linux-kernel - Re: [PATCH v4] Introduce v3 namespaced file capabilities

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Message-ID: <20170613205312.gre2s6a3zsrjnyos@smitten>
Date:   Tue, 13 Jun 2017 14:53:12 -0600
From:   Tycho Andersen <tycho@...ker.com>
To:     Stefan Berger <stefanb@...ux.vnet.ibm.com>
Cc:     James Bottomley <James.Bottomley@...senPartnership.com>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        containers@...ts.linux-foundation.org,
        LKML <linux-kernel@...r.kernel.org>, xiaolong.ye@...el.com,
        "Eric W. Biederman" <ebiederm@...ssion.com>, lkp@...org
Subject: Re: [PATCH v4] Introduce v3 namespaced file capabilities

On Tue, Jun 13, 2017 at 04:49:03PM -0400, Stefan Berger wrote:
> On 06/13/2017 04:46 PM, Tycho Andersen wrote:
> > On Tue, Jun 13, 2017 at 10:45:02AM -0700, James Bottomley wrote:
> > > On Tue, 2017-06-13 at 11:14 -0600, Tycho Andersen via Containers wrote:
> > > > Hi Stefan,
> > > > 
> > > > On Tue, Jun 13, 2017 at 11:47:26AM -0400, Stefan Berger wrote:
> > > > > On 05/08/2017 02:11 PM, Serge E. Hallyn wrote:
> > > > > > Root in a non-initial user ns cannot be trusted to write a
> > > > > > traditional security.capability xattr.  If it were allowed to do
> > > > > > so, then any unprivileged user on the host could map his own uid
> > > > > > to root in a private namespace, write the xattr, and execute the
> > > > > > file with privilege on the host.
> > > > > > 
> > > > > > However supporting file capabilities in a user namespace is very
> > > > > > desirable.  Not doing so means that any programs designed to run
> > > > > > with limited privilege must continue to support other methods of
> > > > > > gaining and dropping privilege.  For instance a program installer
> > > > > > must detect whether file capabilities can be assigned, and assign
> > > > > > them if so but set setuid-root otherwise.  The program in turn
> > > > > > must know how to drop partial capabilities, and do so only if
> > > > > > setuid-root.
> > > > > Hi Serge,
> > > > > 
> > > > > 
> > > > >    I have been looking at patch below primarily to learn how we
> > > > > could apply a similar technique to security.ima and security.evm
> > > > > for a namespaced IMA. From the paragraphs above I thought that you
> > > > > solved the problem of a shared filesystem where one now can write
> > > > > different security.capability xattrs by effectively supporting for
> > > > > example security.capability[uid=1000] and
> > > > > security.capability[uid=2000] written into the filesystem. Each
> > > > > would then become visible as security.capability if the userns
> > > > > mapping is set appropriately.
> > > > One disadvantage of this approach is that whoever is setting up the
> > > > container would need to go touch the security.ima attribute for each
> > > > file in the contianer, which would slow down container creation time.
> > > > For capabilities this makes sense, because you might want the file to
> > > > have different capabilities in different namespaces, but for IMA,
> > > > since the file hash will be the same in every namespace,
> > > Actually, this isn't necessarily true: IMA may have the hash, you're
> > > right, but I suspect in most container use cases it will have the
> > > signature.  It's definitely a use case that the container will be using
> > > a different keyring from the host, so different signatures are surely
> > > possible for the same underlying image file.
> > > 
> > > One might imagine doing the above via overlays, because the new
> > > signature should override the old.
> > Yes, good point, thanks. Assuming the container and the host are using
> > the same keyring, we could design it in such a way that the container
> > engine doesn't need to touch every file on creation, which would be
> > very nice.
> 
> I don't think this will be the general case. The host may be Ubuntu, the
> guest could be Fedora and you'll have different keys. I don't think you
> would want the container keys on the host keyring.

I guess it depends: if your entire infrastructure needs to be signed
by your ops team, it would (presumably) all be the same ops key. If
you're running off the shelf stuff from the distros or from a vendor,
probably not, I agree.

Cheers,

Tycho