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

Ilpo Järvinen
2025-06-02T10:31:55Z

On Wed, 28 May 2025, Max Staudt wrote:

> 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);

Use guard() so you don't need to change the returns and rollback path.

> +
> 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);
>

--
i.