netfilter: xtables: check for unconditionality of policies

This adds a check that iptables's original author Rusty set forth in
a FIXME comment.

Underflows in iptables are better known as chain policies, and are
required to be unconditional or there would be a stochastical chance
for the policy rule to be skipped if it does not match. If that were
to happen, rule execution would continue in an unexpected spurious
fashion.
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>

net/ipv4/netfilter/arp_tables.c

@@ -8,7 +8,7 @@
  * Copyright (C) 2002 David S. Miller (davem@redhat.com)
  *
  */
-
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 #include <linux/kernel.h>
 #include <linux/skbuff.h>
 #include <linux/netdevice.h>
@@ -563,12 +563,14 @@ static inline int check_entry_size_and_hooks(struct arpt_entry *e,
 			continue;
 		if ((unsigned char *)e - base == hook_entries[h])
 			newinfo->hook_entry[h] = hook_entries[h];
-		if ((unsigned char *)e - base == underflows[h])
+		if ((unsigned char *)e - base == underflows[h]) {
+			if (!unconditional(&e->arp)) {
+				pr_err("Underflows must be unconditional\n");
+				return -EINVAL;
+			}
 			newinfo->underflow[h] = underflows[h];
 		}
-
-	/* FIXME: underflows must be unconditional, standard verdicts
-	   < 0 (not ARPT_RETURN). --RR */
+	}
 
 	/* Clear counters and comefrom */
 	e->counters = ((struct xt_counters) { 0, 0 });

net/ipv4/netfilter/ip_tables.c

@@ -8,6 +8,7 @@
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  */
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 #include <linux/cache.h>
 #include <linux/capability.h>
 #include <linux/skbuff.h>
@@ -738,12 +739,14 @@ check_entry_size_and_hooks(struct ipt_entry *e,
 			continue;
 		if ((unsigned char *)e - base == hook_entries[h])
 			newinfo->hook_entry[h] = hook_entries[h];
-		if ((unsigned char *)e - base == underflows[h])
+		if ((unsigned char *)e - base == underflows[h]) {
+			if (!unconditional(&e->ip)) {
+				pr_err("Underflows must be unconditional\n");
+				return -EINVAL;
+			}
 			newinfo->underflow[h] = underflows[h];
 		}
-
-	/* FIXME: underflows must be unconditional, standard verdicts
-	   < 0 (not IPT_RETURN). --RR */
+	}
 
 	/* Clear counters and comefrom */
 	e->counters = ((struct xt_counters) { 0, 0 });

net/ipv6/netfilter/ip6_tables.c

@@ -8,7 +8,7 @@
  * it under the terms of the GNU General Public License version 2 as
  * published by the Free Software Foundation.
  */
-
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 #include <linux/capability.h>
 #include <linux/in.h>
 #include <linux/skbuff.h>
@@ -771,12 +771,14 @@ check_entry_size_and_hooks(struct ip6t_entry *e,
 			continue;
 		if ((unsigned char *)e - base == hook_entries[h])
 			newinfo->hook_entry[h] = hook_entries[h];
-		if ((unsigned char *)e - base == underflows[h])
+		if ((unsigned char *)e - base == underflows[h]) {
+			if (!unconditional(&e->ipv6)) {
+				pr_err("Underflows must be unconditional\n");
+				return -EINVAL;
+			}
 			newinfo->underflow[h] = underflows[h];
 		}
-
-	/* FIXME: underflows must be unconditional, standard verdicts
-	   < 0 (not IP6T_RETURN). --RR */
+	}
 
 	/* Clear counters and comefrom */
 	e->counters = ((struct xt_counters) { 0, 0 });