For the most part, I agree with you, especially about the strategy being backward - and file encryption being a viable more-easily-implementable direction.

However, you are doing yourself a disservice to compare btrfs' features as a "re-implementation" of existing tools. The existing tools cannot do what btrfs' devs want to implement. See below inline.


On 09/16/2016 03:12 AM, Dave Chinner wrote:
On Tue, Sep 13, 2016 at 09:39:46PM +0800, Anand Jain wrote:

This patchset adds btrfs encryption support.

The main objective of this series is to have bugs fixed and stability.
I have verified with fstests to confirm that there is no regression.

A design write-up is coming next, however here below is the quick example
on the cli usage. Please try out, let me know if I have missed something.

Yup, that best practices say "do not roll your own encryption
infrastructure".

100% agreed


This is just my 2c worth - take it or leave it, don't other flaming.
Keep in mind that I'm not picking on btrfs here - I asked similar
hard questions about the proposed f2fs encryption implementation.
That was a "copy and snowflake" version of the ext4 encryption code -
they made changes and now we have generic code and common
functionality between ext4 and f2fs.

Also would like to mention that a review from the security experts is due,
which is important and I believe those review comments can be accommodated
without major changes from here.

That's a fairly significant red flag to me - security reviews need
to be done at the design phase against specific threat models -
security review is not a code/implementation review...

Also agreed. This is a bit backward.


The ext4 developers got this right by publishing threat models and
design docs, which got quite a lot of review and feedback before
code was published for review.

https://docs.google.com/document/d/1ft26lUQyuSpiu6VleP70_npaWdRfXFoNnB8JYnykNTg/edit#heading=h.qmnirp22ipew

[small reorder of comments]

As of now these patch set supports encryption on per subvolume, as
managing properties on per subvolume is a kind of core to btrfs, which is
easier for data center solution-ing, seamlessly persistent and easy to
manage.

We've got dmcrypt for this sort of transparent "device level"
encryption. Do we really need another btrfs layer that re-implements ...

[snip]
Woah, woah. This is partly addressed by Roman's reply - but ...

Subvolumes:
Subvolumes are not comparable to block devices. This thinking is flawed at best; cancerous at worst.

As a user I tend to think of subvolumes simply as directly-mountable folders.

As a sysadmin I also think of them as snapshottable/send-receiveable folders.

And as a dev I know they're actually not that different from regular folders. They have some extra metadata so aren't as lightweight - but of course they expose very useful flexibility not available in a regular folder.

MD/raid comparison:
In much the same way, comparing btrfs' raid features to md directly is also flawed. Btrfs even re-uses code in md to implement raid-type features in ways that md cannot.

I can't answer for the current raid5/6 stability issues - but I am confident that the overall design is good, and that it will be fixed.


The generic file encryption code is solid, reviewed, tested and
already widely deployed via two separate filesystems. There is a
much wider pool of developers who will maintain it, reveiw changes
and know all the traps that a new implementation might fall into.
There's a much bigger safety net here, which significantly lowers
the risk of zero-day fatal flaws in a new implementation and of
flaws in future modifications and enhancements.

Hence, IMO, the first thing to do is implement and make the generic
file encryption support solid and robust, not tack it on as an
afterthought for the magic btrfs encryption pixies to take care of.

Indeed, with the generic file encryption, btrfs may not even need
the special subvolume encryption pixies. i.e. you can effectively
implement subvolume encryption via configuration of a multi-user
encryption key for each subvolume and apply it to the subvolume tree
root at creation time. Then only users with permission to unlock the
subvolume key can access it.

Once the generic file encryption is solid and fulfils the needs of
most users, then you can look to solving the less common threat
models that neither dmcrypt or per-file encryption address. Only if
the generic code cannot be expanded to address specific threat
models should you then implement something that is unique to
btrfs....


Agreed, this sounds like a far safer and achievable implementation process.

Cheers,

Dave.


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http://www.webafrica.co.za/?AFF1E97
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