ipv6: flowlabel: fl6_sock_lookup() must use atomic_inc_not_zero

BugLink: https://bugs.launchpad.net/bugs/1836666

[ Upstream commit 65a3c497 ]

Before taking a refcount, make sure the object is not already
scheduled for deletion.

Same fix is needed in ipv6_flowlabel_opt()

Fixes: 18367681 ("ipv6 flowlabel: Convert np->ipv6_fl_list to RCU.")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/ipv6/ip6_flowlabel.c

@@ -254,9 +254,9 @@ struct ip6_flowlabel *fl6_sock_lookup(struct sock *sk, __be32 label)
 	rcu_read_lock_bh();
 	for_each_sk_fl_rcu(np, sfl) {
 		struct ip6_flowlabel *fl = sfl->fl;
-		if (fl->label == label) {
+
+		if (fl->label == label && atomic_inc_not_zero(&fl->users)) {
 			fl->lastuse = jiffies;
-			atomic_inc(&fl->users);
 			rcu_read_unlock_bh();
 			return fl;
 		}
@@ -622,7 +622,8 @@ int ipv6_flowlabel_opt(struct sock *sk, char __user *optval, int optlen)
 						goto done;
 					}
 					fl1 = sfl->fl;
-					atomic_inc(&fl1->users);
+					if (!atomic_inc_not_zero(&fl1->users))
+						fl1 = NULL;
 					break;
 				}
 			}