mwifiex: fix invalid memory access in mwifiex_get_power_level()

With "while (length)" check we may end up in accessing invalid
memory in last iteration.
This patch makes sure that tlv length is not less than the length
of structure mwifiex_power_group when min/max power is calculated.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/mwifiex/sta_cmdresp.c

@@ -341,12 +341,16 @@ static int mwifiex_get_power_level(struct mwifiex_private *priv, void *data_buf)
 	pg = (struct mwifiex_power_group *)
 		((u8 *) pg_tlv_hdr + sizeof(struct mwifiex_types_power_group));
 	length = le16_to_cpu(pg_tlv_hdr->length);
-	if (length > 0) {
-		max_power = pg->power_max;
-		min_power = pg->power_min;
-		length -= sizeof(struct mwifiex_power_group);
-	}
-	while (length) {
+
+	/* At least one structure required to update power */
+	if (length < sizeof(struct mwifiex_power_group))
+		return 0;
+
+	max_power = pg->power_max;
+	min_power = pg->power_min;
+	length -= sizeof(struct mwifiex_power_group);
+
+	while (length >= sizeof(struct mwifiex_power_group)) {
 		pg++;
 		if (max_power < pg->power_max)
 			max_power = pg->power_max;
@@ -356,10 +360,8 @@ static int mwifiex_get_power_level(struct mwifiex_private *priv, void *data_buf)
 
 		length -= sizeof(struct mwifiex_power_group);
 	}
-	if (le16_to_cpu(pg_tlv_hdr->length) > 0) {
-		priv->min_tx_power_level = (u8) min_power;
-		priv->max_tx_power_level = (u8) max_power;
-	}
+	priv->min_tx_power_level = (u8) min_power;
+	priv->max_tx_power_level = (u8) max_power;
 
 	return 0;
 }