rps: fix insufficient bounds checking in store_rps_dev_flow_table_cnt()

Setting a large rps_flow_cnt like (1 << 30) on 32-bit platform will
cause a kernel oops due to insufficient bounds checking.

	if (count > 1<<30) {
		/* Enforce a limit to prevent overflow */
		return -EINVAL;
	}
	count = roundup_pow_of_two(count);
	table = vmalloc(RPS_DEV_FLOW_TABLE_SIZE(count));

Note that the macro RPS_DEV_FLOW_TABLE_SIZE(count) is defined as:

	... + (count * sizeof(struct rps_dev_flow))

where sizeof(struct rps_dev_flow) is 8.  (1 << 30) * 8 will overflow
32 bits.

This patch replaces the magic number (1 << 30) with a symbolic bound.
Suggested-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NXi Wang <xi.wang@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/net-sysfs.c

@@ -665,11 +665,14 @@ static ssize_t store_rps_dev_flow_table_cnt(struct netdev_rx_queue *queue,
 	if (count) {
 		int i;
 
-		if (count > 1<<30) {
+		if (count > INT_MAX)
+			return -EINVAL;
+		count = roundup_pow_of_two(count);
+		if (count > (ULONG_MAX - sizeof(struct rps_dev_flow_table))
+				/ sizeof(struct rps_dev_flow)) {
 			/* Enforce a limit to prevent overflow */
 			return -EINVAL;
 		}
-		count = roundup_pow_of_two(count);
 		table = vmalloc(RPS_DEV_FLOW_TABLE_SIZE(count));
 		if (!table)
 			return -ENOMEM;