Date:	Wed, 23 Jul 2014 16:39:43 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Michael Halcrow <mhalcrow@...gle.com>
Cc:	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	zohar@...ux.vnet.ibm.com, herbert@...dor.apana.org.au,
	pavel@....cz, hch@...radead.org, lczerner@...hat.com,
	tytso@....edu, tyhicks@...onical.com, serge.hallyn@...onical.com
Subject: Re: [PATCH 0/5] ext4: RFC: Encryption

On Jul 23, 2014, at 3:23 PM, Michael Halcrow <mhalcrow@...gle.com> wrote:
> This patchset proposes a method for encrypting in EXT4 data read and
> write paths. It's a proof-of-concept/prototype only right now.

Maybe it is worthwhile to take a step back and explain what your overall
goal is?  What is the benefit of implementing crypto at the filesystem
level over at the block device level?  Are you targeting per-user crypto
keys?  Fast secure deletion of files by having per-inode keys that are
encrypted by the filesystem/user key and then clobbered at deletion time?

What is the threat model?  Without knowing that, there isn't any point
in designing or implementing anything.

Hopefully you are already aware of the ext4 metadata checksum feature that
is in newer kernels?  That might be useful for storing your strong crypto
integrity hashes for filesystem metadata.

We've also previously discussed storing file-data checksums in some form.
One of the leading candidates being either a per-block table of checksums
that are statically mapped either for every block in the filesystem, or
only to the "data" blocks of the filesystem (i.e. those that don't contain
fixed metadata that already has its own checksums such as inode tables,
bitmaps, and backup group descriptors/superblocks).  The other possibility
is storing checksums with each extent, with the option to make the extents
as small or large as needed.  See thread starting at:
http://www.spinics.net/lists/linux-ext4/msg42620.html

Once we understand what the actual security goals and threat model are,
then it will be easier to determine the best way to implement this.

Cheers, Andreas

> Outstanding issues:
> 
> * While it seems to work well with complex tasks like a parallel
>   kernel build, fsx is pretty good at reliably breaking it in its
>   current form. I think it's trying to decrypt a page of all zeros
>   when doing a mmap'd write after an falloc. I want to get feedback
>   on the overall approach before I spend too much time bug-hunting.
> 
> * It has not undergone a security audit/review. It isn't IND-CCA2
>   secure, and that's the goal. We need a way to store (at least)
>   page-granular metadata.
> 
> * Only the file data is encrypted. I'd like to look into also
>   encrypting the file system metadata with a mount-wide key. That's
>   for another phase of development.
> 
> * The key management isn't fleshed out. I've hacked in some eCryptfs
>   stuff because it was the fastest way for me to stand up the
>   prototype with real crypto keys. Use ecryptfs-add-passphrase to add
>   a key to the keyring, and then pass the hex sig as the
>   encrypt_key_sig mount option:
> 
> # apt-get install ecryptfs-utils
> # echo -n "hunter2" | ecryptfs-add-passphrase 
> Passphrase: 
> Inserted auth tok with sig [4cb927ea0c564410] into the user session keyring
> # mount -o encrypt_key_sig=4cb927ea0c564410 /dev/sdb1 /mnt/ext4crypt
> 
> * The EXT4 block size must be the same as the page size. I'm not yet
>   sure whether I will want to try to support block-granular
>   encryption or page-granular encryption. There are implications with
>   respect to how much the integrity data occupies in relation to the
>   encrypted data.
> 
> Mimi, maybe an approach like this one will work out for IMA. We've
> just got to figure out where to store the block- or page-granular
> integrity data.
> 
> I've broken up the patches so that not only can each one build after
> application, but discrete steps of functionality can be tested one
> patch at a time.
> 
> A couple of other thoughts:
> 
> * Maybe the write submit path can complete on the encryption
>   callback. Not sure what that might buy us.
> 
> * Maybe a key with a specific descriptor in each user's keyring
>   (e.g. "EXT4_DEFAULT_KEY") can be used when creating new files so
>   that each user can use his own key in a common EXT4 mount. Or maybe
>   we can specify an encryption context in the parent directory xattr.
> 
> Michael Halcrow (5):
>  ext4: Adds callback support for bio read completion
>  ext4: Adds EXT4 encryption facilities
>  ext4: Implements the EXT4 encryption write path
>  ext4: Adds EXT4 encryption read callback support
>  ext4: Implements real encryption in the EXT4 write and read paths
> 
> fs/buffer.c                 |  46 +++-
> fs/ext4/Makefile            |   9 +-
> fs/ext4/crypto.c            | 629 ++++++++++++++++++++++++++++++++++++++++++++
> fs/ext4/ext4.h              |  50 ++++
> fs/ext4/file.c              |   9 +-
> fs/ext4/inode.c             | 196 +++++++++++++-
> fs/ext4/namei.c             |   3 +
> fs/ext4/page-io.c           | 182 ++++++++++---
> fs/ext4/super.c             |  34 ++-
> fs/ext4/xattr.h             |   1 +
> include/linux/bio.h         |   3 +
> include/linux/blk_types.h   |   4 +
> include/linux/buffer_head.h |   8 +
> 13 files changed, 1118 insertions(+), 56 deletions(-)
> create mode 100644 fs/ext4/crypto.c
> 
> -- 
> 2.0.0.526.g5318336
> 
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Cheers, Andreas






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