linux-kernel - Re: [patch 3/3] [PATCH] prctl: Add PR_SET_MM codes to set up mm

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Date:	Mon, 12 Dec 2011 17:24:56 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrew Vagin <avagin@...nvz.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Kees Cook <keescook@...omium.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [patch 3/3] [PATCH] prctl: Add PR_SET_MM codes to set up mm_struct
 entires v3

(12/12/11 4:58 PM), Cyrill Gorcunov wrote:
> On Mon, Dec 12, 2011 at 04:49:38PM -0500, KOSAKI Motohiro wrote:
>> Hi
>>
>>> When we restore a task we need to set up text, data and data
>>> heap sizes from userspace to the values a task had at
>>> checkpoint time. This patch adds auxilary prctl codes for that.
>>>
>>> While most of them have a statistical nature (their values
>>> are involved into calculation of /proc/<pid>/statm output)
>>> the start_brk and brk values are used to compute an allowed
>>> size of program data segment expansion. Which means an arbitrary
>>> changes of this values might be dangerous operation. So to restrict
>>> access the following requirements applied to prctl calls:
>>>
>>>    - The process has to have CAP_SYS_ADMIN capability granted.
>>
>> This is very dangerous feature and useless from regular admins.
>
> Except brk() call I don't see where it might be extremelly
> dangerous at moment but indeed it might become very dangerous
> once code grows. Still if evil minded person got CAP_SYS_ADMIN
> these prctls are least thing one should carry about.

I'm sorry, I misunderstood your code. Your code only allow to change
their own process attribute. So, it's enough harmless. Please ignore
my last mail.



>> Moreover, CAP_SYS_ADMIN has a pretty overweight meanings and
>> we can't disable it on practical. So, I have a question. Why
>> don't you make new capability for checkpoint?
>>
>
> It's not a problem to introduce CAP_CHECKPOINT_RESTORE, but
> would it be accepted? I mean, are we fine with new capability
> introduction? If yes -- I'll add new one and rebase the patch.




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