Re: [RFC PATCH 0/4] dm-default-key: target for filesystem metadata encryption

[Mikulas Patocka]

On Fri, 18 Oct 2024, Eric Biggers wrote:

> This series adds "metadata encryption" support to ext4 and f2fs via a
> new device-mapper target dm-default-key.  dm-default-key encrypts all
> data on a block device that isn't already encrypted by the filesystem.
> 
> Except for the passthrough support, dm-default-key is basically the same
> as the proposed dm-inlinecrypt which omits that feature
> (https://lore.kernel.org/dm-devel/20241016232748.134211-1-ebiggers@xxxxxxxxxx/).
> 
> I am sending this out for reference, as dm-default-key (which Android
> has been using for a while) hasn't previously been sent to the lists in
> full, and there has been interest in it.  However, my current impression
> is that this feature will need to be redesigned as a filesystem native
> feature in order to make it upstream.  If that is indeed the case, then
> IMO it would make sense to merge dm-inlinecrypt in the mean time instead
> (or add its functionality to dm-crypt) so that anyone who just wants
> "dm-crypt + inline encryption hardware" gets a solution for that.

I we merge dm-inlinecrypt, we can't remove it later because users will 
depend on it. I think it is not sensible to have two targets 
(dm-inlinecrypt and dm-default-key) that do almost the same thing.

I've got another idea - what about a new target "dm-metadata-switch" that 
will take two block devices as arguments and it will pass metadata bios to 
the first device and data bios to the second device - so that the logic 
to decide where the bio will go would be decoupled from the encryption. 
Then, you can put dm-crypt or dm-inlinecrypt underneath 
"dm-metadata-switch".

----------------------
|     filesystem     |
----------------------
          |
          V
----------------------
| dm-metadata-switch |
----------------------
      |           |
      V           |
------------      |
| dm-crypt |      |
------------      |
      |           |
      V           V
-------------------------
| physical block device |
-------------------------

Mikulas