[PATCH v2 2/2] tty: Fix race against tty_open() in tty_register_device_attr()

[Max Staudt <max@enpas.org>]

Since the chardev is now created before the class device is registered,
an attempt to tty_[k]open() the chardev between these two steps will
lead to tty->dev being assigned NULL by alloc_tty_struct().

alloc_tty_struct() is called via tty_init_dev() when the tty is firstly
opened, and is entered with tty_mutex held, so let's lock the critical
section in tty_register_device_attr() with the same global mutex.
This guarantees that tty->dev can be assigned a sane value.

Fixes: 6a7e6f78c235 ("tty: close race between device register and open")
Signed-off-by: Max Staudt <max@enpas.org>
Cc: <stable@vger.kernel.org>
---
 drivers/tty/tty_io.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e922b84524d2..94768509e2d2 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -3258,6 +3258,8 @@ struct device *tty_register_device_attr(struct tty_driver *driver,
 	else
 		tty_line_name(driver, index, name);
 
+	mutex_lock(&tty_mutex);
+
 	if (!(driver->flags & TTY_DRIVER_DYNAMIC_ALLOC)) {
 		/*
 		 * Free any saved termios data so that the termios state is
@@ -3271,7 +3273,7 @@ struct device *tty_register_device_attr(struct tty_driver *driver,
 
 		retval = tty_cdev_add(driver, devt, index, 1);
 		if (retval)
-			return ERR_PTR(retval);
+			goto err_unlock;
 
 		cdev_added = true;
 	}
@@ -3294,6 +3296,8 @@ struct device *tty_register_device_attr(struct tty_driver *driver,
 	if (retval)
 		goto err_put;
 
+	mutex_unlock(&tty_mutex);
+
 	return dev;
 
 err_put:
@@ -3309,6 +3313,9 @@ struct device *tty_register_device_attr(struct tty_driver *driver,
 		driver->cdevs[index] = NULL;
 	}
 
+err_unlock:
+	mutex_unlock(&tty_mutex);
+
 	return ERR_PTR(retval);
 }
 EXPORT_SYMBOL_GPL(tty_register_device_attr);
-- 
2.39.5