Re: [PATCH] tty_io: Fix a missing-check bug in drivers/tty/tty_io.c

[Gen Zhang <blackgod016574@gmail.com>]

On Wed, May 22, 2019 at 06:25:36AM +0200, Jiri Slaby wrote:
> On 22. 05. 19, 3:40, Gen Zhang wrote:
> > In alloc_tty_struct(), tty->dev is assigned by tty_get_device(). And it
> > calls class_find_device(). And class_find_device() may return NULL.
> > And tty->dev is dereferenced in the following codes. When 
> > tty_get_device() returns NULL, dereferencing this tty->dev null pointer
> > may cause the kernel go wrong. Thus we should check tty->dev.
> > Further, if tty_get_device() returns NULL, we should free tty and 
> > return NULL.
> > 
> > Signed-off-by: Gen Zhang <blackgod016574@gmail.com>
> > 
> > ---
> > diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> > index 033ac7e..1444b59 100644
> > --- a/drivers/tty/tty_io.c
> > +++ b/drivers/tty/tty_io.c
> > @@ -3008,6 +3008,10 @@ struct tty_struct *alloc_tty_struct(struct tty_driver *driver, int idx)
> >  	tty->index = idx;
> >  	tty_line_name(driver, idx, tty->name);
> >  	tty->dev = tty_get_device(tty);
> > +	if (!tty->dev) {
> > +		kfree(tty);
> > +		return NULL;
> > +	}
> 
> This is incorrect, you introduced an ldisc reference leak.
Thanks for your reply, Jiri!
And what do you mean by an ldisc reference leak? I did't get the reason
of introducing it.
> 
> And can this happen at all?
I think tty_get_device() may happen to return NULL. Because it calls 
class_find_device() and there's a chance class_find_device() returns
NULL.
Thanks
Gen
> 
> thanks,
> -- 
> js
> suse labs