Re: [PATCH] capabilities: add capability cgroup controller

[Topi Miettinen <toiwoton-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On 06/24/16 17:21, Eric W. Biederman wrote:
> "Serge E. Hallyn" <serge-A9i7LUbDfNHQT0dZR+AlfA@public.gmane.org> writes:
> 
>> Quoting Tejun Heo (tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org):
>>> Hello,
>>>
>>> On Fri, Jun 24, 2016 at 10:59:16AM -0500, Serge E. Hallyn wrote:
>>>> Quoting Tejun Heo (tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.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
>