netfilter: nf_conntrack_reasm: properly handle packets fragmented into a single fragment

When an ICMPV6_PKT_TOOBIG message is received with a MTU below 1280,
all further packets include a fragment header.

Unlike regular defragmentation, conntrack also needs to "reassemble"
those fragments in order to obtain a packet without the fragment
header for connection tracking. Currently nf_conntrack_reasm checks
whether a fragment has either IP6_MF set or an offset != 0, which
makes it ignore those fragments.

Remove the invalid check and make reassembly handle fragment queues
containing only a single fragment.
Reported-and-tested-by: Ulrich Weber <uweber@astaro.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>

net/ipv6/netfilter/nf_conntrack_reasm.c

@@ -469,7 +469,7 @@ nf_ct_frag6_reasm(struct nf_ct_frag6_queue *fq, struct net_device *dev)
 
 	/* all original skbs are linked into the NFCT_FRAG6_CB(head).orig */
 	fp = skb_shinfo(head)->frag_list;
-	if (NFCT_FRAG6_CB(fp)->orig == NULL)
+	if (fp && NFCT_FRAG6_CB(fp)->orig == NULL)
 		/* at above code, head skb is divided into two skbs. */
 		fp = fp->next;
 
@@ -595,12 +595,6 @@ struct sk_buff *nf_ct_frag6_gather(struct sk_buff *skb, u32 user)
 	hdr = ipv6_hdr(clone);
 	fhdr = (struct frag_hdr *)skb_transport_header(clone);
 
-	if (!(fhdr->frag_off & htons(0xFFF9))) {
-		pr_debug("Invalid fragment offset\n");
-		/* It is not a fragmented frame */
-		goto ret_orig;
-	}
-
 	if (atomic_read(&nf_init_frags.mem) > nf_init_frags.high_thresh)
 		nf_ct_frag6_evictor();