serial: caif: hold tty->link reference in ldisc_open and ser_release

A reproducer triggers a KASAN slab-use-after-free in pty_write_room()
when caif_serial's TX path calls tty_write_room(). The faulting access
is on tty->link->port.

Hold an extra kref on tty->link for the lifetime of the caif_serial line
discipline: get it in ldisc_open() and drop it in ser_release(), and
also drop it on the ldisc_open() error path.

With this change applied, the reproducer no longer triggers the UAF in
my testing.

Link: https://gist.github.com/shuangpengbai/c898debad6bdf170a84be7e6b3d8707f
Link: https://lore.kernel.org/netdev/[e-mail]
Fixes: e31d5a05948e ("caif: tty's are kref objects so take a reference")
Signed-off-by: Shuangpeng Bai <[e-mail]>
Reviewed-by: Jiayuan Chen <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/caif/caif_serial.c

···
 			dev_close(ser->dev);
 			unregister_netdevice(ser->dev);
 			debugfs_deinit(ser);
+			tty_kref_put(tty->link);
 			tty_kref_put(tty);
 		}
 		rtnl_unlock();
···
 
 	ser = netdev_priv(dev);
 	ser->tty = tty_kref_get(tty);
+	tty_kref_get(tty->link);
 	ser->dev = dev;
 	debugfs_init(ser, tty);
 	tty->receive_room = 4096;
···
 	rtnl_lock();
 	result = register_netdevice(dev);
 	if (result) {
+		tty_kref_put(tty->link);
 		tty_kref_put(tty);
 		rtnl_unlock();
 		free_netdev(dev);