netfilter: Can't fail and free after table replacement

All xtables variants suffer from the defect that the copy_to_user()
to copy the counters to user memory may fail after the table has
already been exchanged and thus exposed. Return an error at this
point will result in freeing the already exposed table. Any
subsequent packet processing will result in a kernel panic.

We can't copy the counters before exposing the new tables as we
want provide the counter state after the old table has been
unhooked. Therefore convert this into a silent error.

Cc: Florian Westphal <[e-mail]>
Signed-off-by: Thomas Graf <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/bridge/netfilter/ebtables.c

···
 	if (repl->num_counters &&
 	   copy_to_user(repl->counters, counterstmp,
 	   repl->num_counters * sizeof(struct ebt_counter))) {
-		ret = -EFAULT;
+		/* Silent error, can't fail, new table is already in place */
+		net_warn_ratelimited("ebtables: counters copy to user failed while replacing table\n");
 	}
-	else
-		ret = 0;
 
 	/* decrease module count and free resources */
 	EBT_ENTRY_ITERATE(table->entries, table->entries_size,

net/ipv4/netfilter/arp_tables.c

···
 
 	xt_free_table_info(oldinfo);
 	if (copy_to_user(counters_ptr, counters,
-			 sizeof(struct xt_counters) * num_counters) != 0)
-		ret = -EFAULT;
+			 sizeof(struct xt_counters) * num_counters) != 0) {
+		/* Silent error, can't fail, new table is already in place */
+		net_warn_ratelimited("arptables: counters copy to user failed while replacing table\n");
+	}
 	vfree(counters);
 	xt_table_unlock(t);
 	return ret;

net/ipv4/netfilter/ip_tables.c

···
 
 	xt_free_table_info(oldinfo);
 	if (copy_to_user(counters_ptr, counters,
-			 sizeof(struct xt_counters) * num_counters) != 0)
-		ret = -EFAULT;
+			 sizeof(struct xt_counters) * num_counters) != 0) {
+		/* Silent error, can't fail, new table is already in place */
+		net_warn_ratelimited("iptables: counters copy to user failed while replacing table\n");
+	}
 	vfree(counters);
 	xt_table_unlock(t);
 	return ret;

net/ipv6/netfilter/ip6_tables.c

···
 
 	xt_free_table_info(oldinfo);
 	if (copy_to_user(counters_ptr, counters,
-			 sizeof(struct xt_counters) * num_counters) != 0)
-		ret = -EFAULT;
+			 sizeof(struct xt_counters) * num_counters) != 0) {
+		/* Silent error, can't fail, new table is already in place */
+		net_warn_ratelimited("ip6tables: counters copy to user failed while replacing table\n");
+	}
 	vfree(counters);
 	xt_table_unlock(t);
 	return ret;