linux-kernel - Re: [PATCH v3] vt: Fix a missing-check bug in con

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Date:   Sun, 9 Jun 2019 23:45:21 -0700
From:   Gen Zhang <blackgod016574@...il.com>
To:     Nicolas Pitre <nico@...xnic.net>
Cc:     Greg KH <gregkh@...uxfoundation.org>, jslaby@...e.com,
        kilobyte@...band.pl, textshell@...uujin.de, mpatocka@...hat.com,
        daniel.vetter@...ll.ch, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] vt: Fix a missing-check bug in con_init()

On Sat, Jun 08, 2019 at 08:15:46PM -0400, Nicolas Pitre wrote:
> On Sat, 8 Jun 2019, Greg KH wrote:
> 
> > 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@...il.com>
> > > > Reviewed-by: Nicolas Pitre <nico@...xnic.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?
> 
> Absolutely.
> 
> > Ugh, that's beyond ugly.
> 
> That was me who suggested to do it like that. To me, this is nicer than 
> the proposed alternatives. For an error path that is rather unlikely to 
> happen, I think this is a very concise and eleguant way to do it.
> 
> > And please provide "real" names for the
> > labels, "fail1" and "fail2" do not tell anything here.
> 
> That I agree with.
> 
> 
> Nicolas
Thanks for your comments. Then am I supposed to revise the patch and
send a v4 version?

Thanks
Gen