cfg80211: wext: avoid copying malformed SSIDs

Ensure the SSID element is bounds-checked prior to invoking memcpy()
with its length field, when copying to userspace.

Cc: <[e-mail]>
Cc: Kees Cook <[e-mail]>
Reported-by: Nicolas Waisman <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[adjust commit log a bit]
Signed-off-by: Johannes Berg <[e-mail]>

net/wireless/wext-sme.c

···
 			       struct iw_point *data, char *ssid)
 {
 	struct wireless_dev *wdev = dev->ieee80211_ptr;
+	int ret = 0;
 
 	/* call only for station! */
 	if (WARN_ON(wdev->iftype != NL80211_IFTYPE_STATION))
···
 		if (ie) {
 			data->flags = 1;
 			data->length = ie[1];
-			memcpy(ssid, ie + 2, data->length);
+			if (data->length > IW_ESSID_MAX_SIZE)
+				ret = -EINVAL;
+			else
+				memcpy(ssid, ie + 2, data->length);
 		}
 		rcu_read_unlock();
 	} else if (wdev->wext.connect.ssid && wdev->wext.connect.ssid_len) {
···
 	}
 	wdev_unlock(wdev);
 
-	return 0;
+	return ret;
 }
 
 int cfg80211_mgd_wext_siwap(struct net_device *dev,