184 lines
9.2 KiB
ReStructuredText
184 lines
9.2 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=================
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Inline Encryption
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=================
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Objective
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=========
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We want to support inline encryption (IE) in the kernel.
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To allow for testing, we also want a crypto API fallback when actual
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IE hardware is absent. We also want IE to work with layered devices
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like dm and loopback (i.e. we want to be able to use the IE hardware
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of the underlying devices if present, or else fall back to crypto API
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en/decryption).
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Constraints and notes
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=====================
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- IE hardware have a limited number of "keyslots" that can be programmed
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with an encryption context (key, algorithm, data unit size, etc.) at any time.
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One can specify a keyslot in a data request made to the device, and the
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device will en/decrypt the data using the encryption context programmed into
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that specified keyslot. When possible, we want to make multiple requests with
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the same encryption context share the same keyslot.
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- We need a way for filesystems to specify an encryption context to use for
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en/decrypting a struct bio, and a device driver (like UFS) needs to be able
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to use that encryption context when it processes the bio.
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- We need a way for device drivers to expose their capabilities in a unified
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way to the upper layers.
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Design
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======
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We add a struct bio_crypt_ctx to struct bio that can represent an
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encryption context, because we need to be able to pass this encryption
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context from the FS layer to the device driver to act upon.
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While IE hardware works on the notion of keyslots, the FS layer has no
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knowledge of keyslots - it simply wants to specify an encryption context to
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use while en/decrypting a bio.
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We introduce a keyslot manager (KSM) that handles the translation from
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encryption contexts specified by the FS to keyslots on the IE hardware.
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This KSM also serves as the way IE hardware can expose their capabilities to
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upper layers. The generic mode of operation is: each device driver that wants
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to support IE will construct a KSM and set it up in its struct request_queue.
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Upper layers that want to use IE on this device can then use this KSM in
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the device's struct request_queue to translate an encryption context into
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a keyslot. The presence of the KSM in the request queue shall be used to mean
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that the device supports IE.
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On the device driver end of the interface, the device driver needs to tell the
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KSM how to actually manipulate the IE hardware in the device to do things like
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programming the crypto key into the IE hardware into a particular keyslot. All
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this is achieved through the :c:type:`struct keyslot_mgmt_ll_ops` that the
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device driver passes to the KSM when creating it.
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It uses refcounts to track which keyslots are idle (either they have no
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encryption context programmed, or there are no in-flight struct bios
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referencing that keyslot). When a new encryption context needs a keyslot, it
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tries to find a keyslot that has already been programmed with the same
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encryption context, and if there is no such keyslot, it evicts the least
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recently used idle keyslot and programs the new encryption context into that
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one. If no idle keyslots are available, then the caller will sleep until there
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is at least one.
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Blk-crypto
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==========
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The above is sufficient for simple cases, but does not work if there is a
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need for a crypto API fallback, or if we are want to use IE with layered
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devices. To these ends, we introduce blk-crypto. Blk-crypto allows us to
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present a unified view of encryption to the FS (so FS only needs to specify
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an encryption context and not worry about keyslots at all), and blk-crypto
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can decide whether to delegate the en/decryption to IE hardware or to the
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crypto API. Blk-crypto maintains an internal KSM that serves as the crypto
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API fallback.
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Blk-crypto needs to ensure that the encryption context is programmed into the
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"correct" keyslot manager for IE. If a bio is submitted to a layered device
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that eventually passes the bio down to a device that really does support IE, we
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want the encryption context to be programmed into a keyslot for the KSM of the
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device with IE support. However, blk-crypto does not know a priori whether a
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particular device is the final device in the layering structure for a bio or
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not. So in the case that a particular device does not support IE, since it is
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possibly the final destination device for the bio, if the bio requires
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encryption (i.e. the bio is doing a write operation), blk-crypto must fallback
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to the crypto API *before* sending the bio to the device.
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Blk-crypto ensures that:
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- The bio's encryption context is programmed into a keyslot in the KSM of the
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request queue that the bio is being submitted to (or the crypto API fallback
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KSM if the request queue doesn't have a KSM), and that the ``bc_ksm``
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in the ``bi_crypt_context`` is set to this KSM
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- That the bio has its own individual reference to the keyslot in this KSM.
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Once the bio passes through blk-crypto, its encryption context is programmed
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in some KSM. The "its own individual reference to the keyslot" ensures that
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keyslots can be released by each bio independently of other bios while
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ensuring that the bio has a valid reference to the keyslot when, for e.g., the
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crypto API fallback KSM in blk-crypto performs crypto on the device's behalf.
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The individual references are ensured by increasing the refcount for the
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keyslot in the ``bc_ksm`` when a bio with a programmed encryption
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context is cloned.
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What blk-crypto does on bio submission
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--------------------------------------
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**Case 1:** blk-crypto is given a bio with only an encryption context that hasn't
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been programmed into any keyslot in any KSM (for e.g. a bio from the FS).
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In this case, blk-crypto will program the encryption context into the KSM of the
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request queue the bio is being submitted to (and if this KSM does not exist,
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then it will program it into blk-crypto's internal KSM for crypto API
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fallback). The KSM that this encryption context was programmed into is stored
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as the ``bc_ksm`` in the bio's ``bi_crypt_context``.
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**Case 2:** blk-crypto is given a bio whose encryption context has already been
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programmed into a keyslot in the *crypto API fallback* KSM.
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In this case, blk-crypto does nothing; it treats the bio as not having
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specified an encryption context. Note that we cannot do here what we will do
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in Case 3 because we would have already encrypted the bio via the crypto API
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by this point.
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**Case 3:** blk-crypto is given a bio whose encryption context has already been
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programmed into a keyslot in some KSM (that is *not* the crypto API fallback
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KSM).
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In this case, blk-crypto first releases that keyslot from that KSM and then
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treats the bio as in Case 1.
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This way, when a device driver is processing a bio, it can be sure that
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the bio's encryption context has been programmed into some KSM (either the
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device driver's request queue's KSM, or blk-crypto's crypto API fallback KSM).
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It then simply needs to check if the bio's ``bc_ksm`` is the device's
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request queue's KSM. If so, then it should proceed with IE. If not, it should
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simply do nothing with respect to crypto, because some other KSM (perhaps the
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blk-crypto crypto API fallback KSM) is handling the en/decryption.
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Blk-crypto will release the keyslot that is being held by the bio (and also
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decrypt it if the bio is using the crypto API fallback KSM) once
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``bio_remaining_done`` returns true for the bio.
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Layered Devices
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===============
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Layered devices that wish to support IE need to create their own keyslot
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manager for their request queue, and expose whatever functionality they choose.
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When a layered device wants to pass a bio to another layer (either by
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resubmitting the same bio, or by submitting a clone), it doesn't need to do
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anything special because the bio (or the clone) will once again pass through
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blk-crypto, which will work as described in Case 3. If a layered device wants
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for some reason to do the IO by itself instead of passing it on to a child
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device, but it also chose to expose IE capabilities by setting up a KSM in its
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request queue, it is then responsible for en/decrypting the data itself. In
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such cases, the device can choose to call the blk-crypto function
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``blk_crypto_fallback_to_kernel_crypto_api`` (TODO: Not yet implemented), which will
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cause the en/decryption to be done via the crypto API fallback.
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Future Optimizations for layered devices
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========================================
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Creating a keyslot manager for the layered device uses up memory for each
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keyslot, and in general, a layered device (like dm-linear) merely passes the
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request on to a "child" device, so the keyslots in the layered device itself
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might be completely unused. We can instead define a new type of KSM; the
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"passthrough KSM", that layered devices can use to let blk-crypto know that
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this layered device *will* pass the bio to some child device (and hence
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through blk-crypto again, at which point blk-crypto can program the encryption
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context, instead of programming it into the layered device's KSM). Again, if
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the device "lies" and decides to do the IO itself instead of passing it on to
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a child device, it is responsible for doing the en/decryption (and can choose
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to call ``blk_crypto_fallback_to_kernel_crypto_api``). Another use case for the
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"passthrough KSM" is for IE devices that want to manage their own keyslots/do
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not have a limited number of keyslots.
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