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

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Date:	Thu, 14 Jul 2016 12:49:18 -0500
From:	"Serge E. Hallyn" <serge@...lyn.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.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>,
	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

Quoting Kees Cook (keescook@...omium.org):
> 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.

Thanks - so it seems to me if we go with CAP_SYS_NICE we are giving
those who can already hose the system another vector to doing so.  But
if we require CAP_SYS_PTRACE then we are giving those who can newly hose
the system also the ability to subvert any task.  It sounds like
CAP_SYS_NICE is the winner.

Kees, you said adding a capability is hard - can you expound on that?