Re: [PATCH] [request for inclusion] Realtime LSM

[Peter Williams <pwil3058@bigpond.net.au>]

Ingo Molnar wrote:
> * Peter Williams <pwil3058@bigpond.net.au> wrote:
> 
> 
>>I don't object to rlimits per se and I think that they are useful but
>>not as a sole solution to this problem.  Being able to give a task
>>preferential treatment is a permissions issue and should be solved as
>>one.
>>
>>Having RT cpu usage limits on tasks is a useful tool to have when
>>granting normal users the privilege of running tasks as RT tasks so
>>that you can limit the damage that they can do BUT the presence of a
>>limit on a task is not a very good criterion for granting that
>>privilege.
> 
> 
> i think you are talking about my rlimit patch (the 'RT CPU limit' patch)
> - but that one is not in discussion here.
> 
> what is being discussed currently is the other rlimit patch (from Chris
> Wright and Matt Mackall) which implements a simple rlimit ceiling for
> the RT (and nice) priorities a task can set. The rlimit defaults to 0,
> meaning no change in behavior by default. A value of 50 means RT
> priority levels 1-50 are allowed. A value of 100 means all 99 privilege
> levels from 1 to 99 are allowed. CAP_SYS_NICE is blanket permission.
> It's all pretty finegrained and and it's a quite straightforward
> extension of what we have today.

OK.  My misunderstanding.

But the patch you describe still seems a little loose to me in that it 
doesn't control both which users AND which programs they can run. 
Although I suppose that can be managed by suitable setting of file 
permissions?

Also I presume that root privileges are needed to set the rlimits which 
means that the program has to be setuid root or run from a setuid root 
wrapper.  In the first of these cases the program will be running for a 
(hopefully) short while with way more privilege than it needs.  This is 
why I'm attracted to mechanisms that allow programs to be given a subset 
of root's privileges and only for specified users.

I would be nice to have a solution to this particular problem that fits 
in with such a generalized "granular" privilege mechanism (when/if such 
a mechanism becomes available in the future) rather than a quirky fix 
that is specific to this problem and doesn't generalize well to similar 
problems when they arise in the future.  However, I agree with your 
opinion that granting CAP_SYS_NICE is dangerous without some limit on 
the priority levels is dangerous and think that a generalized "granular" 
privilege mechanism would need to include such restrictions.

> The patch does not attempt to do any
> "damage control" of abuse caused by RT tasks, and is hence much simpler
> than my patch or Con's SCHED_ISO patch. ("damage control" could be done
> from userspace anyway)

Yes.  In kernel "damage control" is an optional extra not a necessity 
with this solution.  Not so sure about with the RT LSB solution though.

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce