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

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Date:   Thu, 13 Jul 2017 12:14:11 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Theodore Ts'o <tytso@....edu>
Cc:     Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        "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

Theodore Ts'o <tytso@....edu> writes:

> On Thu, Jul 13, 2017 at 07:11:36AM -0500, Eric W. Biederman wrote:
>> The concise summary:
>> 
>> Today we have the xattr security.capable that holds a set of
>> capabilities that an application gains when executed.  AKA setuid root exec
>> without actually being setuid root.
>> 
>> User namespaces have the concept of capabilities that are not global but
>> are limited to their user namespace.  We do not currently have
>> filesystem support for this concept.
>
> So correct me if I am wrong; in general, there will only be one
> variant of the form:
>
>    security.foo@...=15000
>
> It's not like there will be:
>
>    security.foo@...=1000
>    security.foo@...=2000
>
> Except.... if you have an Distribution root directory which is shared
> by many containers, you would need to put the xattrs in the overlay
> inodes.  Worse, each time you launch a new container, with a new
> subuid allocation, you will have to iterate over all files with
> capabilities and do a copy-up operations on the xattrs in overlayfs.
> So that's actually a bit of a disaster.
>
> So for distribution overlays, you will need to do things a different
> way, which is to map the distro subdirectory so you know that the
> capability with the global uid 0 should be used for the container
> "root" uid, right?
>
> So this hack of using security.foo@...=1000 is *only* useful when the
> subcontainer root wants to create the privileged executable.  You
> still have to do things the other way.
>
> So can we make perhaps the assertion that *either*:
>
>    security.foo
>
> exists, *or*
>
>    security.foo@...=BAR
>
> exists, but never both?  And there BAR is exclusive to only one
> instances?
>
> Otherwise, I suspect that the architecture is going to turn around and
> bite us in the *ss eventually, because someone will want to do
> something crazy and the solution will not be scalable.

Yep.  That is what it looks like from here.

Which is why I asked the question about scalability of the xattr
implementations.  It looks like trying to accomodate the general
case just gets us in trouble, and sets unrealistic expectations.

Which strongly suggests that Serge's previous version that
just reved the format of security.capable so that a uid field could
be added is likely to be the better approach.

I want to see what Serge and Stefan have to say but the case looks
pretty clear cut at the moment.

Eric