netfilter: nf_tables: fix race when create new element in dynset

Packets may race when create the new element in nft_hash_update:
CPU0 CPU1
lookup_fast - fail lookup_fast - fail
new - ok new - ok
insert - ok insert - fail(EEXIST)

So when race happened, we reuse the existing element. Otherwise,
these *racing* packets will not be handled properly.

Fixes: 22fe54d5fefc ("netfilter: nf_tables: add support for dynamic set updates")
Signed-off-by: Liping Zhang <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nft_set_hash.c

···
 			    const struct nft_set_ext **ext)
 {
 	struct nft_hash *priv = nft_set_priv(set);
-	struct nft_hash_elem *he;
+	struct nft_hash_elem *he, *prev;
 	struct nft_hash_cmp_arg arg = {
 		.genmask = NFT_GENMASK_ANY,
 		.set	 = set,
···
 	he = new(set, expr, regs);
 	if (he == NULL)
 		goto err1;
-	if (rhashtable_lookup_insert_key(&priv->ht, &arg, &he->node,
-					 nft_hash_params))
+
+	prev = rhashtable_lookup_get_insert_key(&priv->ht, &arg, &he->node,
+						nft_hash_params);
+	if (IS_ERR(prev))
 		goto err2;
+
+	/* Another cpu may race to insert the element with the same key */
+	if (prev) {
+		nft_set_elem_destroy(set, he, true);
+		he = prev;
+	}
+
 out:
 	*ext = &he->ext;
 	return true;