[PATCH] fbcon: Fix user font detection test at fbcon_resize().

[Daniel Vetter]

On Wed, Sep 16, 2020 at 09:01:06AM +0900, Tetsuo Handa wrote:
> Greg, will you pick up this patch?
> 
> It seems that finding the real cause of [3] and actually fixing [3] will be difficult.
> Since I can't reproduce [3] locally, I will have to try flood of "#syz test" requests
> for debug printk() patches.

Grasping for straws, but have you retested with the scrollback code
removed already? That was full of fail and we outright deleted it:

50145474f6ef ("fbcon: remove soft scrollback code")

Cheers, Daniel

> 
> On 2020/09/11 7:57, Tetsuo Handa wrote:
> > syzbot is reporting OOB read at fbcon_resize() [1], for
> > commit 39b3cffb8cf31117 ("fbcon: prevent user font height or width change
> >  from causing potential out-of-bounds access") is by error using
> > registered_fb[con2fb_map[vc->vc_num]]->fbcon_par->p->userfont (which was
> > set to non-zero) instead of fb_display[vc->vc_num].userfont (which remains
> > zero for that display).
> > 
> > We could remove tricky userfont flag [2], for we can determine it by
> > comparing address of the font data and addresses of built-in font data.
> > But since that commit is failing to fix the original OOB read [3], this
> > patch keeps the change minimal in case we decide to revert altogether.
> > 
> > [1] https://syzkaller.appspot.com/bug?id=ebcbbb6576958a496500fee9cf7aa83ea00b5920
> > [2] https://syzkaller.appspot.com/text?tag=Patch&x=14030853900000
> > [3] https://syzkaller.appspot.com/bug?id=6fba8c186d97cf1011ab17660e633b1cc4e080c9
> > 
> > Reported-by: syzbot <syzbot+b38b1ef6edf0c74a8d97 at syzkaller.appspotmail.com>
> > Signed-off-by: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
> > Fixes: 39b3cffb8cf31117 ("fbcon: prevent user font height or width change from causing potential out-of-bounds access")
> > Cc: George Kennedy <george.kennedy at oracle.com>
> > ---
> >  drivers/video/fbdev/core/fbcon.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> > index 66167830fefd..dae7ae7f225a 100644
> > --- a/drivers/video/fbdev/core/fbcon.c
> > +++ b/drivers/video/fbdev/core/fbcon.c
> > @@ -2203,7 +2203,7 @@ static int fbcon_resize(struct vc_data *vc, unsigned int width,
> >  	struct fb_var_screeninfo var = info->var;
> >  	int x_diff, y_diff, virt_w, virt_h, virt_fw, virt_fh;
> >  
> > -	if (ops->p && ops->p->userfont && FNTSIZE(vc->vc_font.data)) {
> > +	if (p->userfont && FNTSIZE(vc->vc_font.data)) {
> >  		int size;
> >  		int pitch = PITCH(vc->vc_font.width);
> >  
> > 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch