[RFC PATCH 0/4] Allow file systems to selectively bypass dm-crypt

[Michael Halcrow <mhalcrow@google.com>]

Several file systems either have already implemented encryption or are
in the process of doing so.  This addresses usability and storage
isolation requirements on mobile devices and in multi-tenant
environments.

While distinct keys locked down to user accounts protect the names and
contents of individual files, a volume-level encryption key should
protect the file system metadata.  When using dm-crypt to do this, the
blocks that the file system encrypts undergo another round of
encryption.  In many contexts this isn't necessary, and it results in
decreased performance and increased power consumption.  This
discourages users and vendors from taking steps to protect file system
metadata in addition to file contents.

This patchset provides a new I/O request flag that suggests to lower
layers that encryption may not be necessary because the file system
has already done it.  The first patch targets the block subsystem and
adds the REQ_NOENCRYPT flag.  The second patch targets dm-crypt and
adds logic to observe the flag only when the user has opted-in by
passing allow_encrypt_override as one of the optional parameters to
the dm-crypt target.  The third patch targets ext4 and sets the
REQ_NOENCRYPT flag for blocks it encrypts and decrypts.  The fourth
patch does the same for f2fs.

In this patchset I'm proposing that we make dm-crypt's observation of
the file system "don't encrypt" request optional, but I'm not sure
that's a good tradeoff.  Under the assumption that encryption in
upstream file systems is sound, the most common way I expect things
can go awry is if the file system keys don't meet security
requirements while dm-crypt keys do.  However I'm not convinced that
will end up being a widespread problem in practice, and there's a real
data corruption concern with making dm-crypt's observation of the flag
optional.

The problem is that once dm-crypt observes the flag, it must always
continue to do so or dm-crypt might decrypt content that it didn't
encrypt.  This can lead to scenarios where a vendor sets the dm-crypt
option to observe the "don't encrypt" flag, and then down the road a
user follows one of the many online guides to manually recover a
dm-crypt partition without setting this new option.

Should there be an encryption disable flag?  I'm interested in
considering other solutions.

Michael Halcrow (4):
  block: Add bio req flag to disable encryption in block
  dm-crypt: Skip encryption of file system-encrypted blocks
  ext4: Set the bio REQ_NOENCRYPT flag
  f2fs: Set the bio REQ_NOENCRYPT flag

 drivers/md/dm-crypt.c     | 17 +++++++++++++----
 fs/crypto/bio.c           |  2 +-
 fs/ext4/ext4.h            |  3 +++
 fs/ext4/inode.c           | 13 ++++++++-----
 fs/ext4/page-io.c         |  5 +++++
 fs/ext4/readpage.c        |  3 ++-
 fs/f2fs/data.c            | 10 ++++++++--
 include/linux/blk_types.h |  2 ++
 8 files changed, 42 insertions(+), 13 deletions(-)

-- 
2.13.1.508.gb3defc5cc-goog