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

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Date:   Tue, 4 Dec 2018 11:17:47 -0500
From:   Vivek Goyal <vgoyal@...hat.com>
To:     Stephen Smalley <sds@...ho.nsa.gov>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Mark Salyzyn <salyzyn@...roid.com>,
        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 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@...hat.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