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Date: Wed, 17 Feb 2016 14:53:37 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: John Stultz <john.stultz@linaro.org>
Cc: Kees Cook <keescook@chromium.org>, Thomas Gleixner <tglx@linutronix.de>,
        Arjan van de Ven <arjan@linux.intel.com>,
        lkml <linux-kernel@vger.kernel.org>, Oren Laadan <orenl@cellrox.com>,
        Ruchi Kandoi <kandoiruchi@google.com>,
        Rom Lemarchand <romlem@android.com>,
        Android Kernel Team <kernel-team@android.com>
Subject: Re: [PATCH 2/2] proc: Add /proc/<pid>/timerslack_ns interface
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On Wed, 17 Feb 2016 14:29:29 -0800 John Stultz <john.stultz@linaro.org> wrote:

> On Wed, Feb 17, 2016 at 12:18 PM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
> > On Wed, 17 Feb 2016 12:09:08 -0800 Kees Cook <keescook@chromium.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?).

I can't immediately think of a problem with it.  Could we check
PTRACE_MODE_ATTACH_FSCREDS in open() to prevent bad guys from reading
our timerslack?