linux-kernel - Re: [PATCH v2] vt: Fix a missing-check bug in drivers/tty/vt/vt.c

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Date:   Tue, 21 May 2019 11:09:05 +0800
From:   Gen Zhang <blackgod016574@...il.com>
To:     Nicolas Pitre <nico@...xnic.net>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vt: Fix a missing-check bug in drivers/tty/vt/vt.c

On Mon, May 20, 2019 at 10:55:40PM -0400, Nicolas Pitre wrote:
> On Tue, 21 May 2019, Gen Zhang wrote:
> 
> > In function con_init(), the pointer variable vc_cons[currcons].d, vc and
> > vc->vc_screenbuf is allocated a memory space via kzalloc(). And they are
> > used in the following codes.
> > However, when there is a memory allocation error, kzalloc() can fail.
> > Thus null pointer (vc_cons[currcons].d, vc and vc->vc_screenbuf)
> > dereference may happen. And it will cause the kernel to crash. Therefore,
> > we should check return value and handle the error.
> > Further,the loop condition MIN_NR_CONSOLES is defined as 1 in
> > include/uapi/linux/vt.h. So there is no need to unwind the loop.
> 
> But what if someone changes that define? It won't be obvious that some 
> code did rely on it to be defined to 1.
I re-examine the source code. MIN_NR_CONSOLES is only defined once and
no other changes to it.

> 
> > Signed-off-by: Gen Zhang <blackgod016574@...il.com>
> > 
> > ---
> > diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> > index fdd12f8..b756609 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_cons[currcons].d || !vc)
> 
> Both vc_cons[currcons].d and vc are assigned the same value on the 
> previous line. You don't have to test them both.
Thanks for this comment!

> 
> > +			goto err_vc;
> >  		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 err_vc_screenbuf;
> >  		vc_init(vc, vc->vc_rows, vc->vc_cols,
> >  			currcons || !vc->vc_sw->con_save_screen);
> >  	}
> > @@ -3375,6 +3379,14 @@ static int __init con_init(void)
> >  	register_console(&vt_console_driver);
> >  #endif
> >  	return 0;
> > +err_vc:
> > +	console_unlock();
> > +	return -ENOMEM;
> > +err_vc_screenbuf:
> > +	console_unlock();
> > +	kfree(vc);
> > +	vc_cons[currcons].d = NULL;
> > +	return -ENOMEM;
> 
> As soon as you release the lock, another thread could come along and 
> start using the memory pointed by vc_cons[currcons].d you're about to 
> free here. This is unlikely for an initcall, but still.
> 
> You should consider this ordering instead:
> 
> err_vc_screenbuf:
> 	kfree(vc);
> 	vc_cons[currcons].d = NULL;
> err_vc:
> 	console_unlock();
> 	return -ENOMEM;
> 
> 
Thanks for your patient reply, Nicolas!
I will work on this patch and resubmit it.
Thanks
Gen
> >  }
> >  console_initcall(con_init);
> >  
> >  ---
> >