Re: [PATCH v2 1/3] dm-inlinecrypt: Add inline encryption support

[Christoph Hellwig <hch@infradead.org>]

On Thu, Oct 17, 2024 at 11:26:49PM -0400, Adrian Vovk wrote:
> Let's say I'm using LVM, so I've got a physical partition that stores a
> couple different virtual partitions. I can use dm-default-key both
> underneath the physical partition, and on top of some of the virtual
> partitions. In such a configuration, the virtual partitions with their own
> dm-default-key instance get encrypted with their own key and passed through
> the lower dm-default-key instance onto the hardware. Virtual partitions that
> lack their own dm-default-key are encrypted once by the lower dm-default-key
> instance. There's no filesystem involved here, and yet to avoid the cost of
> double-encryption we need the passthrough functionality of dm-default-key.
> This scenario is constrained entirely to the block layer.

So just run a target on each partition.

> Other usecases involve loopback devices. This is a real scenario of
> something we do in userspace. I have a loopback file, with a partition table
> inside where some partitions are encrypted and others are not. I would like
> to store this loopback file in a filesystem that sits on top of a dm-crypt
> protected partition. With the current capabilities of the kernel, I'd have
> to double-encrypt. But with dm-default-key, I could encrypt just once.

Which would completely break the security model of the underlying
file system, because it can't trust what you do in the loop device.

This is the prime example of why allowing higher layers to skip
encryption is a no-go.