linux-kernel - Re: [PATCH 05/10] user namespace: clamp down users of cap

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Date:	Mon, 24 Oct 2011 22:03:14 -0500
From:	"Serge E. Hallyn" <serge.hallyn@...onical.com>
To:	"Andrew G. Morgan" <morgan@...nel.org>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org, ebiederm@...ssion.com,
	akpm@...ux-foundation.org, oleg@...hat.com, richard@....at,
	mikevs@...all.net, segoon@...nwall.com, gregkh@...e.de,
	eparis@...hat.com
Subject: Re: [PATCH 05/10] user namespace: clamp down users of cap_raised

Quoting Andrew G. Morgan (morgan@...nel.org):
> On Mon, Oct 24, 2011 at 10:28 AM, Serge E. Hallyn
> <serge.hallyn@...onical.com> wrote:
> > Quoting Andrew G. Morgan (morgan@...nel.org):
> >> Serge,
> >>
> >> It seems as if this whole thing is really idiomatic. How about?
> >>
> >> #define IN_ROOT_USER_NS_CAPABLE(cap)  \
> >>    ((current_user_ns() == &init_user_ns) && cap_raised(current_cap(), cap))
> >
> > My objection to this was that it seems to encourage others to use it :)  I'm
> > not sure we want that.  Also, IN_ROOT_USER_NS seems more generally useful.
> 
> What is driving the choice of when its appropriate? How can a

I'd like to say it's never appropriate.  The reason is that it bypasses
the whole security_ops->capable() sequence, so for instance SELinux is
kept in the dark.

> developer determine this? If you make it hard, presumably folk won't
> do it by default, but will that create a burdon on others to go round
> patching things like this up?
> 
> > But if I'm the only one who feels this way I'll go ahead and do it...
> 
> I'm more of a optimize for a human to read the source code (ie. debug
> a problem) kind of person. If IN_ROOT_USER_NS is useful, you could
> always define IN_ROOT_USER_NS_CAPABLE in terms of IN_ROOT_USER_NS &&

My other objection is that, in contrast to IN_ROOT_USER_NS(), which is
very clear, IN_ROOT_USER_NS_CAPABLE() is not as helpful.  I'm sure a
better name is out there somewhere, though.

> ... and provide both.
> 
> I guess I'm unclear, however, when you want developers to use one or
> the other variant of the basic capable() functionality. Since I'm not
> clear, I'm suspecting this is a fragile situation.

I think only security code (LSMs) should be using cap_raised directly.
Everything else should go through the capable()/has_capability() family
of functions.  Which, incidentally, have been (or are about to be) made
less of a mess and thus less fragile by Eric Paris' patchset starting at
http://www.spinics.net/linux/fedora/linux-security-module/msg11896.html

-serge
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