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

[Phil Sutter]

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 <p...@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