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

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Date:   Sat, 8 Jun 2019 20:15:46 -0400 (EDT)
From:   Nicolas Pitre <nico@...xnic.net>
To:     Greg KH <gregkh@...uxfoundation.org>
cc:     Gen Zhang <blackgod016574@...il.com>, 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, 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