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Date:   Mon, 20 May 2019 23:26:20 -0400 (EDT)
From:   Nicolas Pitre <nico@fluxnic.net>
To:     Gen Zhang <blackgod016574@gmail.com>
cc:     linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] vt: Fix a missing-check bug in drivers/tty/vt/vt.c
In-Reply-To: <20190521030905.GB5263@zhanggen-UX430UQ>
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References: <20190521022940.GA4858@zhanggen-UX430UQ> <nycvar.YSQ.7.76.1905202242410.1558@knanqh.ubzr> <20190521030905.GB5263@zhanggen-UX430UQ>
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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