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Message-ID: <CAMj1kXFE45J9fDJ9XtMyJiUP1F-odZUmH0zLKkuZeWL_bLdDZQ@mail.gmail.com>
Date: Tue, 2 Jul 2024 09:41:19 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: dm-devel@...ts.linux.dev, Alasdair Kergon <agk@...hat.com>,
Mike Snitzer <snitzer@...nel.org>, Mikulas Patocka <mpatocka@...hat.com>, linux-kernel@...r.kernel.org,
Sami Tolvanen <samitolvanen@...gle.com>, Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH 7/7] dm-verity: hash blocks with shash import+finup when possible
Hi Eric,
One question below.
On Tue, 2 Jul 2024 at 08:49, Eric Biggers <ebiggers@...nel.org> wrote:
>
> From: Eric Biggers <ebiggers@...gle.com>
>
> Currently dm-verity computes the hash of each block by using multiple
> calls to the "ahash" crypto API. While the exact sequence depends on
> the chosen dm-verity settings, in the vast majority of cases it is:
>
> 1. crypto_ahash_init()
> 2. crypto_ahash_update() [salt]
> 3. crypto_ahash_update() [data]
> 4. crypto_ahash_final()
>
> This is inefficient for two main reasons:
>
> - It makes multiple indirect calls, which is expensive on modern CPUs
> especially when mitigations for CPU vulnerabilities are enabled.
>
> Since the salt is the same across all blocks on a given dm-verity
> device, a much more efficient sequence would be to do an import of the
> pre-salted state, then a finup.
>
> - It uses the ahash (asynchronous hash) API, despite the fact that
> CPU-based hashing is almost always used in practice, and therefore it
> experiences the overhead of the ahash-based wrapper for shash.
>
> Because dm-verity was intentionally converted to ahash to support
> off-CPU crypto accelerators, a full reversion to shash might not be
> acceptable. Yet, we should still provide a fast path for shash with
> the most common dm-verity settings.
>
> Another reason for shash over ahash is that the upcoming multibuffer
> hashing support, which is specific to CPU-based hashing, is much
> better suited for shash than for ahash. Supporting it via ahash would
> add significant complexity and overhead. And it's not possible for
> the "same" code to properly support both multibuffer hashing and HW
> accelerators at the same time anyway, given the different computation
> models. Unfortunately there will always be code specific to each
> model needed (for users who want to support both).
>
> Therefore, this patch adds a new shash import+finup based fast path to
> dm-verity. It is used automatically when appropriate. This makes
> dm-verity optimized for what the vast majority of users want: CPU-based
> hashing with the most common settings, while still retaining support for
> rarer settings and off-CPU crypto accelerators.
>
> In benchmarks with veritysetup's default parameters (SHA-256, 4K data
> and hash block sizes, 32-byte salt), which also match the parameters
> that Android currently uses, this patch improves block hashing
> performance by about 15% on x86_64 using the SHA-NI instructions, or by
> about 5% on arm64 using the ARMv8 SHA2 instructions. On x86_64 roughly
> two-thirds of the improvement comes from the use of import and finup,
> while the remaining third comes from the switch from ahash to shash.
>
> Note that another benefit of using "import" to handle the salt is that
> if the salt size is equal to the input size of the hash algorithm's
> compression function, e.g. 64 bytes for SHA-256, then the performance is
> exactly the same as no salt. This doesn't seem to be much better than
> veritysetup's current default of 32-byte salts, due to the way SHA-256's
> finalization padding works, but it should be marginally better.
>
> Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>
> Acked-by: Ard Biesheuvel <ardb@...nel.org>
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
> drivers/md/dm-verity-target.c | 169 ++++++++++++++++++++++++----------
> drivers/md/dm-verity.h | 18 ++--
> 2 files changed, 130 insertions(+), 57 deletions(-)
>
> diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> index 4aa140751166..d16c51958465 100644
> --- a/drivers/md/dm-verity-target.c
> +++ b/drivers/md/dm-verity-target.c
> @@ -46,10 +46,13 @@ static unsigned int dm_verity_prefetch_cluster = DM_VERITY_DEFAULT_PREFETCH_SIZE
>
> module_param_named(prefetch_cluster, dm_verity_prefetch_cluster, uint, 0644);
>
> static DEFINE_STATIC_KEY_FALSE(use_bh_wq_enabled);
>
> +/* Is at least one dm-verity instance using ahash_tfm instead of shash_tfm? */
> +static DEFINE_STATIC_KEY_FALSE(ahash_enabled);
> +
> struct dm_verity_prefetch_work {
> struct work_struct work;
> struct dm_verity *v;
> unsigned short ioprio;
> sector_t block;
> @@ -100,11 +103,11 @@ static sector_t verity_position_at_level(struct dm_verity *v, sector_t block,
> int level)
> {
> return block >> (level * v->hash_per_block_bits);
> }
>
> -static int verity_hash_update(struct dm_verity *v, struct ahash_request *req,
> +static int verity_ahash_update(struct dm_verity *v, struct ahash_request *req,
> const u8 *data, size_t len,
> struct crypto_wait *wait)
> {
> struct scatterlist sg;
>
> @@ -133,16 +136,16 @@ static int verity_hash_update(struct dm_verity *v, struct ahash_request *req,
> }
>
> /*
> * Wrapper for crypto_ahash_init, which handles verity salting.
> */
> -static int verity_hash_init(struct dm_verity *v, struct ahash_request *req,
> +static int verity_ahash_init(struct dm_verity *v, struct ahash_request *req,
> struct crypto_wait *wait, bool may_sleep)
> {
> int r;
>
> - ahash_request_set_tfm(req, v->tfm);
> + ahash_request_set_tfm(req, v->ahash_tfm);
> ahash_request_set_callback(req,
> may_sleep ? CRYPTO_TFM_REQ_MAY_SLEEP | CRYPTO_TFM_REQ_MAY_BACKLOG : 0,
> crypto_req_done, (void *)wait);
> crypto_init_wait(wait);
>
> @@ -153,22 +156,22 @@ static int verity_hash_init(struct dm_verity *v, struct ahash_request *req,
> DMERR("crypto_ahash_init failed: %d", r);
> return r;
> }
>
> if (likely(v->salt_size && (v->version >= 1)))
> - r = verity_hash_update(v, req, v->salt, v->salt_size, wait);
> + r = verity_ahash_update(v, req, v->salt, v->salt_size, wait);
>
> return r;
> }
>
> -static int verity_hash_final(struct dm_verity *v, struct ahash_request *req,
> - u8 *digest, struct crypto_wait *wait)
> +static int verity_ahash_final(struct dm_verity *v, struct ahash_request *req,
> + u8 *digest, struct crypto_wait *wait)
> {
> int r;
>
> if (unlikely(v->salt_size && (!v->version))) {
> - r = verity_hash_update(v, req, v->salt, v->salt_size, wait);
> + r = verity_ahash_update(v, req, v->salt, v->salt_size, wait);
>
> if (r < 0) {
> DMERR("%s failed updating salt: %d", __func__, r);
> goto out;
> }
> @@ -181,25 +184,28 @@ static int verity_hash_final(struct dm_verity *v, struct ahash_request *req,
> }
>
> int verity_hash(struct dm_verity *v, struct dm_verity_io *io,
> const u8 *data, size_t len, u8 *digest, bool may_sleep)
> {
> - struct ahash_request *req = verity_io_hash_req(v, io);
> int r;
> - struct crypto_wait wait;
> -
> - r = verity_hash_init(v, req, &wait, may_sleep);
> - if (unlikely(r < 0))
> - goto out;
>
> - r = verity_hash_update(v, req, data, len, &wait);
> - if (unlikely(r < 0))
> - goto out;
> + if (static_branch_unlikely(&ahash_enabled) && !v->shash_tfm) {
Is the static key really worth the hassle? Couldn't this just be
if (unlikely(!v->shash_tfm)) {
so that the ahash logic is moved to the cold path? We need to
dereference v->shash_tfm right away in any case, and if it is never
NULL, the branch predictor should be able to remember that.
> + struct ahash_request *req = verity_io_hash_req(v, io);
> + struct crypto_wait wait;
>
> - r = verity_hash_final(v, req, digest, &wait);
> + r = verity_ahash_init(v, req, &wait, may_sleep) ?:
> + verity_ahash_update(v, req, data, len, &wait) ?:
> + verity_ahash_final(v, req, digest, &wait);
> + } else {
> + struct shash_desc *desc = verity_io_hash_req(v, io);
>
> -out:
> + desc->tfm = v->shash_tfm;
> + r = crypto_shash_import(desc, v->initial_hashstate) ?:
> + crypto_shash_finup(desc, data, len, digest);
> + }
> + if (unlikely(r))
> + DMERR("Error hashing block: %d", r);
> return r;
> }
>
> static void verity_hash_at_level(struct dm_verity *v, sector_t block, int level,
> sector_t *hash_block, unsigned int *offset)
> @@ -932,15 +938,20 @@ static void verity_dtr(struct dm_target *ti)
> if (v->bufio)
> dm_bufio_client_destroy(v->bufio);
>
> kvfree(v->validated_blocks);
> kfree(v->salt);
> + kfree(v->initial_hashstate);
> kfree(v->root_digest);
> kfree(v->zero_digest);
>
> - if (v->tfm)
> - crypto_free_ahash(v->tfm);
> + if (v->ahash_tfm) {
> + static_branch_dec(&ahash_enabled);
> + crypto_free_ahash(v->ahash_tfm);
> + } else {
> + crypto_free_shash(v->shash_tfm);
> + }
>
> kfree(v->alg_name);
>
> if (v->hash_dev)
> dm_put_device(ti, v->hash_dev);
> @@ -990,11 +1001,11 @@ static int verity_alloc_zero_digest(struct dm_verity *v)
> v->zero_digest = kmalloc(v->digest_size, GFP_KERNEL);
>
> if (!v->zero_digest)
> return r;
>
> - io = kmalloc(sizeof(*io) + v->ahash_reqsize, GFP_KERNEL);
> + io = kmalloc(sizeof(*io) + v->hash_reqsize, GFP_KERNEL);
>
> if (!io)
> return r; /* verity_dtr will free zero_digest */
>
> zero_data = kzalloc(1 << v->data_dev_block_bits, GFP_KERNEL);
> @@ -1129,40 +1140,110 @@ static int verity_parse_opt_args(struct dm_arg_set *as, struct dm_verity *v,
>
> static int verity_setup_hash_alg(struct dm_verity *v, const char *alg_name)
> {
> struct dm_target *ti = v->ti;
> struct crypto_ahash *ahash;
> + struct crypto_shash *shash = NULL;
> + const char *driver_name;
>
> v->alg_name = kstrdup(alg_name, GFP_KERNEL);
> if (!v->alg_name) {
> ti->error = "Cannot allocate algorithm name";
> return -ENOMEM;
> }
>
> + /*
> + * Allocate the hash transformation object that this dm-verity instance
> + * will use. The vast majority of dm-verity users use CPU-based
> + * hashing, so when possible use the shash API to minimize the crypto
> + * API overhead. If the ahash API resolves to a different driver
> + * (likely an off-CPU hardware offload), use ahash instead. Also use
> + * ahash if the obsolete dm-verity format with the appended salt is
> + * being used, so that quirk only needs to be handled in one place.
> + */
> ahash = crypto_alloc_ahash(alg_name, 0,
> v->use_bh_wq ? CRYPTO_ALG_ASYNC : 0);
> if (IS_ERR(ahash)) {
> ti->error = "Cannot initialize hash function";
> return PTR_ERR(ahash);
> }
> - v->tfm = ahash;
> -
> - /*
> - * dm-verity performance can vary greatly depending on which hash
> - * algorithm implementation is used. Help people debug performance
> - * problems by logging the ->cra_driver_name.
> - */
> - DMINFO("%s using implementation \"%s\"", alg_name,
> - crypto_hash_alg_common(ahash)->base.cra_driver_name);
> -
> - v->digest_size = crypto_ahash_digestsize(ahash);
> + driver_name = crypto_ahash_driver_name(ahash);
> + if (v->version >= 1 /* salt prepended, not appended? */) {
> + shash = crypto_alloc_shash(alg_name, 0, 0);
> + if (!IS_ERR(shash) &&
> + strcmp(crypto_shash_driver_name(shash), driver_name) != 0) {
> + /*
> + * ahash gave a different driver than shash, so probably
> + * this is a case of real hardware offload. Use ahash.
> + */
> + crypto_free_shash(shash);
> + shash = NULL;
> + }
> + }
> + if (!IS_ERR_OR_NULL(shash)) {
> + crypto_free_ahash(ahash);
> + ahash = NULL;
> + v->shash_tfm = shash;
> + v->digest_size = crypto_shash_digestsize(shash);
> + v->hash_reqsize = sizeof(struct shash_desc) +
> + crypto_shash_descsize(shash);
> + DMINFO("%s using shash \"%s\"", alg_name, driver_name);
> + } else {
> + v->ahash_tfm = ahash;
> + static_branch_inc(&ahash_enabled);
> + v->digest_size = crypto_ahash_digestsize(ahash);
> + v->hash_reqsize = sizeof(struct ahash_request) +
> + crypto_ahash_reqsize(ahash);
> + DMINFO("%s using ahash \"%s\"", alg_name, driver_name);
> + }
> if ((1 << v->hash_dev_block_bits) < v->digest_size * 2) {
> ti->error = "Digest size too big";
> return -EINVAL;
> }
> - v->ahash_reqsize = sizeof(struct ahash_request) +
> - crypto_ahash_reqsize(ahash);
> + return 0;
> +}
> +
> +static int verity_setup_salt_and_hashstate(struct dm_verity *v, const char *arg)
> +{
> + struct dm_target *ti = v->ti;
> +
> + if (strcmp(arg, "-") != 0) {
> + v->salt_size = strlen(arg) / 2;
> + v->salt = kmalloc(v->salt_size, GFP_KERNEL);
> + if (!v->salt) {
> + ti->error = "Cannot allocate salt";
> + return -ENOMEM;
> + }
> + if (strlen(arg) != v->salt_size * 2 ||
> + hex2bin(v->salt, arg, v->salt_size)) {
> + ti->error = "Invalid salt";
> + return -EINVAL;
> + }
> + }
> + if (v->shash_tfm) {
> + SHASH_DESC_ON_STACK(desc, v->shash_tfm);
> + int r;
> +
> + /*
> + * Compute the pre-salted hash state that can be passed to
> + * crypto_shash_import() for each block later.
> + */
> + v->initial_hashstate = kmalloc(
> + crypto_shash_statesize(v->shash_tfm), GFP_KERNEL);
> + if (!v->initial_hashstate) {
> + ti->error = "Cannot allocate initial hash state";
> + return -ENOMEM;
> + }
> + desc->tfm = v->shash_tfm;
> + r = crypto_shash_init(desc) ?:
> + crypto_shash_update(desc, v->salt, v->salt_size) ?:
> + crypto_shash_export(desc, v->initial_hashstate);
> + if (r) {
> + ti->error = "Cannot set up initial hash state";
> + return r;
> + }
> + }
> return 0;
> }
>
> /*
> * Target parameters:
> @@ -1304,25 +1385,13 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
> r = -EINVAL;
> goto bad;
> }
> root_hash_digest_to_validate = argv[8];
>
> - if (strcmp(argv[9], "-")) {
> - v->salt_size = strlen(argv[9]) / 2;
> - v->salt = kmalloc(v->salt_size, GFP_KERNEL);
> - if (!v->salt) {
> - ti->error = "Cannot allocate salt";
> - r = -ENOMEM;
> - goto bad;
> - }
> - if (strlen(argv[9]) != v->salt_size * 2 ||
> - hex2bin(v->salt, argv[9], v->salt_size)) {
> - ti->error = "Invalid salt";
> - r = -EINVAL;
> - goto bad;
> - }
> - }
> + r = verity_setup_salt_and_hashstate(v, argv[9]);
> + if (r)
> + goto bad;
>
> argv += 10;
> argc -= 10;
>
> /* Optional parameters */
> @@ -1420,11 +1489,11 @@ static int verity_ctr(struct dm_target *ti, unsigned int argc, char **argv)
> ti->error = "Cannot allocate workqueue";
> r = -ENOMEM;
> goto bad;
> }
>
> - ti->per_io_data_size = sizeof(struct dm_verity_io) + v->ahash_reqsize;
> + ti->per_io_data_size = sizeof(struct dm_verity_io) + v->hash_reqsize;
>
> r = verity_fec_ctr(v);
> if (r)
> goto bad;
>
> diff --git a/drivers/md/dm-verity.h b/drivers/md/dm-verity.h
> index 0e1dd02a916f..aac3a1b1d94a 100644
> --- a/drivers/md/dm-verity.h
> +++ b/drivers/md/dm-verity.h
> @@ -37,13 +37,15 @@ struct dm_verity {
> struct dm_dev *data_dev;
> struct dm_dev *hash_dev;
> struct dm_target *ti;
> struct dm_bufio_client *bufio;
> char *alg_name;
> - struct crypto_ahash *tfm;
> + struct crypto_ahash *ahash_tfm; /* either this or shash_tfm is set */
> + struct crypto_shash *shash_tfm; /* either this or ahash_tfm is set */
> u8 *root_digest; /* digest of the root block */
> u8 *salt; /* salt: its size is salt_size */
> + u8 *initial_hashstate; /* salted initial state, if shash_tfm is set */
> u8 *zero_digest; /* digest for a zero block */
> unsigned int salt_size;
> sector_t data_start; /* data offset in 512-byte sectors */
> sector_t hash_start; /* hash start in blocks */
> sector_t data_blocks; /* the number of data blocks */
> @@ -54,11 +56,11 @@ struct dm_verity {
> unsigned char levels; /* the number of tree levels */
> unsigned char version;
> bool hash_failed:1; /* set if hash of any block failed */
> bool use_bh_wq:1; /* try to verify in BH wq before normal work-queue */
> unsigned int digest_size; /* digest size for the current hash algorithm */
> - unsigned int ahash_reqsize;/* the size of temporary space for crypto */
> + unsigned int hash_reqsize; /* the size of temporary space for crypto */
> enum verity_mode mode; /* mode for handling verification errors */
> unsigned int corrupted_errs;/* Number of errors for corrupted blocks */
>
> struct workqueue_struct *verify_wq;
>
> @@ -91,19 +93,21 @@ struct dm_verity_io {
>
> u8 real_digest[HASH_MAX_DIGESTSIZE];
> u8 want_digest[HASH_MAX_DIGESTSIZE];
>
> /*
> - * This struct is followed by a variable-sized struct ahash_request of
> - * size v->ahash_reqsize. To access it, use verity_io_hash_req().
> + * This struct is followed by a variable-sized hash request of size
> + * v->hash_reqsize, either a struct ahash_request or a struct shash_desc
> + * (depending on whether ahash_tfm or shash_tfm is being used). To
> + * access it, use verity_io_hash_req().
> */
> };
>
> -static inline struct ahash_request *verity_io_hash_req(struct dm_verity *v,
> - struct dm_verity_io *io)
> +static inline void *verity_io_hash_req(struct dm_verity *v,
> + struct dm_verity_io *io)
> {
> - return (struct ahash_request *)(io + 1);
> + return io + 1;
> }
>
> static inline u8 *verity_io_real_digest(struct dm_verity *v,
> struct dm_verity_io *io)
> {
> --
> 2.45.2
>
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