linux-kernel - Re: [PATCH] capabilities: add capability cgroup controller

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Date:	Sun, 26 Jun 2016 19:03:11 +0000
From:	Topi Miettinen <toiwoton@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Serge E. Hallyn" <serge@...lyn.com>
Cc:	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	luto@...nel.org, keescook@...omium.org,
	Jonathan Corbet <corbet@....net>,
	Li Zefan <lizefan@...wei.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	James Morris <james.l.morris@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	David Woodhouse <David.Woodhouse@...el.com>,
	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Petr Mladek <pmladek@...e.com>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	"open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
	"open list:CAPABILITIES" <linux-security-module@...r.kernel.org>
Subject: Re: [PATCH] capabilities: add capability cgroup controller

On 06/24/16 17:21, Eric W. Biederman wrote:
> "Serge E. Hallyn" <serge@...lyn.com> writes:
> 
>> Quoting Tejun Heo (tj@...nel.org):
>>> Hello,
>>>
>>> On Fri, Jun 24, 2016 at 10:59:16AM -0500, Serge E. Hallyn wrote:
>>>> Quoting Tejun Heo (tj@...nel.org):
>>>>> But isn't being recursive orthogonal to using cgroup?  Why not account
>>>>> usages recursively along the process hierarchy?  Capabilities don't
>>>>> have much to do with cgroup but everything with process hierarchy.
>>>>> That's how they're distributed and modified.  If monitoring their
>>>>> usages is necessary, it makes sense to do it in the same structure.
>>>>
>>>> That was my argument against using cgroups to enforce a new bounding
>>>> set.  For tracking though, the cgroup process tracking seems as applicable
>>>> to this as it does to systemd tracking of services.  It tracks a task and
>>>> the children it forks.
>>>
>>> Just monitoring is less jarring than implementing security enforcement
>>> via cgroup, but it is still jarring.  What's wrong with recursive
>>> process hierarchy monitoring which is in line with the whole facility
>>> is implemented anyway?
>>
>> As I think Topi pointed out, one shortcoming is that if there is a short-lived
>> child task, using its /proc/self/status is racy.  You might just miss that it
>> ever even existed, let alone that the "application" needed it.
>>
>> Another alternative we've both mentioned is to use systemtap.  That's not
>> as nice a solution as a cgroup, but then again this isn't really a common
>> case, so maybe it is precisely what a tracing infrastructure is meant for.
> 
> Hmm.
> 
> We have capability use wired up into auditing.  So we might be able to
> get away with just adding an appropriate audit message in
> commoncap.c:cap_capable that honors the audit flag and logs an audit
> message.  The hook in selinux already appears to do that.
> 
> Certainly audit sounds like the subsystem for this kind of work, as it's
> whole point in life is logging things, then something in userspace can
> just run over the audit longs and build a nice summary.

Even simpler would be to avoid the complexity of audit subsystem and
just printk() when a task starts using a capability first time (not on
further uses by same task). There are not that many capability bits nor
privileged processes, meaning not too many log entries. I know as this
was actually my first approach. But it's also far less user friendly
than just reading a summarized value which could be directly fed back to
configuration.

Logging/auditing approach also doesn't work well for other things I'd
like to present meaningful values for the user. For example, consider
RLIMIT_AS, where my goal is also to enable the users to be able to
configure this limit for a service. Should there be an audit message
whenever the address space limit grows (i.e. each mmap())? What about
when it shrinks? For RLIMIT_NOFILE we'd have to report each
open()/close()/dup()/socket()/etc. and track how many are opened at the
same time. I think it's better to store the fully cooked (meaningful to
user) value in kernel and present it only when asked.

-Topi

> 
> Eric
>