Re: [nf PATCH v2 1/2] net: nf_tables: Make nft_meta expression more robust

[Pablo Neira Ayuso <pablo@netfilter.org>]

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On Sat, Jul 20, 2019 at 05:15:02PM +0200, Phil Sutter wrote:
> Hi,
> 
> On Fri, Jul 19, 2019 at 06:35:21PM +0200, Pablo Neira Ayuso wrote:
> > On Fri, Jul 19, 2019 at 02:39:20PM +0200, Phil Sutter wrote:
> > > 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.
> > > 
> > > {BRI_,}{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).
> > 
> > What could it be done with?
> > 
> > NFT_META_BRI_IIFPVID
> > NFT_META_BRI_IIFPVPROTO
> > 
> > Those will still not work for
> > 
> >         meta ibrpvid != 40
> > 
> > if interface is not available.
> > 
> > For VPROTO probably it's possible. I don't have a solution for
> > IIFPVID.
> 
> VLAN IDs 0 and 4095 are reserved, we could use those. I refrained from
> changing bridge VLAN matches because of IIFPVPROTO, no idea if there's
> an illegal value we could use for that. If you have an idea, I'm all for
> it. :)

I think we can add something like:

        NFT_META_BRI_IIFVLAN

just to check for br_vlan_enabled(), from userspace we can check for
exists/missing as a boolean, so we don't have to worry on assuming an
unused value for things like this. This can be added in the next
release cycle.

Regarding IIFTYPE / OIFTYPE, if there is no in-trees interfaces using
"", I think we are fine, probably send a patch to propose to disallow
this to net.

These ones are missing:

        NFT_META_IIFGROUP
        NFT_META_OIFGROUP

For these two, the default group (0) should be fine since every
interface is falling under this category by default.

I can squash this small patch to this one and push it one.

Thanks.

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diff --git a/net/netfilter/nft_meta.c b/net/netfilter/nft_meta.c
index d8d04ec1face..e2c6857c0f68 100644
--- a/net/netfilter/nft_meta.c
+++ b/net/netfilter/nft_meta.c
@@ -177,14 +177,10 @@ void nft_meta_get_eval(const struct nft_expr *expr,
 		*dest = raw_smp_processor_id();
 		break;
 	case NFT_META_IIFGROUP:
-		if (in == NULL)
-			goto err;
-		*dest = in->group;
+		*dest = in ? in->group : 0;
 		break;
 	case NFT_META_OIFGROUP:
-		if (out == NULL)
-			goto err;
-		*dest = out->group;
+		*dest = out ? out->group : 0;
 		break;
 #ifdef CONFIG_CGROUP_NET_CLASSID
 	case NFT_META_CGROUP: