Re: [RFC PATCH v2 0/3] ceph: add support for snapshot names encryption

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2022-03-17 at 20:31 +0800, Xiubo Li wrote:
> On 3/17/22 8:01 PM, Jeff Layton wrote:
> > On Thu, 2022-03-17 at 11:11 +0000, Luís Henriques wrote:
> > > Xiubo Li <xiubli@redhat.com> writes:
> > > 
> > > > On 3/17/22 6:01 PM, Jeff Layton wrote:
> > > > > I'm not sure we want to worry about .snap directories here since they
> > > > > aren't "real". IIRC, snaps are inherited from parents too, so you could
> > > > > do something like
> > > > > 
> > > > >       mkdir dir1
> > > > >       mkdir dir1/.snap/snap1
> > > > >       mkdir dir1/dir2
> > > > >       fscrypt encrypt dir1/dir2
> > > > > 
> > > > > There should be nothing to prevent encrypting dir2, but I'm pretty sure
> > > > > dir2/.snap will not be empty at that point.
> > > > If we don't take care of this. Then we don't know which snapshots should do
> > > > encrypt/dencrypt and which shouldn't when building the path in lookup and when
> > > > reading the snapdir ?
> > > In my patchset (which I plan to send a new revision later today, I think I
> > > still need to rebase it) this is handled by using the *real* snapshot
> > > parent inode.  If we're decrypting/encrypting a name for a snapshot that
> > > starts with a '_' character, we first find the parent inode for that
> > > snapshot and only do the operation if that parent is encrypted.
> > > 
> > > In the other email I suggested that we could prevent enabling encryption
> > > in a directory when there are snapshots above in the hierarchy.  But now
> > > that I think more about it, it won't solve any problem because you could
> > > create those snapshots later and then you would still need to handle these
> > > (non-encrypted) "_name_xxxx" snapshots anyway.
> > > 
> > Yeah, that sounds about right.
> > 
> > What happens if you don't have the snapshot parent's inode in cache?
> > That can happen if you (e.g.) are running NFS over ceph, or if you get
> > crafty with name_to_handle_at() and open_by_handle_at().
> > 
> > Do we have to do a LOOKUPINO in that case or does the trace contain that
> > info? If it doesn't then that could really suck in a big hierarchy if
> > there are a lot of different snapshot parent inodes to hunt down.
> > 
> > I think this is a case where the client just doesn't have complete
> > control over the dentry name. It may be better to just not encrypt them
> > if it's too ugly.
> > 
> > Another idea might be to just use the same parent inode (maybe the
> > root?) for all snapshot names. It's not as secure, but it's probably
> > better than nothing.
> 
> Does it allow to have different keys for the subdirs in the hierarchy ? 
>  From my test it doesn't.
> 

No. Once you set a key on directory you can't set another on a subtree
of it.

> If so we can always use the same oldest ancestor in the hierarchy, who 
> has encryption key, to encyrpt/decrypt all the .snap in the hierarchy, 
> then just need to lookup oldest ancestor inode only once.
> 

That's a possibility.
-- 
Jeff Layton <jlayton@kernel.org>