linux-kernel - Re: [PATCH v2] gpu: vga: limit kmalloc'ed memory size

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Date:	Wed, 24 Nov 2010 21:33:00 +0300
From:	Vasiliy Kulikov <segoon@...nwall.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	kernel-janitors@...r.kernel.org, Dave Airlie <airlied@...hat.com>,
	Tiago Vignatti <tiago.vignatti@...ia.com>,
	Mike Travis <travis@....com>, "H. Peter Anvin" <hpa@...or.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] gpu: vga: limit kmalloc'ed memory size

Andrew,

On Tue, Nov 23, 2010 at 12:46 -0800, Andrew Morton wrote:
> What I'm suggesting is that we simply do
> 
> 	kbuf = strndup_user(buf, count);
> 
> and make strndup_user() do the right thing if `count' turned out to be
> crazy large.  THis way we don't have to sprinkle decisions about "crazy
> largeness" all over the kernel.
> 
> And the way in which I suggest that strndup_user() decides whether the
> length is too great is to try to kmalloc that amount of memory. 
> If it succeeds then fine, proceed.

I don't think that it is a good idea - the process would have an ability
to allocate too much system memory bypassing any limits.  Assuming that
the kernel would only double the memory is not right - even if the
process is limited in physical memory it may pass address of e.g. mapped file.


Also this specific driver is happy with very low limit of copied string.

> If it fails then return an error,
> probably ENOMEM.

It is already done in strndup_user().

> And that attempt to invoke kmalloc() shouldn't spew a
> warning.

It is not obvious for me to change strndup_user's behaviour, I'm not
familiar with this code.


-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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