linux-kernel - Re: [PATCH] mm: Enable MAP_UNINITIALIZED for archs with mmu

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Date:	Fri, 24 Feb 2012 11:11:12 -0800
From:	Arun Sharma <asharma@...com>
To:	Balbir Singh <bsingharora@...il.com>
CC:	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] mm: Enable MAP_UNINITIALIZED for archs with mmu

On 2/24/12 6:51 AM, Balbir Singh wrote:
> On Fri, Feb 24, 2012 at 8:17 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com>  wrote:
>>> They don't have access to each other's VMAs, but if "accidentally" one
>>> of them comes across an uninitialized page with data from another task,
>>> it's not a violation of the security model.
>
> Can you expand more on the single address space model?

I haven't thought this through yet. But I know that just adding

&& (cgroup_task_count() == 1)

to page_needs_clearing() is not going to do it. We'll have to design a 
new mechanism (cgroup_mm_count_all()?) and make sure that it doesn't 
race with fork() and inadvertently expose pages from the new address 
space to the existing one.

A uid based approach such as the one implemented by Davide Libenzi

http://thread.gmane.org/gmane.linux.kernel/548928
http://thread.gmane.org/gmane.linux.kernel/548926

would probably apply the optimization to more use cases - but 
conceptually a bit more complex. If we go with this more relaxed 
approach, we'll have to design a race-free cgroup_uid_count() based 
mechanism.

  -Arun

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