Re: [PATCH 4/4] ubifs: Implement new mount option, fscrypt_key_required

From: Richard Weinberger <richard@nod.at>
To: Theodore Ts'o <tytso@mit.edu>
Cc: Eric Biggers <ebiggers@kernel.org>,
	linux-mtd@lists.infradead.org, linux-fscrypt@vger.kernel.org,
	jaegeuk@kernel.org, linux-unionfs@vger.kernel.org,
	miklos@szeredi.hu, amir73il@gmail.com,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	paullawrence@google.com, James.Bottomley@hansenpartnership.com
Subject: Re: [PATCH 4/4] ubifs: Implement new mount option, fscrypt_key_required
Date: Fri, 15 Mar 2019 14:59:26 +0100	[thread overview]
Message-ID: <11194150.36rPM0hNvo@blindfold> (raw)
In-Reply-To: <20190315135128.GL11334@mit.edu>

Ted,

Am Freitag, 15. März 2019, 14:51:28 CET schrieb Theodore Ts'o:
> On Fri, Mar 15, 2019 at 08:48:10AM +0100, Richard Weinberger wrote:
> > Ted,
> > 
> > Am Freitag, 15. März 2019, 00:07:02 CET schrieb Theodore Ts'o:
> > > Richard --- stepping back for a moment, in your use case, are you
> > > assuming that the encryption key is always going to be present while
> > > the system is running?
> > 
> > it is not a hard requirement, it is something what is common on embedded
> > systems that utilize UBIFS and fscrypt.
> > 
> > Well, fscrypt was chosen as UBIFS encryption backend because per-file encryption
> > with derived keys makes a lot of sense.
> > Also the implementation was not super hard, David and I weren't keen to reinvent
> > dm-crypt für UBI/MTD.
> > 
> > That said, I'm happy with fscrypt, it works well in production.
> 
> OK, but please note that fscrypt leaks i_size and timestamp
> information; dm-crypt doesn't.  An enterprising attacker could very
> easily be able to do something interesting with that information, so
> be sure you've thought through what the threat model for users of
> ubifs is going to be.

No need to worry, I'm fully aware of all this.

> If you need per-user keying, and you need to be able to mount the file
> system and access some of the files without having any keys, and if
> it's useful for an admin to be able to delete files without having the
> key, then fscrypt is a great fit.
> 
> You are proposing changes that (optionally) eliminate that last
> advantage of fscrypt.  So I just wanted to sanity check whether or not
> the other advantages are useful to you, and worth the security
> tradeoffs that are inherent in such a choice.  If it's worth it, then
> great.  But if it isn't, I'd much rather that you appropriately
> protect your users and your customers rather than be an additional
> user of fscrypt.  :-)

Like I said, this patch series is an RFC, the mount option was suggested
by Amir and Miklos, so I assumed showing some code is a good base for further
discussion.
For most of *my* use-cases it works but having general support for fscrypt+overlayfs
would be the ultimate goal.

Thanks,
//richard