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

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Date:   Thu, 13 Dec 2018 15:26:08 -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 Thu, Dec 13, 2018 at 03:09:55PM -0500, Stephen Smalley wrote:
> On 12/13/18 1:54 PM, Vivek Goyal wrote:
> > 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.
> 
> Do we need to support such metadata changes to device nodes through an
> overlay mount?  Is that required for some legitimate purpose (and if so,
> what is the use case?)?  If not, just deny it up front.  Much simpler and no
> potential for a leak.

This will be overlay specific behavior and further from POSIX
like filesystem behavior. Don't know which workloads depend on changing
ownership of devices of changing metadata of devices.

Thanks
Vivek