netfilter: tproxy: fix deadlock due to missing BH disable

The xtables packet traverser performs an unconditional local_bh_disable(),
but the nf_tables evaluation loop does not.

Functions that are called from either xtables or nftables must assume
that they can be called in process context.

inet_twsk_deschedule_put() assumes that no softirq interrupt can occur.
If tproxy is used from nf_tables its possible that we'll deadlock
trying to aquire a lock already held in process context.

Add a small helper that takes care of this and use it.

Link: https://lore.kernel.org/netfilter-devel/401bd6ed-314a-a196-1cdc-e13c720cc8f2@balasys.hu/
Fixes: 4ed8eb6 ("netfilter: nf_tables: Add native tproxy support")
Reported-and-tested-by: Major Dávid <major.david@balasys.hu>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Author: Florian Westphal

include/net/netfilter/nf_tproxy.h

@@ -17,6 +17,13 @@ static inline bool nf_tproxy_sk_is_transparent(struct sock *sk)
 	return false;
 }
 
+static inline void nf_tproxy_twsk_deschedule_put(struct inet_timewait_sock *tw)
+{
+	local_bh_disable();
+	inet_twsk_deschedule_put(tw);
+	local_bh_enable();
+}
+
 /* assign a socket to the skb -- consumes sk */
 static inline void nf_tproxy_assign_sock(struct sk_buff *skb, struct sock *sk)
 {

net/ipv4/netfilter/nf_tproxy_ipv4.c

@@ -38,7 +38,7 @@ nf_tproxy_handle_time_wait4(struct net *net, struct sk_buff *skb,
 					    hp->source, lport ? lport : hp->dest,
 					    skb->dev, NF_TPROXY_LOOKUP_LISTENER);
 		if (sk2) {
-			inet_twsk_deschedule_put(inet_twsk(sk));
+			nf_tproxy_twsk_deschedule_put(inet_twsk(sk));
 			sk = sk2;
 		}
 	}

net/ipv6/netfilter/nf_tproxy_ipv6.c

@@ -63,7 +63,7 @@ nf_tproxy_handle_time_wait6(struct sk_buff *skb, int tproto, int thoff,
 					    lport ? lport : hp->dest,
 					    skb->dev, NF_TPROXY_LOOKUP_LISTENER);
 		if (sk2) {
-			inet_twsk_deschedule_put(inet_twsk(sk));
+			nf_tproxy_twsk_deschedule_put(inet_twsk(sk));
 			sk = sk2;
 		}
 	}