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

Milan Broz gmazyland at gmail.com
Thu Jun 15 09:33:39 CEST 2017


On 06/15/2017 01:40 AM, Michael Halcrow wrote:
> 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.

The whole reason for full disk encryption (FDE) is the it is FULL disk
encryption.

If you do not need encryption on dmcrypt level, just do not use it
by properly configuring storage stack!

The file-level encryption and dm-crypt encryption can have completely different
purpose, even one can be authenticated one (providing integrity)
and second just providing confidentiality.

It is not "redundant" encryption, it is the encryption for different purpose
on different layer.

IMO what you are proposing is incredible security hack and very ugly
layer violation.

If this is accepted, we basically allow attacker to trick system to write
plaintext to media just by setting this flag. This must never ever happen
with FDE - BY DESIGN.

For me, ABSOLUTE NACK to this approach.

(cc dmcrypt list as well)

Milan


> 
> 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(-)
> 



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