On Wed, Mar 13, 2019 at 04:06:16PM +0000, Al Viro wrote: > 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? Isn't this handled by d_splice_alias() already, by moving the old dentry to the new name? - Eric
