Re: overlayfs vs. fscrypt

Theodore Ts'o
2019-03-13T15:16:51Z

On Wed, Mar 13, 2019 at 04:26:54PM +0200, Amir Goldstein wrote:
> AFAIK, this feature was born to tailor Android's file based encryption.
> https://source.android.com/security/encryption#file-based
> It is meant to protect data at rest and what happens when user enters
> the screen lock password IIRC, is that some service will get restarted.
> IOW, there should NOT be any processes in Android accessing the
> encrypted user data folders with and without the key simultaneously.
> Also, like OpenWRT, in Android the key does not get removed
> (until boot) AFAIK(?).

Actually, the original use was for ChromeOS, but the primary
assumption is that keying is per user (or profile), and that users are
mutually distrustful. So when Alice logs out of the system, her keys
will be invalidated and removed from the kernel. We can (and do) try
to flush cache entries via "echo 3 > /proc/sys/vm/drop_caches" on
logout. However, this does not guarantee that all dcache entries will
be removed --- a dcache entry can be pinned due to an open file, a
process's current working directory, a bind mount, etc.

The other issue is negative dentries; if you try open a file in an
encrypted file, the file system won't even *know* whether or not a
file exists, since the directory entries are encrypted; hence, there
may be some negative dentries that need to be invalidated.

So a fundamental assumption with fscrypt is that keys will be added
and removed, and that when this happens, dentries will need to be
invalidated. This is going to surprise overlayfs, so if overlayfs is
going to support fscrypt it *has* to be aware of the fact that this
can happen. It's not even clear what the proper security semantics
should be; *especially* if the upper and lower directories aren't
similarly protected using the same fscrypt encryption key. Suppose
the lower directory is encrypted, and the upper is not. Now on a copy
up operation, the previously encrypted file, which might contain
credit card numbers, medical records, or other things that would cause
a GDPR regulator to have a freak out attack, would *poof* become
decrypted.

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?

- Ted