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, 6 insertions, 0 deletions

diff --git a/include/net/netfilter/nft_fib.h b/include/net/netfilter/nft_fib.h
index cbedda077db2..5ceb2205e4e3 100644
--- a/include/net/netfilter/nft_fib.h
+++ b/include/net/netfilter/nft_fib.h
@@ -9,6 +9,12 @@ struct nft_fib {
 
 extern const struct nla_policy nft_fib_policy[];
 
+static inline bool
+nft_fib_is_loopback(const struct sk_buff *skb, const struct net_device *in)
+{
+	return skb->pkt_type == PACKET_LOOPBACK || in->flags & IFF_LOOPBACK;
+}
+
 int nft_fib_dump(struct sk_buff *skb, const struct nft_expr *expr);
 int nft_fib_init(const struct nft_ctx *ctx, const struct nft_expr *expr,
 		 const struct nlattr * const tb[]);