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

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Date:   Tue, 13 Jun 2017 10:45:02 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Tycho Andersen <tycho@...ker.com>,
        Stefan Berger <stefanb@...ux.vnet.ibm.com>
Cc:     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, 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.

James

>  it would be nice to use a design that avoids touching each file on 
> new ns creation.