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Date: Wed, 17 Feb 2016 12:18:13 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Kees Cook <keescook@chromium.org>
Cc: John Stultz <john.stultz@linaro.org>, Thomas Gleixner <tglx@linutronix.de>,
        Arjan van de Ven <arjan@linux.intel.com>,
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        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 12:09:08 -0800 Kees Cook <keescook@chromium.org> wrote:

> On Wed, Feb 17, 2016 at 11:35 AM, Andrew Morton
> <akpm@linux-foundation.org> wrote:
> > On Tue, 16 Feb 2016 17:06:31 -0800 John Stultz <john.stultz@linaro.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.
> >
> > hm.  What the heck is PTRACE_MODE_ATTACH_FSCREDS and why was it chosen?
> 
> This says the writer needs to have ptrace "attach" level of access,
> and that it should be checked with fscreds, as is the standard for
> most /proc things like that.

The only place where PTRACE_MODE_ATTACH_FSCREDS is used in all of Linux
is /prc/pid/stack.  Makes me curious!

> > 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.