[NETFILTER]: ip_conntrack: fix NAT helper unload races

The NAT helpr hooks are protected by RCU, but all of the
conntrack helpers test and use the global pointers instead
of copying them first using rcu_dereference()

Also replace synchronize_net() by synchronize_rcu() for clarity
since sychronizing only with packet receive processing is
insufficient to prevent races.

Signed-off-by: Patrick McHardy <kaber@trash.net>

net/ipv4/netfilter/ip_conntrack_amanda.c

@@ -92,6 +92,7 @@ static int help(struct sk_buff **pskb,
 	char pbuf[sizeof("65535")], *tmp;
 	u_int16_t port, len;
 	int ret = NF_ACCEPT;
+	typeof(ip_nat_amanda_hook) ip_nat_amanda;
 
 	/* Only look at packets from the Amanda server */
 	if (CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL)
@@ -161,9 +162,11 @@ static int help(struct sk_buff **pskb,
 		exp->mask.dst.protonum = 0xFF;
 		exp->mask.dst.u.tcp.port = htons(0xFFFF);
 
-		if (ip_nat_amanda_hook)
-			ret = ip_nat_amanda_hook(pskb, ctinfo, off - dataoff,
-						 len, exp);
+		/* RCU read locked by nf_hook_slow */
+		ip_nat_amanda = rcu_dereference(ip_nat_amanda_hook);
+		if (ip_nat_amanda)
+			ret = ip_nat_amanda(pskb, ctinfo, off - dataoff,
+					    len, exp);
 		else if (ip_conntrack_expect_related(exp) != 0)
 			ret = NF_DROP;
 		ip_conntrack_expect_put(exp);

net/ipv4/netfilter/ip_conntrack_ftp.c

@@ -310,6 +310,7 @@ static int help(struct sk_buff **pskb,
 	struct ip_conntrack_expect *exp;
 	unsigned int i;
 	int found = 0, ends_in_nl;
+	typeof(ip_nat_ftp_hook) ip_nat_ftp;
 
 	/* Until there's been traffic both ways, don't look in packets. */
 	if (ctinfo != IP_CT_ESTABLISHED
@@ -433,9 +434,10 @@ static int help(struct sk_buff **pskb,
 
 	/* Now, NAT might want to mangle the packet, and register the
 	 * (possibly changed) expectation itself. */
-	if (ip_nat_ftp_hook)
-		ret = ip_nat_ftp_hook(pskb, ctinfo, search[dir][i].ftptype,
-				      matchoff, matchlen, exp, &seq);
+	ip_nat_ftp = rcu_dereference(ip_nat_ftp_hook);
+	if (ip_nat_ftp)
+		ret = ip_nat_ftp(pskb, ctinfo, search[dir][i].ftptype,
+				 matchoff, matchlen, exp, &seq);
 	else {
 		/* Can't expect this?  Best to drop packet now. */
 		if (ip_conntrack_expect_related(exp) != 0)