netfilter: rpfilter: fix incorrect loopback packet judgment

Currently, we check the existing rtable in PREROUTING hook, if RTCF_LOCAL
is set, we assume that the packet is loopback.

But this assumption is incorrect, for example, a packet encapsulated
in ipsec transport mode was received and routed to local, after
decapsulation, it would be delivered to local again, and the rtable
was not dropped, so RTCF_LOCAL check would trigger. But actually, the
packet was not loopback.

So for these normal loopback packets, we can check whether the in device
is IFF_LOOPBACK or not. For these locally generated broadcast/multicast,
we can check whether the skb->pkt_type is PACKET_LOOPBACK or not.

Finally, there's a subtle difference between nft fib expr and xtables
rpfilter extension, user can add the following nft rule to do strict
rpfilter check:
  # nft add rule x y meta iif eth0 fib saddr . iif oif != eth0 drop

So when the packet is loopback, it's better to store the in device
instead of the LOOPBACK_IFINDEX, otherwise, after adding the above
nft rule, locally generated broad/multicast packets will be dropped
incorrectly.

Fixes: f83a7ea2075c ("netfilter: xt_rpfilter: skip locally generated broadcast/multicast, too")
Fixes: f6d0cbcf09c5 ("netfilter: nf_tables: add fib expression")
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 4 insertions, 9 deletions

diff --git a/net/ipv6/netfilter/nft_fib_ipv6.c b/net/ipv6/netfilter/nft_fib_ipv6.c
index c947aad8bcc6..765facf03d45 100644
--- a/net/ipv6/netfilter/nft_fib_ipv6.c
+++ b/net/ipv6/netfilter/nft_fib_ipv6.c
@@ -18,13 +18,6 @@
 #include <net/ip6_fib.h>
 #include <net/ip6_route.h>
 
-static bool fib6_is_local(const struct sk_buff *skb)
-{
-	const struct rt6_info *rt = (const void *)skb_dst(skb);
-
-	return rt && (rt->rt6i_flags & RTF_LOCAL);
-}
-
 static int get_ifindex(const struct net_device *dev)
 {
 	return dev ? dev->ifindex : 0;
@@ -164,8 +157,10 @@ void nft_fib6_eval(const struct nft_expr *expr, struct nft_regs *regs,
 
 	lookup_flags = nft_fib6_flowi_init(&fl6, priv, pkt, oif);
 
-	if (nft_hook(pkt) == NF_INET_PRE_ROUTING && fib6_is_local(pkt->skb)) {
-		nft_fib_store_result(dest, priv->result, pkt, LOOPBACK_IFINDEX);
+	if (nft_hook(pkt) == NF_INET_PRE_ROUTING &&
+	    nft_fib_is_loopback(pkt->skb, nft_in(pkt))) {
+		nft_fib_store_result(dest, priv->result, pkt,
+				     nft_in(pkt)->ifindex);
 		return;
 	}