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.

This issue becomes reproducible on top of 308e7e4d0a84. Before that, the
reproducer typically hits another bug first, so this UAF is not
observable there.

Link: https://gist.github.com/shuangpengbai/c898debad6bdf170a84be7e6b3d8707f
Link: https://lore.kernel.org/all/20260215025141.1106576-1-shuangpeng.kernel@gmail.com/T/#maee804ef687b4442f18c74e8801f5cde421ab000 (local)
Fixes: 308e7e4d0a84 ("serial: caif: fix use-after-free in caif_serial ldisc_close()")
Signed-off-by: Shuangpeng Bai <redacted>
---
 drivers/net/caif/caif_serial.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/caif/caif_serial.c b/drivers/net/caif/caif_serial.c
index b90890030751..1873d8287bb9 100644
--- a/drivers/net/caif/caif_serial.c
+++ b/drivers/net/caif/caif_serial.c

@@ -297,6 +297,7 @@ static void ser_release(struct work_struct *work)
 			dev_close(ser->dev);
 			unregister_netdevice(ser->dev);
 			debugfs_deinit(ser);
+			tty_kref_put(tty->link);
 			tty_kref_put(tty);
 		}
 		rtnl_unlock();

@@ -331,6 +332,7 @@ static int ldisc_open(struct tty_struct *tty)
 
 	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;

@@ -339,6 +341,7 @@ static int ldisc_open(struct tty_struct *tty)
 	rtnl_lock();
 	result = register_netdevice(dev);
 	if (result) {
+		tty_kref_put(tty->link);
 		tty_kref_put(tty);
 		rtnl_unlock();
 		free_netdev(dev);

-- 
2.34.1