linux-kernel - Re: [PATCH v2] xattr: Enable security.capability in user namespaces

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Message-ID: <20170714133437.GA16737@mail.hallyn.com>
Date:   Fri, 14 Jul 2017 08:34:37 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     Stefan Berger <stefanb@...ux.vnet.ibm.com>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        Theodore Ts'o <tytso@....edu>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        containers@...ts.linux-foundation.org, lkp@...org,
        linux-kernel@...r.kernel.org, zohar@...ux.vnet.ibm.com,
        tycho@...ker.com, James.Bottomley@...senPartnership.com,
        vgoyal@...hat.com, christian.brauner@...lbox.org,
        amir73il@...il.com, linux-security-module@...r.kernel.org,
        casey@...aufler-ca.com
Subject: Re: [PATCH v2] xattr: Enable security.capability in user namespaces

Quoting Stefan Berger (stefanb@...ux.vnet.ibm.com):
> On 07/13/2017 08:38 PM, Eric W. Biederman wrote:
> >Stefan Berger <stefanb@...ux.vnet.ibm.com> writes:
> >
> >>On 07/13/2017 01:49 PM, Eric W. Biederman wrote:
> >>
> >>>My big question right now is can you implement Ted's suggested
> >>>restriction.  Only one security.foo or secuirty.foo@... attribute ?
> >>We need to raw-list the xattrs and do the check before writing them. I am fairly sure this can be done.
> >>
> >>So now you want to allow security.foo and one security.foo@...=<> or just a single one security.foo(@[[:print:]]*)?
> >>
> >The latter.
> 
> That case would prevent a container user from overriding the xattr
> on the host. Is that what we want? For limiting the number of xattrs

Not really.  If the file is owned by a uid mapped into the container,
then the container root can chown the file which will clear the file
capability, after which he can set a new one.  If the file is not
owned by a uid mapped into the container, then container root could
not set a filecap anyway.

-serge