Re: overlayfs access checks on underlying layers

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
Date: Thu, 13 Dec 2018 13:54:56 -0500	[thread overview]
Message-ID: <20181213185456.GC4384@redhat.com> (raw)
In-Reply-To: <846eb23e-1188-9e45-ee0a-676d26cc715e@tycho.nsa.gov>

On Thu, Dec 13, 2018 at 11:12:31AM -0500, Stephen Smalley wrote:

[..]
> > > > Can you elaborate a bit more on how this is leaking data through overlay
> > > > mount. If it is, then why accessing file on lower is not equivalent of
> > > > leaking of data.
> > > 
> > > In the container use case, retaining the lower label on copy-up for a
> > > context-mounted overlay permits a process in the container to leak the
> > > container data out to host files not labeled with the container label and
> > > thus potentially accessible to other containers or host processes.
> > 
> > > The
> > > container process appears to just be writing to files labeled with the
> > > container label via the overlay, but the written data and/or metadata is
> > > directly accessible through the lower label, which is likely readable to
> > > all/many containers and host processes.
> > > 
> > > In the multi-level security (MLS) use case, an analogy would a situation
> > > where you have an unclassified lower dir with some content to be shared
> > > read-only across all levels, and an overlay is context-mounted at each level
> > > with a corresponding upper dir and work dir private to that level.  If a
> > > client process at secret performs a write to a file via the secret overlay,
> > > and if the written data is stored in a file in the upper dir that inherits
> > > the label from the lower file (unclassified), then the secret process can
> > > leak data to unclassified processes at will, violating the MLS policy.
> > 
> > For the case of devices, its already happening. One might change metadata
> > of a device (hence trigger copy up). Now all writes to upper device file
> > from secret process still go to same underlying device and are still
> > readable from lower device file.
> 
> This is an argument for not copying up device files IMHO, not for preserving
> the lower label on them.

How do we handle metadata change to device node (like timestamp, ownership
change) without copy up.

Vivek