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?
