[NETFILTER]: {ip,ip6,arp}_tables: fix exponential worst-case search for loops

If we come to node we'd already marked as seen and it's not a part of path
(i.e. we don't have a loop right there), we already know that it isn't a
part of any loop, so we don't need to revisit it.

That speeds the things up if some chain is refered to from several places
and kills O(exp(table size)) worst-case behaviour (without sleeping,
at that, so if you manage to self-LART that way, you are SOL for a long
time)...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/arp_tables.c

@@ -358,6 +358,7 @@ static int mark_source_chains(struct xt_table_info *newinfo,
 		for (;;) {
 			struct arpt_standard_target *t
 				= (void *)arpt_get_target(e);
+			int visited = e->comefrom & (1 << hook);
 
 			if (e->comefrom & (1 << NF_ARP_NUMHOOKS)) {
 				printk("arptables: loop hook %u pos %u %08X.\n",
@@ -368,11 +369,11 @@ static int mark_source_chains(struct xt_table_info *newinfo,
 				|= ((1 << hook) | (1 << NF_ARP_NUMHOOKS));
 
 			/* Unconditional return/END. */
-			if (e->target_offset == sizeof(struct arpt_entry)
+			if ((e->target_offset == sizeof(struct arpt_entry)
 			    && (strcmp(t->target.u.user.name,
 				       ARPT_STANDARD_TARGET) == 0)
 			    && t->verdict < 0
-			    && unconditional(&e->arp)) {
+			    && unconditional(&e->arp)) || visited) {
 				unsigned int oldpos, size;
 
 				if (t->verdict < -NF_MAX_VERDICT - 1) {

net/ipv4/netfilter/ip_tables.c

@@ -384,6 +384,7 @@ mark_source_chains(struct xt_table_info *newinfo,
 		for (;;) {
 			struct ipt_standard_target *t
 				= (void *)ipt_get_target(e);
+			int visited = e->comefrom & (1 << hook);
 
 			if (e->comefrom & (1 << NF_IP_NUMHOOKS)) {
 				printk("iptables: loop hook %u pos %u %08X.\n",
@@ -394,11 +395,11 @@ mark_source_chains(struct xt_table_info *newinfo,
 				|= ((1 << hook) | (1 << NF_IP_NUMHOOKS));
 
 			/* Unconditional return/END. */
-			if (e->target_offset == sizeof(struct ipt_entry)
+			if ((e->target_offset == sizeof(struct ipt_entry)
 			    && (strcmp(t->target.u.user.name,
 				       IPT_STANDARD_TARGET) == 0)
 			    && t->verdict < 0
-			    && unconditional(&e->ip)) {
+			    && unconditional(&e->ip)) || visited) {
 				unsigned int oldpos, size;
 
 				if (t->verdict < -NF_MAX_VERDICT - 1) {

net/ipv6/netfilter/ip6_tables.c

@@ -413,6 +413,7 @@ mark_source_chains(struct xt_table_info *newinfo,
 		unsigned int pos = newinfo->hook_entry[hook];
 		struct ip6t_entry *e
 			= (struct ip6t_entry *)(entry0 + pos);
+		int visited = e->comefrom & (1 << hook);
 
 		if (!(valid_hooks & (1 << hook)))
 			continue;
@@ -433,11 +434,11 @@ mark_source_chains(struct xt_table_info *newinfo,
 				|= ((1 << hook) | (1 << NF_IP6_NUMHOOKS));
 
 			/* Unconditional return/END. */
-			if (e->target_offset == sizeof(struct ip6t_entry)
+			if ((e->target_offset == sizeof(struct ip6t_entry)
 			    && (strcmp(t->target.u.user.name,
 				       IP6T_STANDARD_TARGET) == 0)
 			    && t->verdict < 0
-			    && unconditional(&e->ipv6)) {
+			    && unconditional(&e->ipv6)) || visited) {
 				unsigned int oldpos, size;
 
 				if (t->verdict < -NF_MAX_VERDICT - 1) {