linux-kernel - Re: overlayfs access checks on underlying layers

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Message-ID: <fe78aaed-5fb1-3c51-1330-39de46ae39c5@android.com>
Date:   Mon, 4 Mar 2019 09:01:08 -0800
From:   Mark Salyzyn <salyzyn@...roid.com>
To:     Vivek Goyal <vgoyal@...hat.com>, Miklos Szeredi <miklos@...redi.hu>
Cc:     Stephen Smalley <sds@...ho.nsa.gov>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Paul Moore <paul@...l-moore.com>, linux-kernel@...r.kernel.org,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, selinux@...r.kernel.org,
        Daniel J Walsh <dwalsh@...hat.com>
Subject: Re: overlayfs access checks on underlying layers

On 11/29/2018 05:49 AM, Vivek Goyal wrote:
> So will override_creds=off solve the NFS issue also where all access will
> happen with the creds of task now? Though it will stil require more
> priviliges in task for other operations in overlay to succeed.

NFS problems seems to have ended the discussion, too many stakeholders? 
too many outstanding questions?

Do we accept the limitations of the override_creds patch as is, and then 
have the folks more familiar with the NFS scenario(s) build on it?

[TL;DR]

After looking at all this discussion, it feels like a larger audited 
rewrite of the security model is in order and override_creds=off may be 
a disservice (although expediently deals with Android's needs) to a 
correct general solution. I admit I have little idea where to go from 
here for a general solution.

As far as I see it, the model of creator && caller credentials is a 
problem for any non-overlapping (MAC) privilege models. This patch 
allows one to drop any creator privilege escalation, re-introducing the 
"caller" to the lower layers.

As such I would expect a better model is to _always_ check the caller 
credentials again in the lower layers, and only check the creator 
credentials, some without caller credentials, for some special cases? 
Change an && to an || for some of the checks? What are those special 
cases? I must admit _none_ of those special cases need attention in the 
Android usage models though making it difficult for me to do the fight 
thing for the associated stakeholders.

The lower privileged application access to the directory cache inherited 
by other callers troubles me (not for Android, but in general) and feels 
troublesome (flush out the directory cache? how to tag the privileges 
associated with the current instance of the directory cache?). Some 
operations (eg: delete a file for incoming, create mknod in upperdir) 
are special cases requiring the checking of caller credentaisl to 
function (not a problem for Android as the caller that deletes a file 
just so happens to have the necessary privileges).

Also, mount namespaces (in upper, lower, etc), how will they affect this 
all, is there a need for more attention to this as well?

-- Mark