Re: [PATCH 2/2] serdev: use tty_port_tty_get() in ttyport_write_buf() to prevent UAF

[Greg Kroah-Hartman]

On Fri, Jul 31, 2026 at 10:24:02AM +0200, Jiri Slaby wrote:
> On 31. 07. 26, 10:06, Greg Kroah-Hartman wrote:
> > From: Joshua Rogers <linux@xxxxxxxxx>
> > 
> > ttyport_write_buf() snapshots serport->tty as a raw pointer without
> > taking a reference, while ttyport_close() can concurrently release the
> > tty via tty_release_struct(), leading to a use-after-free. Use
> > tty_port_tty_get() to obtain a reference-counted tty pointer, matching
> > the pattern already used by ttyport_write_wakeup().
> > 
> > Assisted-by: AISLE:Snapshot
> > Cc: stable <stable@xxxxxxxxxx>
> > Signed-off-by: Joshua Rogers <linux@xxxxxxxxx>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > ---
> >   drivers/tty/serdev/serdev-ttyport.c | 12 ++++++++++--
> >   1 file changed, 10 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/tty/serdev/serdev-ttyport.c b/drivers/tty/serdev/serdev-ttyport.c
> > index 48ce5b3f8308..4d37c8130dd7 100644
> > --- a/drivers/tty/serdev/serdev-ttyport.c
> > +++ b/drivers/tty/serdev/serdev-ttyport.c
> > @@ -85,13 +85,21 @@ static const struct tty_port_client_operations client_ops = {
> >   static ssize_t ttyport_write_buf(struct serdev_controller *ctrl, const u8 *data, size_t len)
> >   {
> >   	struct serport *serport = serdev_controller_get_drvdata(ctrl);
> > -	struct tty_struct *tty = serport->tty;
> > +	struct tty_struct *tty;
> > +	ssize_t ret;
> >   	if (!test_bit(SERPORT_ACTIVE, &serport->flags))
> >   		return 0;
> > +	tty = tty_port_tty_get(serport->port);
> > +	if (!tty)
> > +		return 0;
> > +
> >   	set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
> > -	return tty->ops->write(serport->tty, data, len);
> > +	ret = tty->ops->write(tty, data, len);
> > +	tty_kref_put(tty);
> 
> Here,
> 
>   scope_guard(tty_port_tty, serport->port) {
>     struct tty_struct *tty = scoped_tty();
> 
>     set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
>     return tty->ops->write(serport->tty, data, len);
>   }
> 
>   return 0;
> 
> appears to be cleaner.

Yes, much cleaner.  LLMs really don't know about "modern" kernel coding
styles (they always use min_t() and don't like guard() code).

I'll respin this and do a new version in a few days, thanks.

greg k-h