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

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Message-ID: <f7272dda-0383-c7d0-1a8a-4a70a1aadb77@gmail.com>
Date:   Thu, 8 Oct 2020 11:47:00 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Hangbin Liu <liuhangbin@...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 10/8/20 10:30 AM, Hangbin Liu wrote:
> Hi Eric,
> 
> Thanks for the comments. I should add "RFC" in subject next time for the
> uncertain fix patch.
> 
> On Wed, Oct 07, 2020 at 11:35:41AM +0200, Eric Dumazet wrote:
>>
>>
>> On 10/7/20 5:55 AM, Hangbin Liu wrote:
>>
>>>  		kfree_skb(skb);
>>> @@ -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 ?

End points must handle NEXTHDR_FRAGMENT, in ipv6_frag_rcv()


> 
>>
>> Also, ipv6_skip_exthdr() does not pull anything in skb->head, it would be strange
>> to force a pull of hundreds of bytes just because you want to check if an extra byte is there,
>> if the packet could be forwarded as is, without additional memory allocations.
>>
>> Testing skb->len should be more than enough at this stage.
> 
> Ah, yes, I shouldn't call pskb_may_pull here.
>>
>> 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 ?

The (offset + 1) expression will become 0, and surely the test will be false,
so you wont send the ICMP...

> 
> Thanks
> Hangbin
>