linux-kernel - Re: [RFC 01/18] capabilities: track actually used capabilities

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Date:	Mon, 13 Jun 2016 14:12:37 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Topi Miettinen <toiwoton@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Christoph Lameter <cl@...ux.com>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	"Richard W.M. Jones" <rjones@...hat.com>,
	Iago López Galeiras <iago@...ocode.com>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Andy Lutomirski <luto@...nel.org>, Jann Horn <jann@...jh.net>,
	"open list:FILESYSTEMS (VFS and infrastructure)" 
	<linux-fsdevel@...r.kernel.org>,
	"open list:CAPABILITIES" <linux-security-module@...r.kernel.org>
Subject: Re: [RFC 01/18] capabilities: track actually used capabilities

On Mon, Jun 13, 2016 at 1:45 PM, Topi Miettinen <toiwoton@...il.com> wrote:
> On 06/13/16 20:32, Andy Lutomirski wrote:
>> On Mon, Jun 13, 2016 at 12:44 PM, Topi Miettinen <toiwoton@...il.com> wrote:
>>> Track what capabilities are actually used and present the current
>>> situation in /proc/self/status.
>>
>> What for?
>

>
> Capabilities
> [RFC 01/18] capabilities: track actually used capabilities
>
> Currently, there is no way to know which capabilities are actually used.
> Even
> the source code is only implicit, in-depth knowledge of each capability must
> be used when analyzing a program to judge which capabilities the program
> will
> exercise."
>
> Should I perhaps cite some of this in the commit?

Yes, but you should also clarify what users are supposed to do with
this.  Given ambient capabilities, I suspect that you'll find that
your patch doesn't actually work very well.  For example, if you run a
shell script with ambient caps, then you won't notice caps used by
short-lived helper processes.

>
>>
>> What is the intended behavior on fork()?  Whatever the intended
>> behavior is, there should IMO be a selftest for it.
>>
>> --Andy
>>
>
> The capabilities could be tracked from three points of daemon
> initialization sequence onwards:
> fork()
> setpcap()
> exec()
>
> fork() case would be logical as the /proc entry is per task. But if you
> consider the tools to set the capabilities (for example systemd unit
> files), there can be between fork() and exec() further preparations
> which need more capabilities than the program itself needs.
>
> setpcap() is probably the real point after which we are interested if
> the capabilities are enough.
>
> The amount of setup between setpcap() and exec() is probably very low.

When I asked "what is the intended behavior on fork()?", I mean "what
should CapUsed be after fork()?".  The answer should be about four
words long and should have a test case.  There should maybe also be an
explanation of why the intended behavior is useful.

But, as I said above, I think that you may need to rethink this
entirely to make it useful.  You might need to do it per process tree
or per cgroup or something.

--Andy