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Subject: Re: overlayfs vs. fscrypt
From:   James Bottomley <James.Bottomley@HansenPartnership.com>
To:     Eric Biggers <ebiggers@kernel.org>
Cc:     Theodore Ts'o <tytso@mit.edu>, Amir Goldstein <amir73il@gmail.com>,
        Richard Weinberger <richard@nod.at>,
        Miklos Szeredi <miklos@szeredi.hu>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-fscrypt@vger.kernel.org,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Paul Lawrence <paullawrence@google.com>
Date:   Wed, 13 Mar 2019 09:13:52 -0700
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On Wed, 2019-03-13 at 08:51 -0700, Eric Biggers wrote:
> Hi James,
> 
> On Wed, Mar 13, 2019 at 08:36:34AM -0700, James Bottomley wrote:
> > On Wed, 2019-03-13 at 11:16 -0400, Theodore Ts'o wrote:
> > > So before we talk about how to make things work from a technical
> > > perspective, we should consider what the use case happens to be,
> > > and what are the security requirements.  *Why* are we trying to
> > > use the combination of overlayfs and fscrypt, and what are the
> > > security properties we are trying to provide to someone who is
> > > relying on this combination?
> > 
> > I can give one: encrypted containers:
> > 
> > https://github.com/opencontainers/image-spec/issues/747
> > 
> > The current proposal imagines that the key would be delivered to
> > the physical node and the physical node containerd would decrypt
> > all the layers before handing them off to to the kubelet.  However,
> > one could imagine a slightly more secure use case where the layers
> > were constructed as an encrypted filesystem tar and so the key
> > would go into the kernel and the layers would be constructed with
> > encryption in place using fscrypt.
> > 
> > Most of the desired security properties are in image at rest but
> > one can imagine that the running image wants some protection
> > against containment breaches by other tenants and using fscrypt
> > could provide that.
> > 
> 
> What do you mean by "containment breaches by other tenants"?  Note
> that while the key is added, fscrypt doesn't prevent access to the
> encrypted files.

You mean it's not multiuser safe?  Even if user a owns the key they add
user b can still see the decrypted contents?

>   fscrypt is orthogonal to OS-level access control (UNIX mode bits,
> ACLs, SELinux, etc.), which can and should be used alongside
> fscrypt.  fscrypt is a storage encryption mechanism, not an OS-level
> access control mechanism.

I was assuming in the multi-user case that if you don't own the keyring
you can't see the files. I suppose absent that it boils down to a
possible way to do the layering then as an fscrypt image rather than
tar then encrypt.

James