netfilter: nf_tables: Make nft_meta expression more robust

nft_meta_get_eval()'s tendency to bail out setting NFT_BREAK verdict in
situations where required data is missing leads to unexpected behaviour
with inverted checks like so:

| meta iifname != eth0 accept

This rule will never match if there is no input interface (or it is not
known) which is not intuitive and, what's worse, breaks consistency of
iptables-nft with iptables-legacy.

Fix this by falling back to placing a value in dreg which never matches
(avoiding accidental matches), i.e. zero for interface index and an
empty string for interface name.
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/bridge/netfilter/nft_meta_bridge.c

@@ -30,13 +30,9 @@ static void nft_meta_bridge_get_eval(const struct nft_expr *expr,
 	switch (priv->key) {
 	case NFT_META_BRI_IIFNAME:
 		br_dev = nft_meta_get_bridge(in);
-		if (!br_dev)
-			goto err;
 		break;
 	case NFT_META_BRI_OIFNAME:
 		br_dev = nft_meta_get_bridge(out);
-		if (!br_dev)
-			goto err;
 		break;
 	case NFT_META_BRI_IIFPVID: {
 		u16 p_pvid;
@@ -64,7 +60,7 @@ static void nft_meta_bridge_get_eval(const struct nft_expr *expr,
 		goto out;
 	}
 
-	strncpy((char *)dest, br_dev->name, IFNAMSIZ);
+	strncpy((char *)dest, br_dev ? br_dev->name : "", IFNAMSIZ);
 	return;
 out:
 	return nft_meta_get_eval(expr, regs, pkt);

net/netfilter/nft_meta.c

@@ -60,24 +60,16 @@ void nft_meta_get_eval(const struct nft_expr *expr,
 		*dest = skb->mark;
 		break;
 	case NFT_META_IIF:
-		if (in == NULL)
-			goto err;
-		*dest = in->ifindex;
+		*dest = in ? in->ifindex : 0;
 		break;
 	case NFT_META_OIF:
-		if (out == NULL)
-			goto err;
-		*dest = out->ifindex;
+		*dest = out ? out->ifindex : 0;
 		break;
 	case NFT_META_IIFNAME:
-		if (in == NULL)
-			goto err;
-		strncpy((char *)dest, in->name, IFNAMSIZ);
+		strncpy((char *)dest, in ? in->name : "", IFNAMSIZ);
 		break;
 	case NFT_META_OIFNAME:
-		if (out == NULL)
-			goto err;
-		strncpy((char *)dest, out->name, IFNAMSIZ);
+		strncpy((char *)dest, out ? out->name : "", IFNAMSIZ);
 		break;
 	case NFT_META_IIFTYPE:
 		if (in == NULL)