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

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Date:	Wed, 17 Feb 2016 14:29:29 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	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>
Subject: Re: [PATCH 2/2] proc: Add /proc/<pid>/timerslack_ns interface

On Wed, Feb 17, 2016 at 12:18 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Wed, 17 Feb 2016 12:09:08 -0800 Kees Cook <keescook@...omium.org> wrote:
>> On Wed, Feb 17, 2016 at 11:35 AM, Andrew Morton
>> > The procfs file's permissions are 0644, yes?  So a process's
>> > timer_slack is world-readable?  hm.
>>
>> This should be 600, IMO.
>
> Sounds safer.

So I've gone ahead and addressed this and the other feedback you had.
But this bit made me realize that I may have missed a key aspect to
the interface that Android needs.

In particular, the whole point here is to allow a controlling task to
modify other tasks' timerslack to limit background tasks' power usage
(and to modify them back to normal when the background tasks become
foreground tasks). Note that on android different tasks run as
different users.

Currently, the controlling process has minimally elevated privileges
(CAP_SYS_NICE). The initial review suggested those privileges should
be higher (PTRACE_MODE_ATTACH), which I've implemented.  However, I'm
realizing that by moving to the proc interface, the filesystem
permissions here put yet another barrier in the way.

While the 600 permissions makes initial sense, it does limit these
controlling tasks with extra privileges (though not root) from
modifying the timerslack, since they cannot open the file to begin
with.

So.... Does world writable (plus the PTRACE_MODE_ATTACH_FSCREDS check)
make more sense here?  Or is there a better way for a system to tweak
the default permissions for procfs entries? (And if so, does that
render the PTRACE_MODE_ATTACH... check unnecessary?).

Apologies. I'm fighting a head-cold, so I'm not feeling particularly sharp here.

thanks
-john