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

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Date:	Wed, 13 Jul 2016 16:47:15 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	Oren Laadan <orenl@...lrox.com>,
	Ruchi Kandoi <kandoiruchi@...gle.com>,
	Rom Lemarchand <romlem@...roid.com>,
	Kees Cook <keescook@...omium.org>,
	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 Tue, Feb 16, 2016 at 5:06 PM, John Stultz <john.stultz@...aro.org> wrote:
> This patch provides a proc/PID/timerslack_ns interface which
> exposes a task's timerslack value in nanoseconds and allows it
> to be changed.
>
> This allows power/performance management software to set timer
> slack for other threads according to its policy for the thread
> (such as when the thread is designated foreground vs. background
> activity)
>
> If the value written is non-zero, slack is set to that value.
> Otherwise sets it to the default for the thread.
>
> This interface checks that the calling task has permissions to
> to use PTRACE_MODE_ATTACH_FSCREDS on the target task, so that we
> can ensure arbitrary apps do not change the timer slack for other
> apps.

Sigh.

So I wanted to pull this thread up again, because when I originally
proposed upstreaming the PR_SET_TIMERSLACK_PID feature from the AOSP
common.git tree, the first objection from Arjan was that it only
required CAP_SYS_NICE:
   http://lkml.iu.edu/hypermail/linux/kernel/1506.3/01491.html

And reasonably, setting timerslack to very large values does have the
potential to effect applications much further then what a task could
do previously with CAP_SYS_NICE.

CAP_SYS_PTRACE was suggested instead, as that allows applications to
manipulate other tasks more drastically.

(At the time, I checked with some of the Android developers, and got
no objection to changing to use this capability.)

However, after submitting the changes to Android required to support
the upstreamed /proc/<tid>/timerslack_ns interface, I've gotten some
objections with adding CAP_SYS_PTRACE to the system_server, as this
would allow the system_server to be able to inspect and modify memory
on any task in the system. This gives the system_server privileged to
effect applications much further then what it could do previously.

So I worry I'm a bit stuck here. For general systems, CAP_SYS_NICE is
too low a level of privilege  to set a tasks timerslack, but
apparently CAP_SYS_PTRACE is too high a privilege for Android's
system_server to require just to set a tasks timerslack value.

So I wanted to ask again if we might consider backing this down to
CAP_SYS_NICE, or if we can instead introduce a new CAP_SYS_TIMERSLACK
or something to provide the needed in-between capability level.

Thoughts?

thanks
-john