[PATCH v4 4/5] tty: clear cdev pointer after cdev_add() failure

[Karl Mehltretter <kmehltretter@gmail.com>]

tty_cdev_add() drops the cdev reference when cdev_add() fails, but
leaves driver->cdevs[index] pointing to freed memory.
tty_unregister_device() later passes that stale pointer to cdev_del(),
causing a use-after-free.

Clear the slot after dropping the reference.

Fixes: c1a752ba2d6b ("tty: don't leak cdev in tty_cdev_add()")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
 drivers/tty/tty_io.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e742bf9d8631..4889076b975f 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -3167,8 +3167,10 @@ static int tty_cdev_add(struct tty_driver *driver, dev_t dev,
 	driver->cdevs[index]->ops = &tty_fops;
 	driver->cdevs[index]->owner = driver->owner;
 	err = cdev_add(driver->cdevs[index], dev, count);
-	if (err)
+	if (err) {
 		kobject_put(&driver->cdevs[index]->kobj);
+		driver->cdevs[index] = NULL;
+	}
 	return err;
 }
 
-- 
2.53.0