linux-kernel - Re: [PATCH v5 2/4] fuse: Support fuse filesystems outside of init_user

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Date:	Fri, 21 Nov 2014 10:44:41 -0600
From:	Seth Forshee <seth.forshee@...onical.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Serge H. Hallyn" <serge.hallyn@...ntu.com>,
	Andy Lutomirski <luto@...capital.net>,
	Michael j Theall <mtheall@...ibm.com>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v5 2/4] fuse: Support fuse filesystems outside of
 init_user_ns

On Wed, Nov 19, 2014 at 03:09:11PM +0100, Serge E. Hallyn wrote:
> Quoting Miklos Szeredi (miklos@...redi.hu):
> > On Wed, Nov 19, 2014 at 9:50 AM, Miklos Szeredi <miklos@...redi.hu> wrote:
> > > On Tue, Nov 18, 2014 at 4:21 PM, Seth Forshee
> > > <seth.forshee@...onical.com> wrote:
> > >>> I asked around a bit, and it turns out there are use cases for nested
> > >> containers (i.e. a container within a container) where the rootfs for
> > >> the outer container mounts a filesystem containing the rootfs for the
> > >> inner container. If that mount is nosuid then suid utilities like ping
> > >> aren't going to work in the inner container.
> > >>
> > >> So since there's a use case for suid in a userns mount and we have what
> > >> we belive are sufficient protections against using this as a vector to
> > >> get privileges outside the container, I'm planning to move ahead without
> > >> the MNT_NOSUID restriction. Any objections?
> > >
> > > In the general case how'd we prevent suid executable being tricked to
> > > do something it shouldn't do by unprivileged mounting into sensitive
> > > places (i.e. config files) inside the container?
> 
> The design of the namespaces would prevent that.  You cannot manipulate your
> mounts namespace unless you own it.  You cannot manipulate the mounts namespace
> for a task whose user namespace you do not own.  If you can, for instance,
> bind mount $HOME/shadow onto /etc/shadow, then you already own your user
> namespace and are root there, so any suid-root program which you mount through
> fuse will only subjegate your own namespace.  Any task which running in the
> parent user-ns (and therefore parent mount-ns) will not see your bind mount.
> 
> > > Allowing SUID looks like a slippery slope to me.  And there are plenty
> > > of solutions to the "ping" problem, AFAICS, that don't involve the
> > > suid bit.
> > 
> > ping isn't even suid on my system, it has security.capability xattr instead.
> 
> security.capability xattrs that will have the exact same concerns wrt
> confusion through bind mounts as suid.
> 
> > Please just get rid of SUID/SGID.  It's a legacy, it's a hack, not
> > worth the complexity and potential problems arising from that
> > complexity.
> 
> Oh boy, I don't know which side to sit on here :)  I'm all for replacing
> suid with some use of file capabilities, but realistically there are reasons
> why that hasn't happened more widely than it has - tar, package managers,
> cpio, nfs, etc.

Miklos: I we're all generally in agreement here that suid/sgid is not
the best solution, but as Serge points out we are unfortunately not yet
in a place where it can be completely dropped in favor of capabilities.
In light of this can I convince you to reconsider your position?

Thanks,
Seth
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