netfilter: nft_set_rbtree: fix bogus EEXIST with NLM_F_CREATE with null interval

Userspace adds a non-matching null element to the kernel for historical
reasons. This null element is added when the set is populated with
elements. Inclusion of this element is conditional, therefore,
userspace needs to dump the set content to check for its presence.

If the NLM_F_CREATE flag is turned on, this becomes an issue because
kernel bogusly reports EEXIST.

Add special case to ignore NLM_F_CREATE in this case, therefore,
re-adding the nul-element never fails.

Fixes: c016c7e ("netfilter: nf_tables: honor NLM_F_EXCL flag in set element insertion")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>

Author: ummakynes

net/netfilter/nf_tables_api.c

@@ -7636,6 +7636,11 @@ static int nft_add_set_elem(struct nft_ctx *ctx, struct nft_set *set,
 			 * and an existing one.
 			 */
 			err = -EEXIST;
+		} else if (err == -ECANCELED) {
+			/* ECANCELED reports an existing nul-element in
+			 * interval sets.
+			 */
+			err = 0;
 		}
 		goto err_element_clash;
 	}

net/netfilter/nft_set_rbtree.c

@@ -57,6 +57,13 @@ static bool nft_rbtree_interval_start(const struct nft_rbtree_elem *rbe)
 	return !nft_rbtree_interval_end(rbe);
 }
 
+static bool nft_rbtree_interval_null(const struct nft_set *set,
+				     const struct nft_rbtree_elem *rbe)
+{
+	return (!memchr_inv(nft_set_ext_key(&rbe->ext), 0, set->klen) &&
+		nft_rbtree_interval_end(rbe));
+}
+
 static int nft_rbtree_cmp(const struct nft_set *set,
 			  const struct nft_rbtree_elem *e1,
 			  const struct nft_rbtree_elem *e2)
@@ -373,6 +380,12 @@ static int __nft_rbtree_insert(const struct net *net, const struct nft_set *set,
 	 */
 	if (rbe_le && !nft_rbtree_cmp(set, new, rbe_le) &&
 	    nft_rbtree_interval_end(rbe_le) == nft_rbtree_interval_end(new)) {
+		/* - ignore null interval, otherwise NLM_F_CREATE bogusly
+		 *   reports EEXIST.
+		 */
+		if (nft_rbtree_interval_null(set, new))
+			return -ECANCELED;
+
 		*elem_priv = &rbe_le->priv;
 		return -EEXIST;
 	}