linux-kernel - Re: [PATCH v3 09/25] fs: add is_userns

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Date:   Thu, 20 Feb 2020 08:26:24 -0600
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Stéphane Graber <stgraber@...ntu.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Aleksa Sarai <cyphar@...har.com>, Jann Horn <jannh@...gle.com>,
        smbarber@...omium.org, Seth Forshee <seth.forshee@...onical.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Phil Estes <estesp@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v3 09/25] fs: add is_userns_visible() helper

On Wed, Feb 19, 2020 at 09:18:51AM -0800, Andy Lutomirski wrote:
> On Wed, Feb 19, 2020 at 4:06 AM Christian Brauner
> <christian.brauner@...ntu.com> wrote:
> >
> > On Tue, Feb 18, 2020 at 08:42:33PM -0600, Serge Hallyn wrote:
> > > On Tue, Feb 18, 2020 at 03:33:55PM +0100, Christian Brauner wrote:
> > > > Introduce a helper which makes it possible to detect fileystems whose
> > > > superblock is visible in multiple user namespace. This currently only
> > > > means proc and sys. Such filesystems usually have special semantics so their
> > > > behavior will not be changed with the introduction of fsid mappings.
> > >
> > > Hi,
> > >
> > > I'm afraid I've got a bit of a hangup about the terminology here.  I
> > > *think* what you mean is that SB_I_USERNS_VISIBLE is an fs whose uids are
> > > always translated per the id mappings, not fsid mappings.  But when I see
> >
> > Correct!
> >
> > > the name it seems to imply that !SB_I_USERNS_VISIBLE filesystems can't
> > > be seen by other namespaces at all.
> > >
> > > Am I right in my first interpretation?  If so, can we talk about the
> > > naming?
> >
> > Yep, your first interpretation is right. What about: wants_idmaps()
> 
> Maybe fsidmap_exempt()?

Yeah, and maybe SB_USERNS_FSID_EXEMPT ?

> I still haven't convinced myself that any of the above is actually
> correct behavior, especially when people do things like creating
> setuid binaries.

The only place that would be a problem is if the child userns has an
fsidmapping from X to 0 in the parent userns, right?  Yeah I'm sure
many people would ignore all advice to the contrary and do this anyway,
but I would try hard to suggest that people use an intermediary userns
for storing filesystems for the "docker share" case.  So the host fsid
range would start at say 200000.  So a setuid binary would just be
setuid-200000.