linux-kernel - Re: [PATCH 2/2] proc: Add /proc/<pid>/timerslack

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Date:	Thu, 14 Jul 2016 10:48:17 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Kees Cook <keescook@...omium.org>,
	John Stultz <john.stultz@...aro.org>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	lkml <linux-kernel@...r.kernel.org>,
	Oren Laadan <orenl@...lrox.com>,
	Ruchi Kandoi <kandoiruchi@...gle.com>,
	Rom Lemarchand <romlem@...roid.com>,
	Android Kernel Team <kernel-team@...roid.com>,
	Todd Kjos <tkjos@...gle.com>, Colin Cross <ccross@...roid.com>,
	Nick Kralevich <nnk@...gle.com>,
	Dmitry Shmidt <dimitrysh@...gle.com>,
	Elliott Hughes <enh@...gle.com>
Subject: Re: [PATCH 2/2] proc: Add /proc/<pid>/timerslack_ns interface

On 7/14/2016 10:45 AM, Kees Cook wrote:
> On Thu, Jul 14, 2016 at 9:09 AM, John Stultz <john.stultz@...aro.org> wrote:
>> On Thu, Jul 14, 2016 at 5:48 AM, Serge E. Hallyn <serge@...lyn.com> wrote:
>>> Quoting Kees Cook (keescook@...omium.org):
>>>> I think the original CAP_SYS_NICE should be fine. A malicious
>>>> CAP_SYS_NICE process can do plenty of insane things, I don't feel like
>>>> the timer slack adds to any realistic risks.
>>>
>>> Can someone give a detailed explanation of what you could do with
>>> the new timerslack feature and compare it to what you can do with
>>> sys_nice?
>>
>> Looking at the man page for CAP_SYS_NICE, it looks like such a task
>> can set a task as SCHED_FIFO, so they could fork some spinning
>> processes and set them all SCHED_FIFO 99, in effect delaying all other
>> tasks for an infinite amount of time.
>>
>> So one might argue setting large timerslack vlaues isn't that
>> different risk wise?
>
> Right -- you can hose a system with CAP_SYS_NICE already; I don't
> think timerslack realistically changes that.

fair enough

the worry of being able to time attack things is there already with the SCHED_FIFO
so... purist objection withdrawn in favor of the pragmatic