netfilter: x_tables: add counters allocation wrapper

allows to have size checks in a single spot.
This is supposed to reduce oom situations when fuzz-testing xtables.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Author: Florian Westphal

include/linux/netfilter/x_tables.h

@@ -301,6 +301,7 @@ int xt_data_to_user(void __user *dst, const void *src,
 
 void *xt_copy_counters_from_user(const void __user *user, unsigned int len,
 				 struct xt_counters_info *info, bool compat);
+struct xt_counters *xt_counters_alloc(unsigned int counters);
 
 struct xt_table *xt_register_table(struct net *net,
 				   const struct xt_table *table,

net/ipv4/netfilter/arp_tables.c

@@ -883,7 +883,7 @@ static int __do_replace(struct net *net, const char *name,
 	struct arpt_entry *iter;
 
 	ret = 0;
-	counters = vzalloc(num_counters * sizeof(struct xt_counters));
+	counters = xt_counters_alloc(num_counters);
 	if (!counters) {
 		ret = -ENOMEM;
 		goto out;

net/ipv4/netfilter/ip_tables.c

@@ -1045,7 +1045,7 @@ __do_replace(struct net *net, const char *name, unsigned int valid_hooks,
 	struct ipt_entry *iter;
 
 	ret = 0;
-	counters = vzalloc(num_counters * sizeof(struct xt_counters));
+	counters = xt_counters_alloc(num_counters);
 	if (!counters) {
 		ret = -ENOMEM;
 		goto out;

net/ipv6/netfilter/ip6_tables.c

@@ -1063,7 +1063,7 @@ __do_replace(struct net *net, const char *name, unsigned int valid_hooks,
 	struct ip6t_entry *iter;
 
 	ret = 0;
-	counters = vzalloc(num_counters * sizeof(struct xt_counters));
+	counters = xt_counters_alloc(num_counters);
 	if (!counters) {
 		ret = -ENOMEM;
 		goto out;

net/netfilter/x_tables.c

@@ -1290,6 +1290,21 @@ static int xt_jumpstack_alloc(struct xt_table_info *i)
 	return 0;
 }
 
+struct xt_counters *xt_counters_alloc(unsigned int counters)
+{
+	struct xt_counters *mem;
+
+	if (counters == 0 || counters > INT_MAX / sizeof(*mem))
+		return NULL;
+
+	counters *= sizeof(*mem);
+	if (counters > XT_MAX_TABLE_SIZE)
+		return NULL;
+
+	return vzalloc(counters);
+}
+EXPORT_SYMBOL(xt_counters_alloc);
+
 struct xt_table_info *
 xt_replace_table(struct xt_table *table,
 	      unsigned int num_counters,