wifi: cfg80211: fix use-after-free in cmp_bss()

Following bss_free() quirk introduced in commit 776b3580178f
("cfg80211: track hidden SSID networks properly"), adjust
cfg80211_update_known_bss() to free the last beacon frame
elements only if they're not shared via the corresponding
'hidden_beacon_bss' pointer.

Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=30754ca335e6fb7e3092
Fixes: 3ab8227d3e7d ("cfg80211: refactor cfg80211_bss_update")
Signed-off-by: Dmitry Antipov <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

net/wireless/scan.c

···
 			 */
 
 			f = rcu_access_pointer(new->pub.beacon_ies);
-			kfree_rcu((struct cfg80211_bss_ies *)f, rcu_head);
+			if (!new->pub.hidden_beacon_bss)
+				kfree_rcu((struct cfg80211_bss_ies *)f, rcu_head);
 			return false;
 		}