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

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Date:   Tue, 27 Oct 2020 10:20:17 +0000
From:   "Georg Kohmann (geokohma)" <geokohma@...co.com>
To:     Hangbin Liu <liuhangbin@...il.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 27.10.2020 10:57, Hangbin Liu 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.
>
>> 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)

Almost, add some parentheses:

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

>
> Thanks
> Hangbin