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.

Reported-by: Ben Hawkes <[e-mail]>
Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/x_tables.c

···
 	struct xt_table_info *info = NULL;
 	size_t sz = sizeof(*info) + size;
 
+	if (sz < sizeof(*info))
+		return NULL;
+
 	/* Pedantry: prevent them from hitting BUG() in vmalloc.c --RR */
 	if ((SMP_ALIGN(size) >> PAGE_SHIFT) + 2 > totalram_pages)
 		return NULL;