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

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Date:	Tue, 25 Oct 2011 13:33:29 -0400
From:	Eric Paris <eparis@...hat.com>
To:	"Serge E. Hallyn" <serge.hallyn@...onical.com>
Cc:	"Andrew G. Morgan" <morgan@...nel.org>,
	"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
Subject: Re: [PATCH 05/10] user namespace: clamp down users of cap_raised

On Mon, 2011-10-24 at 22:03 -0500, Serge E. Hallyn wrote:
> 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

(sorry out all last week)

I was going to ask why we have these user at all.  Is there a reason
they are bypassing the LSM and not setting PF_PRIV?  Is the best
solution to just bring them back into the capable fold?

-Eric

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