net: fix possible user-after-free in skb_ext_add()

On cow we can free the old extension: we must avoid dereferencing
such extension after skb_ext_maybe_cow(). Since 'new' contents
are always equal to 'old' after the copy, we can fix the above
accessing the relevant data using 'new'.

Fixes: df5042f4c5 ("sk_buff: add skb extension infrastructure")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -5666,13 +5666,13 @@ void *skb_ext_add(struct sk_buff *skb, enum skb_ext_id id)
 		if (!new)
 			return NULL;
 
-		if (__skb_ext_exist(old, id)) {
+		if (__skb_ext_exist(new, id)) {
 			if (old != new)
 				skb->extensions = new;
 			goto set_active;
 		}
 
-		newoff = old->chunks;
+		newoff = new->chunks;
 	} else {
 		newoff = SKB_EXT_CHUNKSIZEOF(*new);