Re: [syzbot] [bluetooth?] KASAN: slab-use-after-free Read in l2cap_send_cmd

[Hillf Danton <hdanton@sina.com>]

On Mon, 06 Jan 2025 01:04:23 -0800
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    ab75170520d4 Merge tag 'linux-watchdog-6.13-rc6' of git://..
> git tree:       upstream
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17fa36f8580000

#syz test

--- x/net/bluetooth/l2cap_core.c
+++ y/net/bluetooth/l2cap_core.c
@@ -1747,6 +1747,8 @@ static void l2cap_unregister_all_users(s
 	}
 }
 
+static DEFINE_SPINLOCK(l2cap_conn_del_lock);
+
 static void l2cap_conn_del(struct hci_conn *hcon, int err)
 {
 	struct l2cap_conn *conn = hcon->l2cap_data;
@@ -1797,7 +1799,9 @@ static void l2cap_conn_del(struct hci_co
 	if (conn->info_state & L2CAP_INFO_FEAT_MASK_REQ_SENT)
 		cancel_delayed_work_sync(&conn->info_timer);
 
+	spin_lock(&l2cap_conn_del_lock);
 	hcon->l2cap_data = NULL;
+	spin_unlock(&l2cap_conn_del_lock);
 	conn->hchan = NULL;
 	l2cap_conn_put(conn);
 }
@@ -1819,6 +1823,8 @@ EXPORT_SYMBOL(l2cap_conn_get);
 
 void l2cap_conn_put(struct l2cap_conn *conn)
 {
+	if (!conn)
+		return;
 	kref_put(&conn->ref, l2cap_conn_free);
 }
 EXPORT_SYMBOL(l2cap_conn_put);
@@ -7477,6 +7483,13 @@ void l2cap_recv_acldata(struct hci_conn
 	if (!conn)
 		goto drop;
 
+	spin_lock(&l2cap_conn_del_lock);
+	conn = hcon->l2cap_data;
+	if (conn)
+		l2cap_conn_get(conn);
+	spin_unlock(&l2cap_conn_del_lock);
+	if (!conn)
+		goto drop;
 	BT_DBG("conn %p len %u flags 0x%x", conn, skb->len, flags);
 
 	switch (flags) {
@@ -7503,6 +7516,7 @@ void l2cap_recv_acldata(struct hci_conn
 		if (len == skb->len) {
 			/* Complete frame received */
 			l2cap_recv_frame(conn, skb);
+			l2cap_conn_put(conn);
 			return;
 		}
 
@@ -7566,6 +7580,7 @@ void l2cap_recv_acldata(struct hci_conn
 	}
 
 drop:
+	l2cap_conn_put(conn);
 	kfree_skb(skb);
 }
 
--