linux-ext4 - Re: [PATCH v5 00/42] idmapped mounts

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Date:   Sat, 16 Jan 2021 01:27:18 +0100
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Dave Chinner <david@...morbit.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org,
        John Johansen <john.johansen@...onical.com>,
        James Morris <jmorris@...ei.org>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Arnd Bergmann <arnd@...db.de>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
        Geoffrey Thomas <geofft@...reload.com>,
        Mrunal Patel <mpatel@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alban Crequy <alban@...volk.io>,
        Tycho Andersen <tycho@...ho.ws>,
        David Howells <dhowells@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        St?phane Graber <stgraber@...ntu.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Aleksa Sarai <cyphar@...har.com>,
        Lennart Poettering <lennart@...ttering.net>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, smbarber@...omium.org,
        Phil Estes <estesp@...il.com>, Serge Hallyn <serge@...lyn.com>,
        Kees Cook <keescook@...omium.org>,
        Todd Kjos <tkjos@...gle.com>, Paul Moore <paul@...l-moore.com>,
        Jonathan Corbet <corbet@....net>,
        containers@...ts.linux-foundation.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-integrity@...r.kernel.org, selinux@...r.kernel.org
Subject: Re: [PATCH v5 00/42] idmapped mounts

On Fri, Jan 15, 2021 at 12:51:20PM -0500, Theodore Ts'o wrote:
> On Fri, Jan 15, 2021 at 04:24:23PM +0000, Christoph Hellwig wrote:
> > 
> > That is what the capabilities are designed for and we already check
> > for them.
> 
> So perhaps I'm confused, but my understanding is that in the
> containers world, capabilities are a lot more complicated.  There is:
> 
> 1) The initial namespace capability set
> 
> 2) The container's user-namespace capability set
> 
> 3) The namespace in which the file system is mounted --- which is
>       "usually, but not necessarily the initial namespace" and
>       presumably could potentially not necessarily be the current
>       container's user name space, is namespaces can be hierarchically
>       arranged.
> 
> Is that correct?  If so, how does this patch set change things (if
> any), and and how does this interact with quota administration
> operations?

The cases you listed are correct. The patchset doesn't change them.
Simply put, the patchset doesn't alter capability checking in any way.

> 
> On a related note, ext4 specifies a "reserved user" or "reserved
> group" which can access the reserved blocks.  If we have a file system
> which is mounted in a namespace running a container which is running
> RHEL or SLES, and in that container, we have a file system mounted (so
> it was not mounted in the initial namespace), with id-mapping --- and
> then there is a further sub-container created with its own user
> sub-namespace further mapping uids/gids --- will the right thing
> happen?  For that matter, how *is* the "right thing" defined?

In short, nothing changes. Whatever happened before happens now.

Specifically s_resuid/s_resgid are superblock mount options and so never
change on a per-mount basis and thus also aren't affected by idmapped
mounts.

> 
> Sorry if this is a potentially stupid question, but I find user
> namespaces and id and capability mapping to be hopefully confusing for
> my tiny brain.  :-)

No, I really appreciate the questions. :) My brain can most likely
handle less. :)

Christian