nl80211: fix another nl80211_fam.attrbuf race

This is similar to the race Linus had reported, but in this case
it's an older bug: nl80211_prepare_wdev_dump() uses the wiphy
index in cb->args[0] as it is and thus parses the message over
and over again instead of just once because 0 is the first valid
wiphy index. Similar code in nl80211_testmode_dump() correctly
offsets the wiphy_index by 1, do that here as well.

Cc: stable@vger.kernel.org
Reported-by: NBen Hutchings <ben@decadent.org.uk>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -441,10 +441,12 @@ static int nl80211_prepare_wdev_dump(struct sk_buff *skb,
 			goto out_unlock;
 		}
 		*rdev = wiphy_to_dev((*wdev)->wiphy);
-		cb->args[0] = (*rdev)->wiphy_idx;
+		/* 0 is the first index - add 1 to parse only once */
+		cb->args[0] = (*rdev)->wiphy_idx + 1;
 		cb->args[1] = (*wdev)->identifier;
 	} else {
-		struct wiphy *wiphy = wiphy_idx_to_wiphy(cb->args[0]);
+		/* subtract the 1 again here */
+		struct wiphy *wiphy = wiphy_idx_to_wiphy(cb->args[0] - 1);
 		struct wireless_dev *tmp;
 
 		if (!wiphy) {