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

[Hangbin Liu <liuhangbin@gmail.com>]

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