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

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Date:   Fri, 14 Jul 2017 13:39:59 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Stefan Berger <stefanb@...ux.vnet.ibm.com>,
        Mimi Zohar <zohar@...ibm.com>
Cc:     containers@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        casey@...aufler-ca.com, Theodore Ts'o <tytso@....edu>, lkp@...org
Subject: Re: [PATCH v2] xattr: Enable security.capability in user namespaces

On Fri, 2017-07-14 at 16:03 -0400, Mimi Zohar wrote:
> On Fri, 2017-07-14 at 11:52 -0700, James Bottomley wrote:
> > 
> > On Fri, 2017-07-14 at 14:48 -0400, Mimi Zohar wrote:
> > > 
> > > The concern is with a shared filesystems.  In that case, for IMA
> > > it would make sense to support a native and a namespace xattr.
> > >  If due to xattr space limitations we have to limit the number of
> > > xattrs, then we should limit it to two - a native and a namespace
> > > version, with a "uid=" tag - first namespace gets permission to
> > > write the namespace xattr.  Again, like in the layered case, if
> > > the namespace xattr doesn't exist, fall back to using the native
> > > xattr.
> > 
> > Just on this point: if we're really concerned about the need on
> > shared filesystems to have multiple IMA signatures per file, might
> > it not make sense simply to support multiple signatures within the
> > security.ima xattr? The rules for writing signature updates within
> > user namespaces would be somewhat complex (say only able to replace
> > a signature for which you demonstrate you possess the key) but it
> > would lead to an implementation which would work for traditional
> > shared filesystems (like NFS) as well as containerised bind mounts.
> 
> Writing security.ima requires being root with CAP_SYS_ADMIN
> privileges.  I wouldn't want to give root within the namespace
> permission to over write or just extend the native security.ima.

but why?  That's partly the point of all of this: some security.
attributes can't be written by container root without some supervision
(the capability ones are the hugely problematic ones from this point of
view), but for some there's no reason they shouldn't be.  What would be
the reason that root in a container shouldn't be able to write the ima
xattr the same as host root could on its filesystem?

James