Re: [PATCH v3 09/25] fs: add is_userns_visible() helper

From: "Serge E. Hallyn" <serge@hallyn.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: "Christian Brauner" <christian.brauner@ubuntu.com>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	"Stéphane Graber" <stgraber@ubuntu.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	"Aleksa Sarai" <cyphar@cyphar.com>,
	"Jann Horn" <jannh@google.com>,
	smbarber@chromium.org,
	"Seth Forshee" <seth.forshee@canonical.com>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Alexey Dobriyan" <adobriyan@gmail.com>,
	"James Morris" <jmorris@namei.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Phil Estes" <estesp@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"Linux FS Devel" <linux-fsdevel@vger.kernel.org>,
	"Linux Containers" <containers@lists.linux-foundation.org>,
	"LSM List" <linux-security-module@vger.kernel.org>,
	"Linux API" <linux-api@vger.kernel.org>
Subject: Re: [PATCH v3 09/25] fs: add is_userns_visible() helper
Date: Thu, 20 Feb 2020 08:26:24 -0600	[thread overview]
Message-ID: <20200220142624.GA5249@mail.hallyn.com> (raw)
In-Reply-To: <CALCETrW-XPkBMs30vk+Aiv+jA5i7TjHOYCgz0Ud6d0geaYte=g@mail.gmail.com>

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@ubuntu.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.