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

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Message-ID: <20190522112449.GA6666@zhanggen-UX430UQ>
Date:   Wed, 22 May 2019 19:24:49 +0800
From:   Gen Zhang <blackgod016574@...il.com>
To:     Johan Hovold <johan@...nel.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty_io: Fix a missing-check bug in drivers/tty/tty_io.c

On Wed, May 22, 2019 at 01:19:49PM +0200, Johan Hovold wrote:
> On Wed, May 22, 2019 at 07:13:54PM +0800, Gen Zhang wrote:
> > On Wed, May 22, 2019 at 12:29:00PM +0200, Johan Hovold wrote:
> > > Where do you see that the kernel is dereferencing tty->dev without
> > > checking for NULL first? If you can find that, then that would indeed be
> > > a bug that needs fixing.
> > Thanks for your reply, Johan!
> > I examined the code but failed to find this situation.
> 
> Ok, so your claim in the commit message was incorrect:
> 
> 	And tty->dev is dereferenced in the following codes.
> 
> > Anyway, checking return value of tty_get_device() is theoritically
> > right. But tty->dev is never dereferenced, so checking is not needed.
> 
> No, sorry, it's not even theoretically correct. Our current code depends
> on tty->dev sometimes being NULL. Your patch would specifically break
> pseudo terminals.
Thanks for your comments. I have to be very proficient in this module
to know this. Of course I am not.
Anyway, appreciate your replies, Johan.
Thanks
Gen
> 
> > However, what if in later kernels tty->dev is dereferenced by some
> > codes? Is it better to apply this check for this reason?
> 
> So for the above reason, no.
> 
> Johan