Re: [syzbot] [net?] KASAN: slab-use-after-free Write in l2tp_session_delete

[Hillf Danton <hdanton@sina.com>]

On 25/06/2024 14:25, syzbot wrote:
> syzbot found the following issue on:
> 
> HEAD commit:    185d72112b95 net: xilinx: axienet: Enable multicast by def..
> git tree:       net-next
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1062bd46980000

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git  185d72112b95

--- x/net/l2tp/l2tp_ppp.c
+++ y/net/l2tp/l2tp_ppp.c
@@ -446,6 +446,7 @@ static int pppol2tp_release(struct socke
 	if (session) {
 		struct pppol2tp_session *ps;
 
+		schedule_timeout_idle(HZ/10);
 		l2tp_session_delete(session);
 
 		ps = l2tp_session_priv(session);
--- x/net/l2tp/l2tp_core.c
+++ y/net/l2tp/l2tp_core.c
@@ -1290,17 +1290,20 @@ static void l2tp_session_unhash(struct l
 static void l2tp_tunnel_closeall(struct l2tp_tunnel *tunnel)
 {
 	struct l2tp_session *session;
-	struct list_head __rcu *pos;
-	struct list_head *tmp;
 
 	spin_lock_bh(&tunnel->list_lock);
 	tunnel->acpt_newsess = false;
-	list_for_each_safe(pos, tmp, &tunnel->session_list) {
-		session = list_entry(pos, struct l2tp_session, list);
+	for (;;) {
+		session = list_first_entry_or_null(&tunnel->session_list,
+						   struct l2tp_session, list);
+		if (!session)
+			break;
+		l2tp_session_inc_refcount(session);
 		list_del_init(&session->list);
 		spin_unlock_bh(&tunnel->list_lock);
 		l2tp_session_delete(session);
 		spin_lock_bh(&tunnel->list_lock);
+		l2tp_session_dec_refcount(session);
 	}
 	spin_unlock_bh(&tunnel->list_lock);
 }
--