Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 4 Dec 2018 11:17:47 -0500
From:   Vivek Goyal <vgoyal@redhat.com>
To:     Stephen Smalley <sds@tycho.nsa.gov>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        Ondrej Mosnacek <omosnace@redhat.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Mark Salyzyn <salyzyn@android.com>,
        Paul Moore <paul@paul-moore.com>, linux-kernel@vger.kernel.org,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-fsdevel@vger.kernel.org, selinux@vger.kernel.org,
        Daniel J Walsh <dwalsh@redhat.com>
Subject: Re: overlayfs access checks on underlying layers
Message-ID: <20181204161747.GC16818@redhat.com>
References: <CAJfpegv6y56k1-Tewu-Pu3x1W75LL6qYB6Hb6=n+2BJoNfEigA@mail.gmail.com>
 <c993dba4-5129-7f04-2724-a82a1f1cafa3@tycho.nsa.gov>
 <f84bbe0b-f4c1-50e0-8c84-a6589154b3ae@tycho.nsa.gov>
 <CAJfpegtBDTMbmKd6eDxRmtSJjGN6CnpGK_QPNSsxjkOoeu=1pQ@mail.gmail.com>
 <4c20a261-5ce1-f0a2-8d40-c6032a023216@tycho.nsa.gov>
 <20181204151549.GA21509@redhat.com>
 <20181204152248.GB21509@redhat.com>
 <CAJfpegtFVvq286HAorGLg6ywMsfs51KqaQwFkcuV7Dtu8TLaLg@mail.gmail.com>
 <20181204154243.GA16818@redhat.com>
 <665ec6f3-f16d-681f-30d5-eface14c9808@tycho.nsa.gov>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <665ec6f3-f16d-681f-30d5-eface14c9808@tycho.nsa.gov>
User-Agent: Mutt/1.9.1 (2017-09-22)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Dec 04, 2018 at 11:05:46AM -0500, Stephen Smalley wrote:
> On 12/4/18 10:42 AM, Vivek Goyal wrote:
> > On Tue, Dec 04, 2018 at 04:31:09PM +0100, Miklos Szeredi wrote:
> > > On Tue, Dec 4, 2018 at 4:22 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> > > 
> > > > Having said that, this still create little anomaly when mknod to client
> > > > is not allowed on context label. So a device file, which is on lower
> > > > and client can not open it for read/write on host, it can now be opened
> > > > for read/write because mounter will allow access. So why it is different
> > > > that regular copy up. Well, in regular copy up, we created a copy of
> > > > the original object and allowed writing to that object (cp --preserve=all)
> > > > model. But in case of device file, writes will go to same original
> > > > object. (And not a separate copy).
> > > 
> > > That's true.
> > > 
> > > In that sense copy up of special file should result in upper having
> > > the same label as of lower, right?
> > 
> > I guess that might be reasonable (if this behavior is a concern). So even
> > after copy up, client will not be able to read/write a device if it was
> > not allowed on lower.
> > 
> > Stephen, what do you think about retaining label of lower for device
> > files during copy up. What about socket/fifo.
> 
> We don't check client task access to the upper inode label, only to the
> overlay, right?  So the client is still free to access the device through
> the overlay even if we preserve the lower inode label on the upper inode?
> What do we gain?

That's only with latest code and Miklos said he will revert it for 4.20.

IOW, I am assuming that we will continue to check access to a file
on upper in the context of mounter. Otherwise, client will be able to access
files on upper/ which even mounter can't access.

Thanks
Vivek