linux-kernel - Re: [PATCH 0/3] add support for metadata encryption to F2FS

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Date:   Wed, 7 Oct 2020 14:00:40 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Satya Tangirala <satyat@...gle.com>
Cc:     "Theodore Y . Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        linux-kernel@...r.kernel.org, linux-fscrypt@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [PATCH 0/3] add support for metadata encryption to F2FS

On Mon, Oct 05, 2020 at 07:36:03AM +0000, Satya Tangirala wrote:
> This patch series adds support for metadata encryption to F2FS using
> blk-crypto.

This patch series needs more explanation about what "metadata encryption" is,
why people will want to use it (as opposed to either not using it, or using
fscrypt + dm-crypt instead), and why this is the best implementation of it.

> Patch 2 introduces some functions to fscrypt that help filesystems perform
> metadata encryption. Any filesystem that wants to use metadata encryption
> can call fscrypt_setup_metadata_encryption() with the super_block of the
> filesystem, the encryption algorithm and the descriptor of the encryption
> key. The descriptor is looked up in the logon keyring of the current
> session with "fscrypt:" as the prefix of the descriptor.

I notice this is missing the step I suggested to include the metadata encryption
key in the HKDF application-specific info string when deriving subkeys from the
fscrypt master keys.

The same effect could also be achieved by adding an additional level to the key
hierarchy: each HKDF key would be derived from a fscrypt master key and the
metadata encryption key.

We need one of those, to guarantee that the file contents encryption is at least
as strong as the "metadata encryption".

- Eric