cfg80211: scan: fix RCU in cfg80211_add_nontrans_list()

The SSID pointer is pointing to RCU protected data, so we
need to have it under rcu_read_lock() for the entire use.
Fix this.

Cc: stable@vger.kernel.org
Fixes: 0b8fb823 ("cfg80211: Parsing of Multiple BSSID information in scanning")
Link: https://lore.kernel.org/r/20210930131120.6ddfc603aa1d.I2137344c4e2426525b1a8e4ce5fca82f8ecbfe7e@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/scan.c

@@ -418,14 +418,17 @@ cfg80211_add_nontrans_list(struct cfg80211_bss *trans_bss,
 	}
 	ssid_len = ssid[1];
 	ssid = ssid + 2;
-	rcu_read_unlock();
 
 	/* check if nontrans_bss is in the list */
 	list_for_each_entry(bss, &trans_bss->nontrans_list, nontrans_list) {
-		if (is_bss(bss, nontrans_bss->bssid, ssid, ssid_len))
+		if (is_bss(bss, nontrans_bss->bssid, ssid, ssid_len)) {
+			rcu_read_unlock();
 			return 0;
+		}
 	}
 
+	rcu_read_unlock();
+
 	/* add to the list */
 	list_add_tail(&nontrans_bss->nontrans_list, &trans_bss->nontrans_list);
 	return 0;