Re: [PATCH v3] vt: Fix a missing-check bug in con_init()

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Jun 09, 2019 at 12:01:38AM +0800, Gen Zhang wrote:
> On Tue, May 28, 2019 at 08:45:29AM +0800, Gen Zhang wrote:
> > In function con_init(), the pointer variable vc_cons[currcons].d, vc and
> > vc->vc_screenbuf is allocated by kzalloc(). And they are used in the 
> > following codes. However, kzalloc() returns NULL when fails, and null 
> > pointer dereference may happen. And it will cause the kernel to crash. 
> > Therefore, we should check the return value and handle the error.
> > 
> > Further, since the allcoation is in a loop, we should free all the 
> > allocated memory in a loop.
> > 
> > Signed-off-by: Gen Zhang <blackgod016574@gmail.com>
> > Reviewed-by: Nicolas Pitre <nico@fluxnic.net>
> > ---
> > diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> > index fdd12f8..d50f68f 100644
> > --- a/drivers/tty/vt/vt.c
> > +++ b/drivers/tty/vt/vt.c
> > @@ -3350,10 +3350,14 @@ static int __init con_init(void)
> >  
> >  	for (currcons = 0; currcons < MIN_NR_CONSOLES; currcons++) {
> >  		vc_cons[currcons].d = vc = kzalloc(sizeof(struct vc_data), GFP_NOWAIT);
> > +		if (!vc)
> > +			goto fail1;
> >  		INIT_WORK(&vc_cons[currcons].SAK_work, vc_SAK);
> >  		tty_port_init(&vc->port);
> >  		visual_init(vc, currcons, 1);
> >  		vc->vc_screenbuf = kzalloc(vc->vc_screenbuf_size, GFP_NOWAIT);
> > +		if (!vc->vc_screenbuf)
> > +			goto fail2;
> >  		vc_init(vc, vc->vc_rows, vc->vc_cols,
> >  			currcons || !vc->vc_sw->con_save_screen);
> >  	}
> > @@ -3375,6 +3379,16 @@ static int __init con_init(void)
> >  	register_console(&vt_console_driver);
> >  #endif
> >  	return 0;
> > +fail1:
> > +	while (currcons > 0) {
> > +		currcons--;
> > +		kfree(vc_cons[currcons].d->vc_screenbuf);
> > +fail2:
> > +		kfree(vc_cons[currcons].d);
> > +		vc_cons[currcons].d = NULL;
> > +	}

Wait, will that even work?  You can jump into the middle of a while
loop?

Ugh, that's beyond ugly.  And please provide "real" names for the
labels, "fail1" and "fail2" do not tell anything here.

Also, have you actually be able to trigger this?

thanks,

greg k-h