linux-kernel - Re: overlayfs vs. fscrypt

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Date:   Wed, 13 Mar 2019 08:51:45 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     James Bottomley <James.Bottomley@...senpartnership.com>
Cc:     Theodore Ts'o <tytso@....edu>, 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

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.
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.

- Eric