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

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Message-ID: <20201007220500.GA2544297@google.com>
Date:   Wed, 7 Oct 2020 22:05:00 +0000
From:   Satya Tangirala <satyat@...gle.com>
To:     Eric Biggers <ebiggers@...nel.org>
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 Wed, Oct 07, 2020 at 02:00:40PM -0700, Eric Biggers wrote:
> 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.
> 
Sure, I'll add that in the next version
> > 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".
>
Yes - I didn't get around to that in the first version, but I'll add
that too in the next version. I was going to go with the first approach
before I saw your comment - is there one method you'd recommend going
with over the other?
> - Eric