Re: overlayfs vs. fscrypt

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Mar 13, 2019 at 11:16:33AM -0400, Theodore Ts'o wrote:
> 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.

Just to make sure - you do realize that ban on multiple dentries refering
to the same directory inode is *NOT* conditional upon those dentries being
hashed, right?