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/20260301220525.1546355-1-shuangpeng.kernel@gmail.com
Fixes: e31d5a05948e ("caif: tty's are kref objects so take a reference")
Signed-off-by: Shuangpeng Bai <shuangpeng.kernel@gmail.com>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://patch.msgid.link/20260306034006.3395740-1-shuangpeng.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/drivers/net/caif/caif_serial.c b/drivers/net/caif/caif_serial.c
index b90890030751f1ec079d017d1c4caec8e13f3ba4..1873d8287bb9bed10d96eb54af8e618e30b0fc7b 100644 (file)
--- 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);