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

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Date:   Thu, 1 Apr 2021 09:21:54 +0300
From:   Hassan Shahbazi <h.shahbazi.git@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
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,
        h.shahbazi.git@...il.com
Subject: Re: [PATCH] fix NULL pointer deference crash

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.


> Also your subject line can use some work, please make it reflect the
> driver subsystem you are looking at.
 
This was a mistake, I did not intend to change the subject. I will ensure
the next patch reflects the subsystem.

> thanks,
> 
> greg k-h

Best,
Hassan Shahbazi