Dumazet <eric.dumazet@gmail.com>
Subject: Re: [PATCHv5 net 2/2] IPv6: reply ICMP error if the first fragment don't include all headers
Date: Fri, 30 Oct 2020 11:31:55 -0400	[thread overview]
Message-ID: <CA+FuTSfSUE8M+TuKkBQbEL7L5Bfd=wrZHEqQ67nWZy8oex1JCw@mail.gmail.com> (raw)
In-Reply-To: <20201027095712.GP2531@dhcp-12-153.nay.redhat.com>

On Tue, Oct 27, 2020 at 5:57 AM Hangbin Liu <liuhangbin@gmail.com> wrote:
>
> 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.

Sorry, what are atomic fragments?

Do you mean packets with a fragment header that encapsulates the
entire packet? If so, isn't that handled in the branch right above?
("/* It is not a fragmented frame */"). That said, the test based on
IP6_OFFSET LGTM.