From: ebiederm@xmission.com (Eric W. Biederman)
To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Stefan Berger <stefanb@linux.vnet.ibm.com>,
        Mimi Zohar <zohar@us.ibm.com>, containers@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org,
        casey@schaufler-ca.com, "Theodore Ts'o" <tytso@mit.edu>, lkp@01.org
References: <87y3rscz9j.fsf@xmission.com>
        <20170713164012.brj2flnkaaks2oci@thunk.org>
        <87k23cb6os.fsf@xmission.com>
        <847ccb2a-30c0-a94c-df6f-091c8901eaa0@linux.vnet.ibm.com>
        <87bmoo8bxb.fsf@xmission.com>
        <9a3010e5-ca2b-5e7a-656b-fcc14f7bec4e@linux.vnet.ibm.com>
        <87h8yf7szd.fsf@xmission.com>
        <65dbe654-0d99-03fa-c838-5a726b462826@linux.vnet.ibm.com>
        <20170714133437.GA16737@mail.hallyn.com>
        <596f808b-e21d-8296-5fef-23c1ce7ab778@linux.vnet.ibm.com>
        <20170714173556.GA19669@mail.hallyn.com>
        <1500058090.3583.28.camel@linux.vnet.ibm.com>
        <1500058362.2853.28.camel@HansenPartnership.com>
        <1500062619.3583.71.camel@linux.vnet.ibm.com>
        <1500064799.2853.36.camel@HansenPartnership.com>
Date: Fri, 14 Jul 2017 18:53:16 -0500
In-Reply-To: <1500064799.2853.36.camel@HansenPartnership.com> (James
        Bottomley's message of "Fri, 14 Jul 2017 13:39:59 -0700")
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Subject: Re: [PATCH v2] xattr: Enable security.capability in user namespaces
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James Bottomley <James.Bottomley@HansenPartnership.com> writes:

> 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?

Mimi said she ``native''.  It competely makes sense for the things that
the container doesn't ``own'' to not be allowed to be written/updated by
the container.

James you are making the case here for root in the container to write
to the ima and evm attributes that are for the container.

So I don't actually see any disagreement here except perhaps for
terminology.

Eric