netns: publish net_generic correctly

Publishing net_generic pointer is done with silly mistake: new array is
published BEFORE setting freshly acquired pernet subsystem pointer.

	memcpy
	rcu_assign_pointer
	kfree_rcu
	ng->ptr[id - 1] = data;

This bug was introduced with commit dec827d1
("[NETNS]: The generic per-net pointers.") in the glorious days of
chopping networking stack into containers proper 8.5 years ago (whee...)

How it didn't trigger for so long?
Well, you need quite specific set of conditions:

*) race window opens once per pernet subsystem addition
   (read: modprobe or boot)

*) not every pernet subsystem is eligible (need ->id and ->size)

*) not every pernet subsystem is vulnerable (need incorrect or absense
   of ordering of register_pernet_sybsys() and actually using net_generic())

*) to hide the bug even more, default is to preallocate 13 pointers which
   is actually quite a lot. You need IPv6, netfilter, bridging etc together
   loaded to trigger reallocation in the first place. Trimmed down
   config are OK.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -64,9 +64,10 @@ static int net_assign_generic(struct net *net, unsigned int id, void *data)
 
 	old_ng = rcu_dereference_protected(net->gen,
 					   lockdep_is_held(&net_mutex));
-	ng = old_ng;
-	if (old_ng->len >= id)
-		goto assign;
+	if (old_ng->len >= id) {
+		old_ng->ptr[id - 1] = data;
+		return 0;
+	}
 
 	ng = net_alloc_generic();
 	if (ng == NULL)
@@ -84,11 +85,10 @@ static int net_assign_generic(struct net *net, unsigned int id, void *data)
 	 */
 
 	memcpy(&ng->ptr, &old_ng->ptr, old_ng->len * sizeof(void*));
+	ng->ptr[id - 1] = data;
 
 	rcu_assign_pointer(net->gen, ng);
 	kfree_rcu(old_ng, rcu);
-assign:
-	ng->ptr[id - 1] = data;
 	return 0;
 }