Re: [PATCH v3 0/7] User namespace mount updates

From: Al Viro <viro@ZenIV.linux.org.uk>
To: Seth Forshee <seth.forshee@canonical.com>
Cc: Austin S Hemmelgarn <ahferroin7@gmail.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	linux-bcache@vger.kernel.org, dm-devel@redhat.com,
	linux-raid@vger.kernel.org, linux-mtd@lists.infradead.org,
	linux-fsdevel@vger.kernel.org,
	linux-security-module@vger.kernel.org, selinux@tycho.nsa.gov,
	Serge Hallyn <serge.hallyn@canonical.com>,
	Andy Lutomirski <luto@amacapital.net>,
	linux-kernel@vger.kernel.org, Theodore Ts'o <tytso@mit.edu>
Subject: Re: [PATCH v3 0/7] User namespace mount updates
Date: Wed, 18 Nov 2015 14:58:18 +0000	[thread overview]
Message-ID: <20151118145818.GC22011@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20151118142238.GB134139@ubuntu-hedt>

On Wed, Nov 18, 2015 at 08:22:38AM -0600, Seth Forshee wrote:

> But it still requires the admin set it up that way, no? And aren't
> privileges required to set up those devices in the first place?
> 
> I'm not saying that it wouldn't be a good idea to lock down the backing
> stores for those types of devices too, just that it isn't something that
> a regular user could exploit without an admin doing something to
> facilitate it.

Sigh...  If it boils down to "all admins within all containers must be
trusted not to try and break out" (along with "roothole in any container
escalates to kernel-mode code execution on host"), then what the fuck
is the *point* of bothering with containers, userns, etc. in the first
place?  If your model is basically "you want isolation, just use kvm",
fine, but where's the place for userns in all that?

And if you are talking about the _host_ admin, then WTF not have him just
mount what's needed as part of setup and to hell with mounting those
inside the container?

Look at that from the hosting company POV - they are offering a bunch of
virtual machines on one physical system.  And you want the admins on those
virtual machines independent from the host admin.  Fine, but then you
really need to keep them unable to screw each other or gain kernel-mode
execution on the host.

Again, what's the point of all that?  I assumed the model where containers
do, you know, contain what's in them, regardless of trust.  You guys seem
to assume something different and I really wonder what it _is_...