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

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Message-ID: <20190521040019.GD5263@zhanggen-UX430UQ>
Date:   Tue, 21 May 2019 12:00:19 +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 11:26:20PM -0400, Nicolas Pitre wrote:
> On Tue, 21 May 2019, Gen Zhang wrote:
> 
> > 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.
> 
> Yes, that is true today.  But if someone changes that in the future, how 
> will that person know that you relied on it to be 1 for not needing to 
> unwind the loop?
> 
> 
> Nicolas
Hi Nicolas,
Thanks for your explaination! And I got your point. And is this way 
proper?

err_vc_screenbuf:
        kfree(vc);
	for (currcons = 0; currcons < MIN_NR_CONSOLES; currcons++)
		vc_cons[currcons].d = NULL;
	return -ENOMEM;
err_vc:
	console_unlock();
	return -ENOMEM;

Thanks
Gen