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

[Hangbin Liu <liuhangbin@gmail.com>]

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?

> 
> 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?

Thanks
Hangbin