netfilter: x_tables: check for size overflow

Ben Hawkes says:
 integer overflow in xt_alloc_table_info, which on 32-bit systems can
 lead to small structure allocation and a copy_from_user based heap
 corruption.

Change-Id: I13c554c630651a37e3f6a195e9a5f40cddcb29a1
Reported-by: Ben Hawkes <hawkes@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/x_tables.c

@@ -664,6 +664,10 @@ struct xt_table_info *xt_alloc_table_info(unsigned int size)
 {
 	struct xt_table_info *newinfo;
 	int cpu;
+	size_t sz = sizeof(*newinfo) + size;
+
+	if (sz < sizeof(*newinfo))
+		return NULL;
 
 	/* Pedantry: prevent them from hitting BUG() in vmalloc.c --RR */
 	if ((SMP_ALIGN(size) >> PAGE_SHIFT) + 2 > totalram_pages)