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

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Message-ID: <20190608163007.GA2492@zhanggen-UX430UQ>
Date:   Sun, 9 Jun 2019 00:30:07 +0800
From:   Gen Zhang <blackgod016574@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     jslaby@...e.com, nico@...xnic.net, 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 06:22:19PM +0200, 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?
I felt like the same when I saw reviewer's advice to write in this way.
But I tested, and it did work.
> 
> Ugh, that's beyond ugly.  And please provide "real" names for the
> labels, "fail1" and "fail2" do not tell anything here.

Sure, I can revise that if needed.

Thanks
Gen
> 
> Also, have you actually be able to trigger this?
> 
> thanks,
> 
> greg k-h