Re: [PATCH] n_tty: add null check for tty->link in packet mode

[Greg Kroah-Hartman]

On Sat, Mar 14, 2026 at 11:10:44PM +0100, Osama Abdelkader wrote:
> Add null check for tty->link before dereferencing in n_tty_read and
> n_tty_poll. When the pty master closes, tty->link can be NULL while
> the slave is still reading, causing a null pointer dereference.

How can that happen?

> Signed-off-by: Osama Abdelkader <osama.abdelkader@xxxxxxxxx>
> ---
>  drivers/tty/n_tty.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
> index e6a0f5b40d0a..dc04b87364f6 100644
> --- a/drivers/tty/n_tty.c
> +++ b/drivers/tty/n_tty.c
> @@ -2232,7 +2232,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file, u8 *kbuf,
>  	add_wait_queue(&tty->read_wait, &wait);
>  	while (nr) {
>  		/* First test for status change. */
> -		if (packet && tty->link->ctrl.pktstatus) {
> +		if (packet && tty->link && tty->link->ctrl.pktstatus) {
>  			u8 cs;
>  			if (kb != kbuf)
>  				break;
> @@ -2444,7 +2444,7 @@ static __poll_t n_tty_poll(struct tty_struct *tty, struct file *file,
>  		if (input_available_p(tty, 1))
>  			mask |= EPOLLIN | EPOLLRDNORM;
>  	}
> -	if (tty->ctrl.packet && tty->link->ctrl.pktstatus)
> +	if (tty->ctrl.packet && tty->link && tty->link->ctrl.pktstatus)

What happens if link changes right after you test it?  Where is the
lock?

And what changed to cause this to show up now?

thanks,

greg k-h