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 00:30:38 -0400 (EDT)
From:   Nicolas Pitre <nico@...xnic.net>
To:     Gen Zhang <blackgod016574@...il.com>
cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] vt: Fix a missing-check bug in drivers/tty/vt/vt.c

On Tue, 21 May 2019, Gen Zhang wrote:

> 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?

Not quite.

> 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;

Now imagine that MIN_NR_CONSOLES is defined to 10 instead of 1.

What happens with allocated memory if the err_vc condition is met on the 
5th loop?

If err_vc_screenbuf condition is encountered on the 5th loop (curcons = 
4), what is the value of vc_cons[4].d? Isn't it the same as vc that you 
just freed?


Nicolas