[nf PATCH] net: nf_tables: Make nft_meta expression more robust

[Phil Sutter <phil@nwl.cc>]

nft_meta_get_eval()'s tendency to bail out setting NFT_BREAK verdict in
situations where required data is missing breaks inverted checks
like e.g.:

| 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,O}IF:
	Use invalid ifindex value zero.

{I,O}IFNAME, {I,O}IFKIND:
	Use an empty string which is neither a valid interface name nor
	kind.

{I,O}IFTYPE:
	Use ARPHRD_VOID (0xFFFF).

Signed-off-by: Phil Sutter <phil@nwl.cc>
---
 net/netfilter/nft_meta.c | 45 +++++++++++++++++-----------------------
 1 file changed, 19 insertions(+), 26 deletions(-)

diff --git a/net/netfilter/nft_meta.c b/net/netfilter/nft_meta.c
index 76866f77e3435..ee3b54692cc7e 100644
--- a/net/netfilter/nft_meta.c
+++ b/net/netfilter/nft_meta.c
@@ -11,6 +11,7 @@
 #include <linux/netlink.h>
 #include <linux/netfilter.h>
 #include <linux/netfilter/nf_tables.h>
+#include <linux/if_arp.h>
 #include <linux/in.h>
 #include <linux/ip.h>
 #include <linux/ipv6.h>
@@ -60,34 +61,22 @@ 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)
-			goto err;
-		nft_reg_store16(dest, in->type);
+		nft_reg_store16(dest, in ? in->type : ARPHRD_VOID);
 		break;
 	case NFT_META_OIFTYPE:
-		if (out == NULL)
-			goto err;
-		nft_reg_store16(dest, out->type);
+		nft_reg_store16(dest, out ? out->type : ARPHRD_VOID);
 		break;
 	case NFT_META_SKUID:
 		sk = skb_to_full_sk(skb);
@@ -216,16 +205,20 @@ void nft_meta_get_eval(const struct nft_expr *expr,
 		nft_reg_store8(dest, secpath_exists(skb));
 		break;
 #endif
-	case NFT_META_IIFKIND:
-		if (in == NULL || in->rtnl_link_ops == NULL)
-			goto err;
-		strncpy((char *)dest, in->rtnl_link_ops->kind, IFNAMSIZ);
+	case NFT_META_IIFKIND: {
+		const struct rtnl_link_ops *rl_ops =
+			in ? in->rtnl_link_ops : NULL;
+
+		strncpy((char *)dest, rl_ops ? rl_ops->kind : "", IFNAMSIZ);
 		break;
-	case NFT_META_OIFKIND:
-		if (out == NULL || out->rtnl_link_ops == NULL)
-			goto err;
-		strncpy((char *)dest, out->rtnl_link_ops->kind, IFNAMSIZ);
+	}
+	case NFT_META_OIFKIND: {
+		const struct rtnl_link_ops *rl_ops =
+			out ? out->rtnl_link_ops : NULL;
+
+		strncpy((char *)dest, rl_ops ? rl_ops->kind : "", IFNAMSIZ);
 		break;
+	}
 	default:
 		WARN_ON(1);
 		goto err;
-- 
2.22.0