linux-kernel - Re: [PATCH] drivers: tty: vt: vt.c: fix NULL dereference crash

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Date:   Fri, 12 Mar 2021 12:07:01 +0300
From:   Fatih Yildirim <yildirim.fatih@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Hassan Shahbazi <h.shahbazi.git@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers: tty: vt: vt.c: fix NULL dereference crash

On Fri, 2021-03-12 at 09:33 +0100, Greg KH wrote:
> On Sun, Mar 07, 2021 at 12:56:43PM +0200, Hassan Shahbazi wrote:
> > Fix a NULL deference crash on hiding the cursor.
> > 
> > Reported by: syzbot
> > https://syzkaller.appspot.com/bug?id=defb47bf56e1c14d5687280c7bb91ce7b608b94b
> > 
> > Signed-off-by: Hassan Shahbazi <h.shahbazi.git@...il.com>
> > ---
> >  drivers/tty/vt/vt.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> > index 284b07224c55..8c3e83c81341 100644
> > --- a/drivers/tty/vt/vt.c
> > +++ b/drivers/tty/vt/vt.c
> > @@ -904,7 +904,9 @@ static void hide_cursor(struct vc_data *vc)
> >  	if (vc_is_sel(vc))
> >  		clear_selection();
> >  
> > -	vc->vc_sw->con_cursor(vc, CM_ERASE);
> > +	if (vc->vc_sw)
> > +		vc->vc_sw->con_cursor(vc, CM_ERASE);
> > +
> >  	hide_softcursor(vc);
> >  }
> >  
> > -- 
> > 2.26.2
> > 
> 
> Are you sure this actually fixes the problem?  How did you test
> it?  Did
> syzbot test this?
> 
> I had a few reports of this patch _not_ solving the problem, so
> getting
> confirmation of this would be good.
> 
> thanks,
> 
> greg k-h

Hi,

I've tested the patch on upstream using the .config and the C
reproducer reported by syzbot. It seems that the patch doesn't fix the
issue.

Maybe you can send a patch test request to syzbot to validate the
patch.

Thanks,
Fatih