Re: RFC: what to do about fscrypt vs block device interaction

Christoph Hellwig
2022-07-22T16:03:55Z

On Fri, Jul 22, 2022 at 01:28:57AM -0700, Eric Biggers wrote:
> Yes, evicting the blk-crypto keys at unmount is the expected behavior.
> And it basically is the actual behavior as well, but as currently
> implemented there can be a slight delay. There are two reasons for the
> delay, both probably solvable.
>
> The first is that ->s_master_keys isn't released until __put_super().
> It probably should be moved earlier, maybe to generic_shutdown_super().

Yes, this does sound like a good idea.

> The second reason is that the keyrings subsystem is being used to keep
> track of the superblock's master keys (for several reasons, such as
> integrating with the key quotas), and a side effect of that we get the
> delay of the keyring's subsystem garbage collector before the destroy
> callbacks of the keys actually run. That delays the eviction of the
> blk-crypto keys.
>
> To avoid that, I think we could go through and evict all the
> blk_crypto_keys (i.e. call fscrypt_destroy_prepared_key() on the
> fscrypt_prepared_keys embedded in each fscrypt_master_key) during the
> unmount itself, separating it from the destruction of the key objects
> from the keyring subsystem's perspective. That could happen in the
> moved call to fscrypt_sb_free().

I'll give this a try.

What would be a good test suite or set of tests to make sure I don't
break fscrypt operation?