[v8,04/11] block: blk-crypto-fallback for Inline Encryption

diff --git a/Documentation/block/index.rst b/Documentation/block/index.rst
index 3fa7a52fafa4..026addfc69bc 100644
--- a/Documentation/block/index.rst
+++ b/Documentation/block/index.rst
@@ -14,6 +14,7 @@  Block
    cmdline-partition
    data-integrity
    deadline-iosched
+   inline-encryption
    ioprio
    kyber-iosched
    null_blk
diff --git a/Documentation/block/inline-encryption.rst b/Documentation/block/inline-encryption.rst
new file mode 100644
index 000000000000..3fa475799ecd
--- /dev/null
+++ b/Documentation/block/inline-encryption.rst
@@ -0,0 +1,195 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+
+=================
+Inline Encryption
+=================
+
+Background
+==========
+
+Inline encryption hardware sits logically between memory and the disk, and can
+en/decrypt data as it goes in/out of the disk. Inline encryption hardware has a
+fixed number of "keyslots" - slots into which encryption contexts (i.e. the
+encryption key, encryption algorithm, data unit size) can be programmed by the
+kernel at any time. Each request sent to the disk can be tagged with the index
+of a keyslot (and also a data unit number to act as an encryption tweak), and
+the inline encryption hardware will en/decrypt the data in the request with the
+encryption context programmed into that keyslot. This is very different from
+full disk encryption solutions like self encrypting drives/TCG OPAL/ATA
+Security standards, since with inline encryption, any block on disk could be
+encrypted with any encryption context the kernel chooses.
+
+
+Objective
+=========
+
+We want to support inline encryption (IE) in the kernel.
+To allow for testing, we also want a crypto API fallback when actual
+IE hardware is absent. We also want IE to work with layered devices
+like dm and loopback (i.e. we want to be able to use the IE hardware
+of the underlying devices if present, or else fall back to crypto API
+en/decryption).
+
+
+Constraints and notes
+=====================
+
+- IE hardware has a limited number of "keyslots" that can be programmed
+  with an encryption context (key, algorithm, data unit size, etc.) at any time.
+  One can specify a keyslot in a data request made to the device, and the
+  device will en/decrypt the data using the encryption context programmed into
+  that specified keyslot. When possible, we want to make multiple requests with
+  the same encryption context share the same keyslot.
+
+- We need a way for upper layers like filesystems to specify an encryption
+  context to use for en/decrypting a struct bio, and a device driver (like UFS)
+  needs to be able to use that encryption context when it processes the bio.
+
+- We need a way for device drivers to expose their capabilities in a unified
+  way to the upper layers.
+
+
+Design
+======
+
+We add a :c:type:`struct bio_crypt_ctx` to :c:type:`struct bio` that can
+represent an encryption context, because we need to be able to pass this
+encryption context from the FS layer to the device driver to act upon.
+
+While IE hardware works on the notion of keyslots, the FS layer has no
+knowledge of keyslots - it simply wants to specify an encryption context to
+use while en/decrypting a bio.
+
+We introduce a keyslot manager (KSM) that handles the translation from
+encryption contexts specified by the FS to keyslots on the IE hardware.
+This KSM also serves as the way IE hardware can expose its capabilities to
+upper layers. The generic mode of operation is: each device driver that wants
+to support IE will construct a KSM and set it up in its struct request_queue.
+Upper layers that want to use IE on this device can then use this KSM in
+the device's struct request_queue to translate an encryption context into
+a keyslot. The presence of the KSM in the request queue shall be used to mean
+that the device supports IE.
+
+On the device driver end of the interface, the device driver needs to tell the
+KSM how to actually manipulate the IE hardware in the device to do things like
+programming the crypto key into the IE hardware into a particular keyslot. All
+this is achieved through the :c:type:`struct keyslot_mgmt_ll_ops` that the
+device driver passes to the KSM when creating it.
+
+It uses refcounts to track which keyslots are idle (either they have no
+encryption context programmed, or there are no in-flight struct bios
+referencing that keyslot). When a new encryption context needs a keyslot, it
+tries to find a keyslot that has already been programmed with the same
+encryption context, and if there is no such keyslot, it evicts the least
+recently used idle keyslot and programs the new encryption context into that
+one. If no idle keyslots are available, then the caller will sleep until there
+is at least one.
+
+
+blk-mq changes, other block layer changes and blk-crypto-fallback
+=================================================================
+
+We add a pointer to a ``bi_crypt_context`` and ``keyslot`` to
+:c:type:`struct request`. These will be referred to as the ``crypto fields``
+for the request. This ``keyslot`` is the keyslot into which the
+``bi_crypt_context`` has been programmed into in the keyslot manager of the
+``request_queue`` that this request is being sent to.
+
+We introduce ``block/blk-crypto-fallback.c``, which allows upper layers to remain
+blissfully unaware of whether or not real inline encryption hardware is present
+underneath.
+
+When a bio is submitted with a target ``request_queue`` that doesn't support the
+encryption context specified with the bio, the block layer will en/decrypt the
+bio with the blk-crypto-fallback.
+
+If the bio is a ``WRITE`` bio, a bounce bio is allocated, and the data in the bio
+is encrypted stored in the bounce bio - blk-mq will then proceed to process the
+bounce bio as if it were not encrypted at all (except when blk-integrity is
+concerned). ``blk-crypto-fallback`` sets the bounce bio's ``bi_end_io`` to an
+internal function that cleans up the bounce bio and ends the original bio.
+
+If the bio is a ``READ`` bio, the bio's ``bi_end_io`` (and also ``bi_private``)
+is saved and overwritten by ``blk-crypto-fallback`` to
+``bio_crypto_fallback_decrypt_bio``.  The bio's ``bi_crypt_context`` is also
+overwritten with ``NULL``, so that to the rest of the stack, the bio looks
+as if it was a regular bio that never had an encryption context specified.
+``bio_crypto_fallback_decrypt_bio`` will decrypt the bio, restore the original
+``bi_end_io`` (and also ``bi_private``) and end the bio again.
+
+If we reach a point when a :c:type:`struct request` needs to be allocated for a
+bio that still has an encryption context, that means that the bio was not
+handled by the ``blk-crypto-fallback``, which means that the underlying inline
+encryption hardware claimed to support the encryption context specified with the
+bio. So in this situation, blk-mq tries to program the encryption context into
+the ``request_queue``'s keyslot_manager, and obtain a keyslot, which it stores
+in its newly added ``keyslot`` field. This keyslot is released when the request
+is completed.
+
+When a bio is added to a request, the request takes over ownership of the
+``bi_crypt_context`` of the bio - in particular, the request keeps the
+``bi_crypt_context`` of the first bio in its bio-list, and frees the rest
+(blk-mq needs to be careful to maintain this invariant during bio and request
+merges).
+
+To make it possible for inline encryption to work with request queue based
+layered devices, when a request is cloned, its ``crypto fields`` are cloned as
+well. When the cloned request is submitted, blk-mq programs the
+``bi_crypt_context`` of the request into the clone's request_queue's keyslot
+manager, and stores the returned keyslot in the clone's ``keyslot``.
+
+
+Layered Devices
+===============
+
+Request queue based layered devices like dm-rq that wish to support IE need to
+create their own keyslot manager for their request queue, and expose whatever
+functionality they choose. When a layered device wants to pass a clone of that
+request to another ``request_queue``, blk-crypto will initialize and prepare the
+clone as necessary - see ``blk_crypto_rq_prep_clone`` and
+``blk_crypto_insert_cloned_request`` in ``blk-crypto.c``.
+
+
+Future Optimizations for layered devices
+========================================
+
+Creating a keyslot manager for a layered device uses up memory for each
+keyslot, and in general, a layered device merely passes the request on to a
+"child" device, so the keyslots in the layered device itself are completely
+unused, and don't need any refcounting or keyslot programming. We can instead
+define a new type of KSM; the "passthrough KSM", that layered devices can use
+to advertise an unlimited number of keyslots, and support for any encryption
+algorithms they choose, while not actually using any memory for each keyslot.
+Another use case for the "passthrough KSM" is for IE devices that do not have a
+limited number of keyslots.
+
+
+Interaction between inline encryption and blk integrity
+=======================================================
+
+At the time of this patch, there is no real hardware that supports both these
+features. However, these features do interact with each other, and it's not
+completely trivial to make them both work together properly. In particular,
+when a WRITE bio wants to use inline encryption on a device that supports both
+features, the bio will have an encryption context specified, after which
+its integrity information is calculated (using the plaintext data, since
+the encryption will happen while data is being written), and the data and
+integrity info is sent to the device. Obviously, the integrity info must be
+verified before the data is encrypted. After the data is encrypted, the device
+must not store the integrity info that it received with the plaintext data
+since that might reveal information about the plaintext data. As such, it must
+re-generate the integrity info from the ciphertext data and store that on disk
+instead. Another issue with storing the integrity info of the plaintext data is
+that it changes the on disk format depending on whether hardware inline
+encryption support is present or the kernel crypto API fallback is used (since
+if the fallback is used, the device will receive the integrity info of the
+ciphertext, not that of the plaintext).
+
+Because there isn't any real hardware yet, it seems prudent to assume that
+hardware implementations might not implement both features together correctly,
+and disallow the combination for now. Whenever a device supports integrity, the
+kernel will pretend that the device does not support hardware inline encryption
+(by essentially setting the keyslot manager in the request_queue of the device
+to NULL). When the crypto API fallback is enabled, this means that all bios with
+and encryption context will use the fallback, and IO will complete as usual.
+When the fallback is disabled, a bio with an encryption context will be failed.
diff --git a/block/Kconfig b/block/Kconfig
index c04a1d500842..0af387623774 100644
--- a/block/Kconfig
+++ b/block/Kconfig
@@ -192,6 +192,16 @@  config BLK_INLINE_ENCRYPTION
 	  block layer handle encryption, so users can take
 	  advantage of inline encryption hardware if present.
 
+config BLK_INLINE_ENCRYPTION_FALLBACK
+	bool "Enable crypto API fallback for blk-crypto"
+	depends on BLK_INLINE_ENCRYPTION
+	select CRYPTO
+	select CRYPTO_SKCIPHER
+	help
+	  Enabling this lets the block layer handle inline encryption
+	  by falling back to the kernel crypto API when inline
+	  encryption hardware is not present.
+
 menu "Partition Types"
 
 source "block/partitions/Kconfig"
diff --git a/block/Makefile b/block/Makefile
index 82f42ca3f769..9464fb6ae423 100644
--- a/block/Makefile
+++ b/block/Makefile
@@ -38,3 +38,4 @@  obj-$(CONFIG_BLK_DEBUG_FS_ZONED)+= blk-mq-debugfs-zoned.o
 obj-$(CONFIG_BLK_SED_OPAL)	+= sed-opal.o
 obj-$(CONFIG_BLK_PM)		+= blk-pm.o
 obj-$(CONFIG_BLK_INLINE_ENCRYPTION)	+= keyslot-manager.o blk-crypto.o
+obj-$(CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK)	+= blk-crypto-fallback.o
diff --git a/block/blk-crypto-fallback.c b/block/blk-crypto-fallback.c
new file mode 100644
index 000000000000..0925325c9433
--- /dev/null
+++ b/block/blk-crypto-fallback.c
@@ -0,0 +1,662 @@ 
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright 2019 Google LLC
+ */
+
+/*
+ * Refer to Documentation/block/inline-encryption.rst for detailed explanation.
+ */
+
+#define pr_fmt(fmt) "blk-crypto-fallback: " fmt
+
+#include <crypto/skcipher.h>
+#include <linux/blk-cgroup.h>
+#include <linux/blk-crypto.h>
+#include <linux/blkdev.h>
+#include <linux/crypto.h>
+#include <linux/keyslot-manager.h>
+#include <linux/mempool.h>
+#include <linux/module.h>
+#include <linux/random.h>
+
+#include "blk-crypto-internal.h"
+
+static unsigned int num_prealloc_bounce_pg = 32;
+module_param(num_prealloc_bounce_pg, uint, 0);
+MODULE_PARM_DESC(num_prealloc_bounce_pg,
+		 "Number of preallocated bounce pages for the blk-crypto crypto API fallback");
+
+static unsigned int blk_crypto_num_keyslots = 100;
+module_param_named(num_keyslots, blk_crypto_num_keyslots, uint, 0);
+MODULE_PARM_DESC(num_keyslots,
+		 "Number of keyslots for the blk-crypto crypto API fallback");
+
+static unsigned int num_prealloc_fallback_crypt_ctxs = 128;
+module_param(num_prealloc_fallback_crypt_ctxs, uint, 0);
+MODULE_PARM_DESC(num_prealloc_crypt_fallback_ctxs,
+		 "Number of preallocated bio fallback crypto contexts for blk-crypto to use during crypto API fallback");
+
+struct bio_fallback_crypt_ctx {
+	struct bio_crypt_ctx crypt_ctx;
+	/*
+	 * Copy of the bvec_iter when this bio was submitted.
+	 * We only want to en/decrypt the part of the bio as described by the
+	 * bvec_iter upon submission because bio might be split before being
+	 * resubmitted
+	 */
+	struct bvec_iter crypt_iter;
+	u64 fallback_dun[BLK_CRYPTO_DUN_ARRAY_SIZE];
+	union {
+		struct {
+			struct work_struct work;
+			struct bio *bio;
+		};
+		struct {
+			void *bi_private_orig;
+			bio_end_io_t *bi_end_io_orig;
+		};
+	};
+};
+
+static struct kmem_cache *bio_fallback_crypt_ctx_cache;
+static mempool_t *bio_fallback_crypt_ctx_pool;
+
+/*
+ * Allocating a crypto tfm during I/O can deadlock, so we have to preallocate
+ * all of a mode's tfms when that mode starts being used. Since each mode may
+ * need all the keyslots at some point, each mode needs its own tfm for each
+ * keyslot; thus, a keyslot may contain tfms for multiple modes.  However, to
+ * match the behavior of real inline encryption hardware (which only supports a
+ * single encryption context per keyslot), we only allow one tfm per keyslot to
+ * be used at a time - the rest of the unused tfms have their keys cleared.
+ */
+static DEFINE_MUTEX(tfms_init_lock);
+static bool tfms_inited[BLK_ENCRYPTION_MODE_MAX];
+
+static struct blk_crypto_keyslot {
+	enum blk_crypto_mode_num crypto_mode;
+	struct crypto_skcipher *tfms[BLK_ENCRYPTION_MODE_MAX];
+} *blk_crypto_keyslots;
+
+static struct keyslot_manager blk_crypto_ksm;
+static struct workqueue_struct *blk_crypto_wq;
+static mempool_t *blk_crypto_bounce_page_pool;
+
+/*
+ * This is the key we set when evicting a keyslot. This *should* be the all 0's
+ * key, but AES-XTS rejects that key, so we use some random bytes instead.
+ */
+static u8 blank_key[BLK_CRYPTO_MAX_KEY_SIZE];
+
+static void blk_crypto_evict_keyslot(unsigned int slot)
+{
+	struct blk_crypto_keyslot *slotp = &blk_crypto_keyslots[slot];
+	enum blk_crypto_mode_num crypto_mode = slotp->crypto_mode;
+	int err;
+
+	WARN_ON(slotp->crypto_mode == BLK_ENCRYPTION_MODE_INVALID);
+
+	/* Clear the key in the skcipher */
+	err = crypto_skcipher_setkey(slotp->tfms[crypto_mode], blank_key,
+				     blk_crypto_modes[crypto_mode].keysize);
+	WARN_ON(err);
+	slotp->crypto_mode = BLK_ENCRYPTION_MODE_INVALID;
+}
+
+static blk_status_t blk_crypto_keyslot_program(struct keyslot_manager *ksm,
+					       const struct blk_crypto_key *key,
+					       unsigned int slot)
+{
+	struct blk_crypto_keyslot *slotp = &blk_crypto_keyslots[slot];
+	const enum blk_crypto_mode_num crypto_mode = key->crypto_mode;
+	int err;
+
+	if (crypto_mode != slotp->crypto_mode &&
+	    slotp->crypto_mode != BLK_ENCRYPTION_MODE_INVALID)
+		blk_crypto_evict_keyslot(slot);
+
+	slotp->crypto_mode = crypto_mode;
+	err = crypto_skcipher_setkey(slotp->tfms[crypto_mode], key->raw,
+				     key->size);
+	if (err) {
+		blk_crypto_evict_keyslot(slot);
+		return BLK_STS_IOERR;
+	}
+	return BLK_STS_OK;
+}
+
+static int blk_crypto_keyslot_evict(struct keyslot_manager *ksm,
+				    const struct blk_crypto_key *key,
+				    unsigned int slot)
+{
+	blk_crypto_evict_keyslot(slot);
+	return 0;
+}
+
+/*
+ * The crypto API fallback KSM ops - only used for a bio when it specifies a
+ * blk_crypto_key that was not supported by the device's inline encryption
+ * hardware.
+ */
+static const struct keyslot_mgmt_ll_ops blk_crypto_ksm_ll_ops = {
+	.keyslot_program	= blk_crypto_keyslot_program,
+	.keyslot_evict		= blk_crypto_keyslot_evict,
+};
+
+static void blk_crypto_fallback_encrypt_endio(struct bio *enc_bio)
+{
+	struct bio *src_bio = enc_bio->bi_private;
+	int i;
+
+	for (i = 0; i < enc_bio->bi_vcnt; i++)
+		mempool_free(enc_bio->bi_io_vec[i].bv_page,
+			     blk_crypto_bounce_page_pool);
+
+	src_bio->bi_status = enc_bio->bi_status;
+
+	bio_put(enc_bio);
+	bio_endio(src_bio);
+}
+
+static struct bio *blk_crypto_clone_bio(struct bio *bio_src)
+{
+	struct bvec_iter iter;
+	struct bio_vec bv;
+	struct bio *bio;
+
+	bio = bio_alloc_bioset(GFP_NOIO, bio_segments(bio_src), NULL);
+	if (!bio)
+		return NULL;
+	bio->bi_disk		= bio_src->bi_disk;
+	bio->bi_opf		= bio_src->bi_opf;
+	bio->bi_ioprio		= bio_src->bi_ioprio;
+	bio->bi_write_hint	= bio_src->bi_write_hint;
+	bio->bi_iter.bi_sector	= bio_src->bi_iter.bi_sector;
+	bio->bi_iter.bi_size	= bio_src->bi_iter.bi_size;
+
+	bio_for_each_segment(bv, bio_src, iter)
+		bio->bi_io_vec[bio->bi_vcnt++] = bv;
+
+	bio_clone_blkg_association(bio, bio_src);
+	blkcg_bio_issue_init(bio);
+
+	return bio;
+}
+
+static void blk_crypto_alloc_cipher_req(struct bio *src_bio,
+					struct blk_ksm_keyslot *slot,
+					struct skcipher_request **ciph_req_ret,
+					struct crypto_wait *wait)
+{
+	struct skcipher_request *ciph_req;
+	const struct blk_crypto_keyslot *slotp;
+	int keyslot_idx = blk_ksm_get_slot_idx(slot);
+
+	slotp = &blk_crypto_keyslots[keyslot_idx];
+	ciph_req = skcipher_request_alloc(slotp->tfms[slotp->crypto_mode],
+					  GFP_NOIO);
+	if (!ciph_req) {
+		src_bio->bi_status = BLK_STS_RESOURCE;
+		return;
+	}
+
+	skcipher_request_set_callback(ciph_req,
+				      CRYPTO_TFM_REQ_MAY_BACKLOG |
+				      CRYPTO_TFM_REQ_MAY_SLEEP,
+				      crypto_req_done, wait);
+	*ciph_req_ret = ciph_req;
+}
+
+static void blk_crypto_split_bio_if_needed(struct bio **bio_ptr)
+{
+	struct bio *bio = *bio_ptr;
+	unsigned int i = 0;
+	unsigned int num_sectors = 0;
+	struct bio_vec bv;
+	struct bvec_iter iter;
+
+	bio_for_each_segment(bv, bio, iter) {
+		num_sectors += bv.bv_len >> SECTOR_SHIFT;
+		if (++i == BIO_MAX_PAGES)
+			break;
+	}
+	if (num_sectors < bio_sectors(bio)) {
+		struct bio *split_bio;
+
+		split_bio = bio_split(bio, num_sectors, GFP_NOIO, NULL);
+		if (!split_bio) {
+			bio->bi_status = BLK_STS_RESOURCE;
+			return;
+		}
+		bio_chain(split_bio, bio);
+		generic_make_request(bio);
+		*bio_ptr = split_bio;
+	}
+}
+
+union blk_crypto_iv {
+	__le64 dun[BLK_CRYPTO_DUN_ARRAY_SIZE];
+	u8 bytes[BLK_CRYPTO_MAX_IV_SIZE];
+};
+
+static void blk_crypto_dun_to_iv(const u64 dun[BLK_CRYPTO_DUN_ARRAY_SIZE],
+				 union blk_crypto_iv *iv)
+{
+	int i;
+
+	for (i = 0; i < BLK_CRYPTO_DUN_ARRAY_SIZE; i++)
+		iv->dun[i] = cpu_to_le64(dun[i]);
+}
+
+/*
+ * The crypto API fallback's encryption routine.
+ * Allocate a bounce bio for encryption, encrypt the input bio using crypto API,
+ * and replace *bio_ptr with the bounce bio. May split input bio if it's too
+ * large. Sets bio->bi_status on error.
+ */
+static void blk_crypto_fallback_encrypt_bio(struct bio **bio_ptr)
+{
+	struct bio *src_bio, *enc_bio;
+	struct bio_crypt_ctx *bc;
+	struct blk_ksm_keyslot *slot;
+	int data_unit_size;
+	struct skcipher_request *ciph_req = NULL;
+	DECLARE_CRYPTO_WAIT(wait);
+	u64 curr_dun[BLK_CRYPTO_DUN_ARRAY_SIZE];
+	struct scatterlist src, dst;
+	union blk_crypto_iv iv;
+	unsigned int i, j;
+	int err = 0;
+	blk_status_t blk_st;
+
+	/* Split the bio if it's too big for single page bvec */
+	blk_crypto_split_bio_if_needed(bio_ptr);
+	if ((*bio_ptr)->bi_status != BLK_STS_OK)
+		return;
+
+	src_bio = *bio_ptr;
+	bc = src_bio->bi_crypt_context;
+	data_unit_size = bc->bc_key->data_unit_size;
+
+	/* Allocate bounce bio for encryption */
+	enc_bio = blk_crypto_clone_bio(src_bio);
+	if (!enc_bio) {
+		src_bio->bi_status = BLK_STS_RESOURCE;
+		return;
+	}
+
+	/*
+	 * Use the crypto API fallback keyslot manager to get a crypto_skcipher
+	 * for the algorithm and key specified for this bio.
+	 */
+	blk_st = blk_ksm_get_slot_for_key(&blk_crypto_ksm, bc->bc_key, &slot);
+	if (blk_st != BLK_STS_OK) {
+		src_bio->bi_status = blk_st;
+		goto out_put_enc_bio;
+	}
+
+	/* and then allocate an skcipher_request for it */
+	blk_crypto_alloc_cipher_req(src_bio, slot, &ciph_req, &wait);
+	if (src_bio->bi_status != BLK_STS_OK)
+		goto out_release_keyslot;
+
+	memcpy(curr_dun, bc->bc_dun, sizeof(curr_dun));
+	sg_init_table(&src, 1);
+	sg_init_table(&dst, 1);
+
+	skcipher_request_set_crypt(ciph_req, &src, &dst, data_unit_size,
+				   iv.bytes);
+
+	/* Encrypt each page in the bounce bio */
+	for (i = 0; i < enc_bio->bi_vcnt; i++) {
+		struct bio_vec *enc_bvec = &enc_bio->bi_io_vec[i];
+		struct page *plaintext_page = enc_bvec->bv_page;
+		struct page *ciphertext_page =
+			mempool_alloc(blk_crypto_bounce_page_pool, GFP_NOIO);
+
+		enc_bvec->bv_page = ciphertext_page;
+
+		if (!ciphertext_page) {
+			src_bio->bi_status = BLK_STS_RESOURCE;
+			goto out_free_bounce_pages;
+		}
+
+		sg_set_page(&src, plaintext_page, data_unit_size,
+			    enc_bvec->bv_offset);
+		sg_set_page(&dst, ciphertext_page, data_unit_size,
+			    enc_bvec->bv_offset);
+
+		/* Encrypt each data unit in this page */
+		for (j = 0; j < enc_bvec->bv_len; j += data_unit_size) {
+			blk_crypto_dun_to_iv(curr_dun, &iv);
+			err = crypto_wait_req(crypto_skcipher_encrypt(ciph_req),
+					      &wait);
+			if (err) {
+				i++;
+				src_bio->bi_status = BLK_STS_IOERR;
+				goto out_free_bounce_pages;
+			}
+			bio_crypt_dun_increment(curr_dun, 1);
+			src.offset += data_unit_size;
+			dst.offset += data_unit_size;
+		}
+	}
+
+	enc_bio->bi_private = src_bio;
+	enc_bio->bi_end_io = blk_crypto_fallback_encrypt_endio;
+	*bio_ptr = enc_bio;
+
+	enc_bio = NULL;
+	goto out_free_ciph_req;
+
+out_free_bounce_pages:
+	while (i > 0)
+		mempool_free(enc_bio->bi_io_vec[--i].bv_page,
+			     blk_crypto_bounce_page_pool);
+out_free_ciph_req:
+	skcipher_request_free(ciph_req);
+out_release_keyslot:
+	blk_ksm_put_slot(slot);
+out_put_enc_bio:
+	if (enc_bio)
+		bio_put(enc_bio);
+}
+
+/*
+ * The crypto API fallback's main decryption routine.
+ * Decrypts input bio in place, and calls bio_endio on the bio.
+ */
+static void blk_crypto_fallback_decrypt_bio(struct work_struct *work)
+{
+	struct bio_fallback_crypt_ctx *f_ctx =
+		container_of(work, struct bio_fallback_crypt_ctx, work);
+	struct bio *bio = f_ctx->bio;
+	struct bio_crypt_ctx *bc = &f_ctx->crypt_ctx;
+	struct blk_ksm_keyslot *slot;
+	struct skcipher_request *ciph_req = NULL;
+	DECLARE_CRYPTO_WAIT(wait);
+	u64 curr_dun[BLK_CRYPTO_DUN_ARRAY_SIZE];
+	union blk_crypto_iv iv;
+	struct scatterlist sg;
+	struct bio_vec bv;
+	struct bvec_iter iter;
+	const int data_unit_size = bc->bc_key->data_unit_size;
+	unsigned int i;
+	blk_status_t blk_st;
+
+	/*
+	 * Use the crypto API fallback keyslot manager to get a crypto_skcipher
+	 * for the algorithm and key specified for this bio.
+	 */
+	blk_st = blk_ksm_get_slot_for_key(&blk_crypto_ksm, bc->bc_key, &slot);
+	if (blk_st != BLK_STS_OK) {
+		bio->bi_status = blk_st;
+		goto out_no_keyslot;
+	}
+
+	/* and then allocate an skcipher_request for it */
+	blk_crypto_alloc_cipher_req(bio, slot, &ciph_req, &wait);
+	if (bio->bi_status != BLK_STS_OK)
+		goto out;
+
+	memcpy(curr_dun, f_ctx->fallback_dun, sizeof(curr_dun));
+	sg_init_table(&sg, 1);
+	skcipher_request_set_crypt(ciph_req, &sg, &sg, data_unit_size,
+				   iv.bytes);
+
+	/* Decrypt each segment in the bio */
+	__bio_for_each_segment(bv, bio, iter, f_ctx->crypt_iter) {
+		struct page *page = bv.bv_page;
+
+		sg_set_page(&sg, page, data_unit_size, bv.bv_offset);
+
+		/* Decrypt each data unit in the segment */
+		for (i = 0; i < bv.bv_len; i += data_unit_size) {
+			blk_crypto_dun_to_iv(curr_dun, &iv);
+			if (crypto_wait_req(crypto_skcipher_decrypt(ciph_req),
+					    &wait)) {
+				bio->bi_status = BLK_STS_IOERR;
+				goto out;
+			}
+			bio_crypt_dun_increment(curr_dun, 1);
+			sg.offset += data_unit_size;
+		}
+	}
+
+out:
+	skcipher_request_free(ciph_req);
+	blk_ksm_put_slot(slot);
+out_no_keyslot:
+	mempool_free(f_ctx, bio_fallback_crypt_ctx_pool);
+	bio_endio(bio);
+}
+
+/**
+ * blk_crypto_fallback_decrypt_endio - clean up bio w.r.t fallback decryption
+ *
+ * @bio: the bio to clean up.
+ *
+ * Restore bi_private and bi_end_io, and queue the bio for decryption into a
+ * workqueue, since this function will be called from an atomic context.
+ */
+static void blk_crypto_fallback_decrypt_endio(struct bio *bio)
+{
+	struct bio_fallback_crypt_ctx *f_ctx = bio->bi_private;
+
+	bio->bi_private = f_ctx->bi_private_orig;
+	bio->bi_end_io = f_ctx->bi_end_io_orig;
+
+	/* If there was an IO error, don't queue for decrypt. */
+	if (bio->bi_status) {
+		mempool_free(f_ctx, bio_fallback_crypt_ctx_pool);
+		bio_endio(bio);
+		return;
+	}
+
+	INIT_WORK(&f_ctx->work, blk_crypto_fallback_decrypt_bio);
+	f_ctx->bio = bio;
+	queue_work(blk_crypto_wq, &f_ctx->work);
+}
+
+/**
+ * blk_crypto_fallback_bio_prep - Prepare a bio to use fallback en/decryption
+ *
+ * @bio_ptr: pointer to the bio to prepare
+ *
+ * If bio is doing a WRITE operation, we split the bio into two parts, resubmit
+ * the second part. Allocates a bounce bio for the first part, encrypts it, and
+ * update bio_ptr to point to the bounce bio.
+ *
+ * For a READ operation, we mark the bio for decryption by using bi_private and
+ * bi_end_io.
+ *
+ * In either case, this function will make the bio look like a regular bio (i.e.
+ * as if no encryption context was ever specified) for the purposes of the rest
+ * of the stack except for blk-integrity (blk-integrity and blk-crypto are not
+ * currently supported together).
+ *
+ * Sets bio->bi_status on error.
+ */
+void blk_crypto_fallback_bio_prep(struct bio **bio_ptr)
+{
+	struct bio *bio = *bio_ptr;
+	struct bio_crypt_ctx *bc = bio->bi_crypt_context;
+	struct bio_fallback_crypt_ctx *f_ctx;
+
+	if (!tfms_inited[bc->bc_key->crypto_mode]) {
+		bio->bi_status = BLK_STS_IOERR;
+		return;
+	}
+
+	if (!blk_ksm_crypto_key_supported(&blk_crypto_ksm, bc->bc_key)) {
+		bio->bi_status = BLK_STS_NOTSUPP;
+		return;
+	}
+
+	if (bio_data_dir(bio) == WRITE) {
+		blk_crypto_fallback_encrypt_bio(bio_ptr);
+		return;
+	}
+
+	/*
+	 * bio READ case: Set up a f_ctx in the bio's bi_private and set the
+	 * bi_end_io appropriately to trigger decryption when the bio is ended.
+	 */
+	f_ctx = mempool_alloc(bio_fallback_crypt_ctx_pool, GFP_NOIO);
+	f_ctx->crypt_ctx = *bc;
+	memcpy(f_ctx->fallback_dun, bc->bc_dun, sizeof(f_ctx->fallback_dun));
+	f_ctx->crypt_iter = bio->bi_iter;
+	f_ctx->bi_private_orig = bio->bi_private;
+	f_ctx->bi_end_io_orig = bio->bi_end_io;
+	bio->bi_private = (void *)f_ctx;
+	bio->bi_end_io = blk_crypto_fallback_decrypt_endio;
+	bio_crypt_free_ctx(bio);
+}
+
+int blk_crypto_fallback_evict_key(const struct blk_crypto_key *key)
+{
+	return blk_ksm_evict_key(&blk_crypto_ksm, key);
+}
+
+static bool blk_crypto_fallback_inited;
+static int blk_crypto_fallback_init(void)
+{
+	int i;
+	int err = -ENOMEM;
+
+	if (blk_crypto_fallback_inited)
+		return 0;
+
+	prandom_bytes(blank_key, BLK_CRYPTO_MAX_KEY_SIZE);
+
+	err = blk_ksm_init(&blk_crypto_ksm, NULL, blk_crypto_num_keyslots);
+	if (err)
+		goto out;
+	err = -ENOMEM;
+
+	blk_crypto_ksm.ksm_ll_ops = blk_crypto_ksm_ll_ops;
+	blk_crypto_ksm.max_dun_bytes_supported = BLK_CRYPTO_MAX_IV_SIZE;
+
+	/* All blk-crypto modes have a crypto API fallback. */
+	for (i = 0; i < BLK_ENCRYPTION_MODE_MAX; i++)
+		blk_crypto_ksm.crypto_modes_supported[i] = 0xFFFFFFFF;
+	blk_crypto_ksm.crypto_modes_supported[BLK_ENCRYPTION_MODE_INVALID] = 0;
+
+	blk_crypto_wq = alloc_workqueue("blk_crypto_wq",
+					WQ_UNBOUND | WQ_HIGHPRI |
+					WQ_MEM_RECLAIM, num_online_cpus());
+	if (!blk_crypto_wq)
+		goto fail_free_ksm;
+
+	blk_crypto_keyslots = kcalloc(blk_crypto_num_keyslots,
+				      sizeof(blk_crypto_keyslots[0]),
+				      GFP_KERNEL);
+	if (!blk_crypto_keyslots)
+		goto fail_free_wq;
+
+	blk_crypto_bounce_page_pool =
+		mempool_create_page_pool(num_prealloc_bounce_pg, 0);
+	if (!blk_crypto_bounce_page_pool)
+		goto fail_free_keyslots;
+
+	bio_fallback_crypt_ctx_cache = KMEM_CACHE(bio_fallback_crypt_ctx, 0);
+	if (!bio_fallback_crypt_ctx_cache)
+		goto fail_free_bounce_page_pool;
+
+	bio_fallback_crypt_ctx_pool =
+		mempool_create_slab_pool(num_prealloc_fallback_crypt_ctxs,
+					 bio_fallback_crypt_ctx_cache);
+	if (!bio_fallback_crypt_ctx_pool)
+		goto fail_free_crypt_ctx_cache;
+
+	blk_crypto_fallback_inited = true;
+
+	return 0;
+fail_free_crypt_ctx_cache:
+	kmem_cache_destroy(bio_fallback_crypt_ctx_cache);
+fail_free_bounce_page_pool:
+	mempool_destroy(blk_crypto_bounce_page_pool);
+fail_free_keyslots:
+	kfree(blk_crypto_keyslots);
+fail_free_wq:
+	destroy_workqueue(blk_crypto_wq);
+fail_free_ksm:
+	blk_ksm_destroy(&blk_crypto_ksm);
+out:
+	return err;
+}
+
+/**
+ * blk_crypto_start_using_key() - Start using a blk_crypto_key on a device
+ * @key: A key to use on the device
+ * @q: the request queue for the device
+ *
+ * Upper layers must call this function to ensure that the crypto API fallback
+ * has transforms for the algorithm/data_unit_size/dun_bytes combo specified by
+ * the key, if it becomes necessary.
+ *
+ * Return: 0 on success and -err on error.
+ */
+int blk_crypto_start_using_key(struct blk_crypto_key *key,
+			       struct request_queue *q)
+{
+	enum blk_crypto_mode_num mode_num = key->crypto_mode;
+	struct blk_crypto_keyslot *slotp;
+	unsigned int i;
+	int err = 0;
+
+	/*
+	 * Fast path
+	 * Ensure that updates to blk_crypto_keyslots[i].tfms[mode_num]
+	 * for each i are visible before we try to access them.
+	 */
+	if (likely(smp_load_acquire(&tfms_inited[mode_num])))
+		return 0;
+
+	/*
+	 * If the keyslot manager of the request queue supports this crypto
+	 * mode, then we don't need to allocate this mode.
+	 */
+	if (blk_ksm_crypto_key_supported(q->ksm, key))
+		return 0;
+
+	mutex_lock(&tfms_init_lock);
+	err = blk_crypto_fallback_init();
+	if (err)
+		goto out;
+
+	if (tfms_inited[mode_num])
+		goto out;
+
+	for (i = 0; i < blk_crypto_num_keyslots; i++) {
+		slotp = &blk_crypto_keyslots[i];
+		slotp->tfms[mode_num] = crypto_alloc_skcipher(
+					blk_crypto_modes[mode_num].cipher_str,
+					0, 0);
+		if (IS_ERR(slotp->tfms[mode_num])) {
+			err = PTR_ERR(slotp->tfms[mode_num]);
+			slotp->tfms[mode_num] = NULL;
+			goto out_free_tfms;
+		}
+
+		crypto_skcipher_set_flags(slotp->tfms[mode_num],
+					  CRYPTO_TFM_REQ_FORBID_WEAK_KEYS);
+	}
+
+	/*
+	 * Ensure that updates to blk_crypto_keyslots[i].tfms[mode_num]
+	 * for each i are visible before we set tfms_inited[mode_num].
+	 */
+	smp_store_release(&tfms_inited[mode_num], true);
+	goto out;
+
+out_free_tfms:
+	for (i = 0; i < blk_crypto_num_keyslots; i++) {
+		slotp = &blk_crypto_keyslots[i];
+		crypto_free_skcipher(slotp->tfms[mode_num]);
+		slotp->tfms[mode_num] = NULL;
+	}
+out:
+	mutex_unlock(&tfms_init_lock);
+	return err;
+}
diff --git a/block/blk-crypto-internal.h b/block/blk-crypto-internal.h
index 5cdf45167117..a8b7c9c4b8da 100644
--- a/block/blk-crypto-internal.h
+++ b/block/blk-crypto-internal.h
@@ -16,6 +16,8 @@  struct blk_crypto_mode {
 	unsigned int ivsize; /* iv size in bytes */
 };
 
+extern const struct blk_crypto_mode blk_crypto_modes[];
+
 #ifdef CONFIG_BLK_INLINE_ENCRYPTION
 
 void bio_crypt_ctx_init(void);
@@ -141,4 +143,26 @@  static inline blk_status_t blk_crypto_insert_cloned_request(struct request *rq)
 
 #endif /* CONFIG_BLK_INLINE_ENCRYPTION */
 
+#ifdef CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK
+
+void blk_crypto_fallback_bio_prep(struct bio **bio_ptr);
+
+int blk_crypto_fallback_evict_key(const struct blk_crypto_key *key);
+
+#else /* CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK */
+
+static inline void blk_crypto_fallback_bio_prep(struct bio **bio_ptr)
+{
+	pr_warn_once("crypto API fallback disabled; failing request.\n");
+	(*bio_ptr)->bi_status = BLK_STS_NOTSUPP;
+}
+
+static inline int
+blk_crypto_fallback_evict_key(const struct blk_crypto_key *key)
+{
+	return 0;
+}
+
+#endif /* CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK */
+
 #endif /* __LINUX_BLK_CRYPTO_INTERNAL_H */
diff --git a/block/blk-crypto.c b/block/blk-crypto.c
index 1c38de053bb6..3b3beed6560f 100644
--- a/block/blk-crypto.c
+++ b/block/blk-crypto.c
@@ -3,6 +3,10 @@ 
  * Copyright 2019 Google LLC
  */
 
+/*
+ * Refer to Documentation/block/inline-encryption.rst for detailed explanation.
+ */
+
 #define pr_fmt(fmt) "blk-crypto: " fmt
 
 #include <linux/bio.h>
@@ -193,7 +197,8 @@  static void bio_crypt_check_alignment(struct bio *bio)
 /**
  * blk_crypto_init_request - Initializes the request's crypto fields based on
  *			     the bio to be added to the request, and prepares
- *			     it for hardware inline encryption.
+ *			     it for hardware inline encryption (as opposed to
+ *			     using the crypto API fallback).
  *
  * @rq: The request to init
  * @bio: The bio that will (eventually) be added to @rq.
@@ -210,6 +215,10 @@  blk_status_t blk_crypto_init_request(struct request *rq, struct bio *bio)
 
 	blk_crypto_rq_set_defaults(rq);
 
+	/*
+	 * We have a bio crypt context here - that means we didn't fallback
+	 * to crypto API, so try to program a keyslot now.
+	 */
 	err = blk_ksm_get_slot_for_key(rq->q->ksm,
 				       bio->bi_crypt_context->bc_key,
 				       &rq->crypt_keyslot);
@@ -240,9 +249,16 @@  void blk_crypto_free_request(struct request *rq)
  *
  * @bio_ptr: pointer to original bio pointer
  *
- * Succeeds if the bio doesn't have inline encryption enabled or if the bio
- * crypt context provided for the bio is supported by the underlying device's
- * inline encryption hardware. Ends the bio with error otherwise.
+ * If the bio crypt context provided for the bio is supported by the underlying
+ * device's inline encryption hardware, do nothing.
+ *
+ * Otherwise, try to perform en/decryption for this bio by falling back to the
+ * kernel crypto API. When the crypto API fallback is used for encryption,
+ * blk-crypto may choose to split the bio into 2 - the first one that will
+ * continue to be processed and the second one that will be resubmitted via
+ * generic_make_request. A bounce bio will be allocated to encrypt the contents
+ * of the aforementioned "first one", and *bio_ptr will be updated to this
+ * bounce bio.
  *
  * Caller must ensure bio has bio_crypt_ctx.
  *
@@ -268,16 +284,16 @@  int blk_crypto_bio_prep(struct bio **bio_ptr)
 		goto fail;
 
 	/*
-	 * Success if device supports the encryption context, and blk-integrity
-	 * isn't supported by device/is turned off.
+	 * Success if device supports the encryption context, or we succeeded
+	 * in falling back to the crypto API.
 	 */
-	if (!blk_ksm_crypto_key_supported(bio->bi_disk->queue->ksm,
-					  bio->bi_crypt_context->bc_key)) {
-		bio->bi_status = BLK_STS_NOTSUPP;
-		goto fail;
-	}
+	if (blk_ksm_crypto_key_supported(bio->bi_disk->queue->ksm,
+					 bio->bi_crypt_context->bc_key))
+		return 0;
 
-	return 0;
+	blk_crypto_fallback_bio_prep(bio_ptr);
+	if ((*bio_ptr)->bi_status == BLK_STS_OK)
+		return 0;
 fail:
 	bio_endio(*bio_ptr);
 	return -EIO;
@@ -301,6 +317,10 @@  void blk_crypto_rq_bio_prep(struct request *rq, struct bio *bio)
 
 void blk_crypto_rq_prep_clone(struct request *dst, struct request *src)
 {
+	/* Don't clone crypto info if src uses fallback en/decryption */
+	if (!src->crypt_keyslot)
+		return;
+
 	dst->crypt_ctx = src->crypt_ctx;
 }
 
@@ -395,6 +415,7 @@  int blk_crypto_init_key(struct blk_crypto_key *blk_key, const u8 *raw_key,
  * is evicted from hardware that it might have been programmed into. This
  * will call blk_ksm_evict_key on the queue's keyslot manager, if one
  * exists, and supports the crypto algorithm with the specified data unit size.
+ * Otherwise, it will evict the key from the blk-crypto-fallback's ksm.
  *
  * Return: 0 on success or if key is not present in the q's ksm, -err on error.
  */
@@ -404,5 +425,5 @@  int blk_crypto_evict_key(struct request_queue *q,
 	if (q->ksm && blk_ksm_crypto_key_supported(q->ksm, key))
 		return blk_ksm_evict_key(q->ksm, key);
 
-	return 0;
+	return blk_crypto_fallback_evict_key(key);
 }
diff --git a/include/linux/blk-crypto.h b/include/linux/blk-crypto.h
index 90a28df26106..9c54bc34044e 100644
--- a/include/linux/blk-crypto.h
+++ b/include/linux/blk-crypto.h
@@ -58,7 +58,7 @@  struct blk_crypto_key {
  *
  * A bio_crypt_ctx specifies that the contents of the bio will be encrypted (for
  * write requests) or decrypted (for read requests) inline by the storage device
- * or controller.
+ * or controller, or by the crypto API fallback.
  */
 struct bio_crypt_ctx {
 	const struct blk_crypto_key	*bc_key;
@@ -110,6 +110,21 @@  static inline void bio_crypt_clone(struct bio *dst, struct bio *src,
 
 #endif /* CONFIG_BLK_INLINE_ENCRYPTION */
 
+#ifdef CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK
+
+int blk_crypto_start_using_key(struct blk_crypto_key *key,
+			       struct request_queue *q);
+
+#else /* CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK */
+
+static inline int blk_crypto_start_using_key(struct blk_crypto_key *key,
+					     struct request_queue *q)
+{
+	return 0;
+}
+
+#endif /* CONFIG_BLK_INLINE_ENCRYPTION_FALLBACK */
+
 #endif /* CONFIG_BLOCK */
 
 #endif /* __LINUX_BLK_CRYPTO_H */