Date: Thu, 3 Feb 2005 22:36:45 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Paul Davis <paul@linuxaudiosystems.com>
Cc: Peter Williams <pwil3058@bigpond.net.au>,
       "Bill Huey (hui)" <bhuey@lnxw.com>, "Jack O'Quin" <joq@io.com>,
       Nick Piggin <nickpiggin@yahoo.com.au>, Con Kolivas <kernel@kolivas.org>,
       linux <linux-kernel@vger.kernel.org>, rlrevell@joe-job.com,
       CK Kernel <ck@vds.kolivas.org>, utz <utz@s2y4n2c.de>,
       Andrew Morton <akpm@osdl.org>, alexn@dsv.su.se,
       Rui Nuno Capela <rncbc@rncbc.org>, Chris Wright <chrisw@osdl.org>,
       Arjan van de Ven <arjanv@redhat.com>
Subject: Re: [patch, 2.6.11-rc2] sched: RLIMIT_RT_CPU_RATIO feature
Message-ID: <20050203213645.GB27255@elte.hu>
References: <42014C10.60407@bigpond.net.au> <200502022303.j12N3nZa002055@localhost.localdomain>
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* Paul Davis <paul@linuxaudiosystems.com> wrote:

> Just a reminder: setuid root is precisely what we are attempting to
> avoid. 
> 
> >If you have the source code for the programs then they could be modified 
> >to drop the root euid after they've changed policy.  Or even do the 
> 
> This is insufficient, since they need to be able to drop RT scheduling
> and then reacquire it again later.

i believe RT-LSM provides a way to solve this cleanly: you can make your
audio app setguid-audio (note: NOT setuid), and make the audio group
have CAP_SYS_NICE-equivalent privilege via the RT-LSM, and then you
could have a finegrained per-app way of enabling SCHED_FIFO scheduling,
without giving _users_ the blanket permission to SCHED_FIFO. Ok?

this way if jackd (or a client) gets run by _any_ user, all jackd
processes will be part of the audio group and can do SCHED_FIFO - but
users are not automatically trusted with SCHED_FIFO.

you are currently using RT-LSM to enable a user to do SCHED_FIFO, right? 
I think the above mechanism is more secure and more finegrained than
that.

	Ingo
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