Re: [PATCH 05/10] user namespace: clamp down users of cap_raised

["Serge E. Hallyn" <serge.hallyn@canonical.com>]

Quoting Eric Paris (eparis@redhat.com):
> On Mon, 2011-10-24 at 22:03 -0500, Serge E. Hallyn wrote:
> > Quoting Andrew G. Morgan (morgan@kernel.org):
> > > On Mon, Oct 24, 2011 at 10:28 AM, Serge E. Hallyn
> > > <serge.hallyn@canonical.com> wrote:
> > > > Quoting Andrew G. Morgan (morgan@kernel.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?

Probably, but I didn't really want to tackle that - and risk
regressions - just then.  The most important issue for now is
that anyone who can create a new user namespace can pass this
test, so I wanted to fix that first.  (Creating a new user ns
takes quite a bit of privilege, but of course userspace loves
to bypass that with setuid)

Maybe, since everyone is calling me on it, I'm wrong and we should
consider it after all :)

-serge