Message-ID: <20210210193327.GA8226@redhat.com>
Date:   Wed, 10 Feb 2021 14:33:27 -0500
From:   Mike Snitzer <snitzer@...hat.com>
To:     Satya Tangirala <satyat@...gle.com>, Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        dm-devel@...hat.com, Alasdair Kergon <agk@...hat.com>,
        Eric Biggers <ebiggers@...gle.com>
Subject: Re: [PATCH v4 0/5] add support for inline encryption to device mapper

On Mon, Feb 01 2021 at 12:10am -0500,
Satya Tangirala <satyat@...gle.com> wrote:

> This patch series adds support for inline encryption to the device mapper.
> 
> Patch 1 introduces the "passthrough" keyslot manager.
> 
> The regular keyslot manager is designed for inline encryption hardware that
> have only a small fixed number of keyslots. A DM device itself does not
> actually have only a small fixed number of keyslots - it doesn't actually
> have any keyslots in the first place, and programming an encryption context
> into a DM device doesn't make much semantic sense. It is possible for a DM
> device to set up a keyslot manager with some "sufficiently large" number of
> keyslots in its request queue, so that upper layers can use the inline
> encryption capabilities of the DM device's underlying devices, but the
> memory being allocated for the DM device's keyslots is a waste since they
> won't actually be used by the DM device.
> 
> The passthrough keyslot manager solves this issue - when the block layer
> sees that a request queue has a passthrough keyslot manager, it doesn't
> attempt to program any encryption context into the keyslot manager. The
> passthrough keyslot manager only allows the device to expose its inline
> encryption capabilities, and a way for upper layers to evict keys if
> necessary.
> 
> There also exist inline encryption hardware that can handle encryption
> contexts directly, and allow users to pass them a data request along with
> the encryption context (as opposed to inline encryption hardware that
> require users to first program a keyslot with an encryption context, and
> then require the users to pass the keyslot index with the data request).
> Such devices can also make use of the passthrough keyslot manager.
> 
> Patch 2 introduces some keyslot manager functions useful for the device
> mapper.
> 
> Patch 3 introduces the changes for inline encryption support for the device
> mapper. A DM device only exposes the intersection of the crypto
> capabilities of its underlying devices. This is so that in case a bio with
> an encryption context is eventually mapped to an underlying device that
> doesn't support that encryption context, the blk-crypto-fallback's cipher
> tfms are allocated ahead of time by the call to blk_crypto_start_using_key.
> 
> Each DM target can now also specify the "DM_TARGET_PASSES_CRYPTO" flag in
> the target type features to opt-in to supporting passing through the
> underlying inline encryption capabilities.  This flag is needed because it
> doesn't make much semantic sense for certain targets like dm-crypt to
> expose the underlying inline encryption capabilities to the upper layers.
> Again, the DM exposes inline encryption capabilities of the underlying
> devices only if all of them opt-in to passing through inline encryption
> support.
> 
> A keyslot manager is created for a table when it is loaded. However, the
> mapped device's exposed capabilities *only* updated once the table is
> swapped in (until the new table is swapped in, the mapped device continues
> to expose the old table's crypto capabilities).
> 
> This patch only allows the keyslot manager's capabilities to *expand*
> because of table changes. Any attempt to load a new table that doesn't
> support a crypto capability that the old table did is rejected.
> 
> This patch also only exposes the intersection of the underlying device's
> capabilities, which has the effect of causing en/decryption of a bio to
> fall back to the kernel crypto API (if the fallback is enabled) whenever
> any of the underlying devices doesn't support the encryption context of the
> bio - it might be possible to make the bio only fall back to the kernel
> crypto API if the bio's target underlying device doesn't support the bio's
> encryption context, but the use case may be uncommon enough in the first
> place not to warrant worrying about it right now.
> 
> Patch 4 makes DM evict a key from all its underlying devices when asked to
> evict a key.
> 
> Patch 5 makes some DM targets opt-in to passing through inline encryption
> support. It does not (yet) try to enable this option with dm-raid, since
> users can "hot add" disks to a raid device, which makes this not completely
> straightforward (we'll need to ensure that any "hot added" disks must have
> a superset of the inline encryption capabilities of the rest of the disks
> in the raid device, due to the way Patch 2 of this series works).
> 
> Changes v3 => v4:
>  - Allocate the memory for the ksm of the mapped device in
>    dm_table_complete(), and install the ksm in the md queue in __bind()
>    (as suggested by Mike). Also drop patch 5 from v3 since it's no longer
>    needed.
>  - Some cleanups
> 
> Changes v2 => v3:
>  - Split up the main DM patch into 4 separate patches
>  - Removed the priv variable added to struct keyslot manager in v2
>  - Use a flag in target type features for opting-in to inline encryption
>    support, instead of using "may_passthrough_inline_crypto"
>  - cleanups, improve docs and restructure code
> 
> Changes v1 => v2:
>  - Introduce private field to struct blk_keyslot_manager
>  - Allow the DM keyslot manager to expand its crypto capabilities if the
>    table is changed.
>  - Make DM reject table changes that would otherwise cause crypto
>    capabilities to be dropped.
>  - Allocate the DM device's keyslot manager only when at least one crypto
>    capability is supported (since a NULL value for q->ksm represents "no
>    crypto support" anyway).
>  - Remove the struct blk_keyslot_manager field from struct mapped_device.
>    This patch now relies on just directly setting up the keyslot manager in
>    the request queue, since each DM device is tied to only 1 queue.
> 
> Satya Tangirala (5):
>   block: keyslot-manager: Introduce passthrough keyslot manager
>   block: keyslot-manager: Introduce functions for device mapper support
>   dm: add support for passing through inline crypto support
>   dm: support key eviction from keyslot managers of underlying devices
>   dm: set DM_TARGET_PASSES_CRYPTO feature for some targets
> 
>  block/blk-crypto.c              |   1 +
>  block/keyslot-manager.c         | 146 ++++++++++++++++++++++
>  drivers/md/dm-core.h            |   5 +
>  drivers/md/dm-flakey.c          |   4 +-
>  drivers/md/dm-linear.c          |   5 +-
>  drivers/md/dm-table.c           | 210 ++++++++++++++++++++++++++++++++
>  drivers/md/dm.c                 |  18 ++-
>  include/linux/device-mapper.h   |  11 ++
>  include/linux/keyslot-manager.h |  11 ++
>  9 files changed, 407 insertions(+), 4 deletions(-)
> 
> -- 
> 2.30.0.365.g02bc693789-goog
> 

This set looks good to me now.

To avoid DM needing another rebase on block: Jens (and others), would
you like to review patches 1 and 2 (and reply with your Reviewed-by) so
I could pickup the DM required keyslot-manager changes along with
patches 3-5?

Thanks,
Mike

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