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

[Osama Abdelkader <osama.abdelkader@gmail.com>]

On Sun, Mar 15, 2026 at 07:57:53AM +0100, Greg Kroah-Hartman wrote:
> 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@gmail.com>
> > ---
> >  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

Hi Greg,

I was just thinking about null dereferencing possiblity in tty->link->ctrl.pktstatus. But, you are right
It’s reasonable to drop this patch and reopen it only if I get a solid reproducer or bug report.

BR,
Osama