linux-kernel - Re: overlayfs vs. fscrypt

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Date:   Wed, 13 Mar 2019 10:45:04 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Amir Goldstein <amir73il@...il.com>,
        Richard Weinberger <richard@....at>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-fscrypt@...r.kernel.org,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Paul Lawrence <paullawrence@...gle.com>
Subject: Re: overlayfs vs. fscrypt

On Wed, 2019-03-13 at 12:44 -0400, Theodore Ts'o wrote:
> 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 kind of containment breaches?  If they can break root, it's all
> over no matter what sort of encryption you are using.

With me it's always unprivileged containers inside a user_ns, so
containment breach means non-root.  I hope eventually this will be the
norm for the container industry as well.

>   If they can't break root, then the OS's user-id based access
> control checks (or SELinux checks if you are using SELinux) will
> still protect you.

Well, that's what one would think about the recent runc exploit as
well.  The thing I was looking to do was reduce the chances that
unencrypted data would be lying around to be discovered.  I suppose the
potentially biggest problem is leaking the image after it's decrypted
by admin means like a badly configured backup, but unencryped data is
potentially discoverable by breakouts as well.

James