linux-kernel - Re: [PATCH] fix NULL pointer deference crash

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Message-ID: <YGVuJ1R02GdYdM5p@kroah.com>
Date:   Thu, 1 Apr 2021 08:54:31 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Hassan Shahbazi <h.shahbazi.git@...il.com>
Cc:     daniel.vetter@...ll.ch, jirislaby@...nel.org,
        yepeilin.cs@...il.com, linux-fbdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fix NULL pointer deference crash

On Thu, Apr 01, 2021 at 09:21:54AM +0300, Hassan Shahbazi wrote:
> On Wed, Mar 31, 2021 at 07:32:06PM +0200, Greg KH wrote:
> > On Wed, Mar 31, 2021 at 07:34:29PM +0300, Hassan Shahbazi wrote:
> > > The patch has fixed a NULL pointer deference crash in hiding the cursor. It 
> > > is verified by syzbot patch tester.
> > > 
> > > Reported by: syzbot
> > > https://syzkaller.appspot.com/bug?id=defb47bf56e1c14d5687280c7bb91ce7b608b94b
> > > 
> > > Signed-off-by: Hassan Shahbazi <h.shahbazi.git@...il.com>
> > > ---
> > >  drivers/video/fbdev/core/fbcon.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> > > index 44a5cd2f54cc..ee252d1c43c6 100644
> > > --- a/drivers/video/fbdev/core/fbcon.c
> > > +++ b/drivers/video/fbdev/core/fbcon.c
> > > @@ -1333,8 +1333,9 @@ static void fbcon_cursor(struct vc_data *vc, int mode)
> > >  
> > >  	ops->cursor_flash = (mode == CM_ERASE) ? 0 : 1;
> > >  
> > > -	ops->cursor(vc, info, mode, get_color(vc, info, c, 1),
> > > -		    get_color(vc, info, c, 0));
> > > +	if (ops && ops->cursor)
> > 
> > As ops obviously is not NULL here (you just used it on the line above),
> > why are you checking it again?
> 
> Yes, that's right. I will remove that check and will submit a new patch.
> 
> 
> > And what makes curser be NULL here?  How can that happen?
> 
> Honestly, I don't know. I reproduced the crash on my local, followed the
> stack trace, and then changed the line to avoid the crash. If you think this
> patch is not the best solution, I can drop it and investigate more to find
> the root cause.

Finding the root cause would be good to do here, so that we can
potentially fix that if it is needed.

thanks,

greg k-h