On Wed, 9 Jul 2025 at 16:48, Eric Biggers <ebigg...@kernel.org> wrote:
>
> The support for asynchronous hashes in dm-verity has outlived its
> usefulness. It adds significant code complexity and opportunity for
> bugs. I don't know of anyone using it practice. (The original
> submitter of the code possibly was, but that was 8 years ago.) Data I
> recently collected for en/decryption shows that using off-CPU crypto
> "accelerators" is consistently much slower than the CPU
> (https://lore.kernel.org/r/20250704070322.20692-1-ebigg...@kernel.org/),
> even on CPUs that lack dedicated cryptographic instructions. Similar
> results are likely to be seen for hashing.
>
> I already removed support for asynchronous hashes from fsverity two
> years ago, and no one ever complained.
>
> Moreover, neither dm-verity, fsverity, nor fscrypt has ever actually
> used the asynchronous crypto algorithms in a truly asynchronous manner.
> The lack of interest in such optimizations provides further evidence
> that it's only the CPU-based crypto that actually matters.
>
> Historically, it's also been common for people to forget to enable the
> optimized SHA-256 code, which could contribute to an off-CPU crypto
> engine being perceived as more useful than it really is. In 6.16 I
> fixed that: the optimized SHA-256 code is now enabled by default.
>
> Therefore, let's drop the support for asynchronous hashes in dm-verity.
>
> Tested with verity-compat-test.
>
> Signed-off-by: Eric Biggers <ebigg...@kernel.org>
> ---
> drivers/md/dm-verity-target.c | 175 +++++-----------------------------
> drivers/md/dm-verity.h | 20 ++--
> 2 files changed, 30 insertions(+), 165 deletions(-)
>
The diffstat speaks for itself
Acked-by: Ard Biesheuvel <a...@kernel.org>
> diff --git a/drivers/md/dm-verity-target.c b/drivers/md/dm-verity-target.c
> index 81186bded1ce7..f94d0b6bd6ba0 100644
> --- a/drivers/md/dm-verity-target.c
> +++ b/drivers/md/dm-verity-target.c
> @@ -17,11 +17,10 @@
> #include "dm-verity-fec.h"
> #include "dm-verity-verify-sig.h"
> #include "dm-audit.h"
> #include <linux/module.h>
> #include <linux/reboot.h>
> -#include <linux/scatterlist.h>
> #include <linux/string.h>
> #include <linux/jump_label.h>
> #include <linux/security.h>
>
> #define DM_MSG_PREFIX "verity"
> @@ -59,13 +58,10 @@ static unsigned int dm_verity_use_bh_bytes[4] = {
>
> module_param_array_named(use_bh_bytes, dm_verity_use_bh_bytes, uint, NULL,
> 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;
> @@ -116,104 +112,25 @@ 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_ahash_update(struct dm_verity *v, struct ahash_request
> *req,
> - const u8 *data, size_t len,
> - struct crypto_wait *wait)
> -{
> - struct scatterlist sg;
> -
> - if (likely(!is_vmalloc_addr(data))) {
> - sg_init_one(&sg, data, len);
> - ahash_request_set_crypt(req, &sg, NULL, len);
> - return crypto_wait_req(crypto_ahash_update(req), wait);
> - }
> -
> - do {
> - int r;
> - size_t this_step = min_t(size_t, len, PAGE_SIZE -
> offset_in_page(data));
> -
> - flush_kernel_vmap_range((void *)data, this_step);
> - sg_init_table(&sg, 1);
> - sg_set_page(&sg, vmalloc_to_page(data), this_step,
> offset_in_page(data));
> - ahash_request_set_crypt(req, &sg, NULL, this_step);
> - r = crypto_wait_req(crypto_ahash_update(req), wait);
> - if (unlikely(r))
> - return r;
> - data += this_step;
> - len -= this_step;
> - } while (len);
> -
> - return 0;
> -}
> -
> -/*
> - * Wrapper for crypto_ahash_init, which handles verity salting.
> - */
> -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->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);
> -
> - r = crypto_wait_req(crypto_ahash_init(req), wait);
> -
> - if (unlikely(r < 0)) {
> - if (r != -ENOMEM)
> - DMERR("crypto_ahash_init failed: %d", r);
> - return r;
> - }
> -
> - if (likely(v->salt_size && (v->version >= 1)))
> - r = verity_ahash_update(v, req, v->salt, v->salt_size, wait);
> -
> - return r;
> -}
> -
> -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_ahash_update(v, req, v->salt, v->salt_size, wait);
> -
> - if (r < 0) {
> - DMERR("%s failed updating salt: %d", __func__, r);
> - goto out;
> - }
> - }
> -
> - ahash_request_set_crypt(req, NULL, digest, 0);
> - r = crypto_wait_req(crypto_ahash_final(req), wait);
> -out:
> - return r;
> -}
> -
> int verity_hash(struct dm_verity *v, struct dm_verity_io *io,
> const u8 *data, size_t len, u8 *digest, bool may_sleep)
> {
> + struct shash_desc *desc = &io->hash_desc;
> int r;
>
> - if (static_branch_unlikely(&ahash_enabled) && !v->shash_tfm) {
> - struct ahash_request *req = verity_io_hash_req(v, io);
> - struct crypto_wait 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);
> + desc->tfm = v->shash_tfm;
> + if (unlikely(v->initial_hashstate == NULL)) {
> + /* version 0: salt at end */
> + r = crypto_shash_init(desc) ?:
> + crypto_shash_update(desc, data, len) ?:
> + crypto_shash_update(desc, v->salt, v->salt_size) ?:
> + crypto_shash_final(desc, digest);
> } else {
> - struct shash_desc *desc = verity_io_hash_req(v, io);
> -
> - desc->tfm = v->shash_tfm;
> + /* version 1: salt at beginning */
> r = crypto_shash_import(desc, v->initial_hashstate) ?:
> crypto_shash_finup(desc, data, len, digest);
> }
> if (unlikely(r))
> DMERR("Error hashing block: %d", r);
> @@ -1090,16 +1007,11 @@ static void verity_dtr(struct dm_target *ti)
> kfree(v->initial_hashstate);
> kfree(v->root_digest);
> kfree(v->zero_digest);
> verity_free_sig(v);
>
> - if (v->ahash_tfm) {
> - static_branch_dec(&ahash_enabled);
> - crypto_free_ahash(v->ahash_tfm);
> - } else {
> - crypto_free_shash(v->shash_tfm);
> - }
> + crypto_free_shash(v->shash_tfm);
>
> kfree(v->alg_name);
>
> if (v->hash_dev)
> dm_put_device(ti, v->hash_dev);
> @@ -1155,11 +1067,12 @@ 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->hash_reqsize, GFP_KERNEL);
> + io = kmalloc(sizeof(*io) + crypto_shash_descsize(v->shash_tfm),
> + GFP_KERNEL);
>
> if (!io)
> return r; /* verity_dtr will free zero_digest */
>
> zero_data = kzalloc(1 << v->data_dev_block_bits, GFP_KERNEL);
> @@ -1322,74 +1235,37 @@ 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;
> + struct crypto_shash *shash;
>
> 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)) {
> + shash = crypto_alloc_shash(alg_name, 0, 0);
> + if (IS_ERR(shash)) {
> ti->error = "Cannot initialize hash function";
> - return PTR_ERR(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);
> + return PTR_ERR(shash);
> }
> + v->shash_tfm = shash;
> + v->digest_size = crypto_shash_digestsize(shash);
> + DMINFO("%s using \"%s\"", alg_name, crypto_shash_driver_name(shash));
> if ((1 << v->hash_dev_block_bits) < v->digest_size * 2) {
> ti->error = "Digest size too big";
> return -EINVAL;
> }
> return 0;
> }
>
> static int verity_setup_salt_and_hashstate(struct dm_verity *v, const char
> *arg)
> {
> struct dm_target *ti = v->ti;
> + SHASH_DESC_ON_STACK(desc, v->shash_tfm);
>
> if (strcmp(arg, "-") != 0) {
> v->salt_size = strlen(arg) / 2;
> v->salt = kmalloc(v->salt_size, GFP_KERNEL);
> if (!v->salt) {
> @@ -1400,12 +1276,11 @@ static int verity_setup_salt_and_hashstate(struct
> dm_verity *v, const char *arg)
> 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);
> + if (v->version) {
> int r;
>
> /*
> * Compute the pre-salted hash state that can be passed to
> * crypto_shash_import() for each block later.
> @@ -1679,11 +1554,12 @@ 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->hash_reqsize;
> + ti->per_io_data_size = sizeof(struct dm_verity_io) +
> + crypto_shash_descsize(v->shash_tfm);
>
> r = verity_fec_ctr(v);
> if (r)
> goto bad;
>
> @@ -1786,14 +1662,11 @@ static int verity_preresume(struct dm_target *ti)
>
> v = ti->private;
> bdev = dm_disk(dm_table_get_md(ti->table))->part0;
> root_digest.digest = v->root_digest;
> root_digest.digest_len = v->digest_size;
> - if (static_branch_unlikely(&ahash_enabled) && !v->shash_tfm)
> - root_digest.alg = crypto_ahash_alg_name(v->ahash_tfm);
> - else
> - root_digest.alg = crypto_shash_alg_name(v->shash_tfm);
> + root_digest.alg = crypto_shash_alg_name(v->shash_tfm);
>
> r = security_bdev_setintegrity(bdev, LSM_INT_DMVERITY_ROOTHASH,
> &root_digest,
> sizeof(root_digest));
> if (r)
> return r;
> diff --git a/drivers/md/dm-verity.h b/drivers/md/dm-verity.h
> index 8cbb57862ae19..d6a0e912f2e18 100644
> --- a/drivers/md/dm-verity.h
> +++ b/drivers/md/dm-verity.h
> @@ -37,15 +37,14 @@ 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 *ahash_tfm; /* either this or shash_tfm is set */
> - struct crypto_shash *shash_tfm; /* either this or ahash_tfm is set */
> + struct crypto_shash *shash_tfm;
> 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 *initial_hashstate; /* salted initial state, if version >= 1 */
> u8 *zero_digest; /* digest for a zero block */
> #ifdef CONFIG_SECURITY
> u8 *root_digest_sig; /* signature of the root digest */
> unsigned int sig_size; /* root digest signature size */
> #endif /* CONFIG_SECURITY */
> @@ -59,11 +58,10 @@ 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 hash_reqsize; /* the size of temporary space for crypto
> */
> enum verity_mode mode; /* mode for handling verification errors */
> enum verity_mode error_mode;/* mode for handling I/O errors */
> unsigned int corrupted_errs;/* Number of errors for corrupted blocks
> */
>
> struct workqueue_struct *verify_wq;
> @@ -98,23 +96,17 @@ struct dm_verity_io {
>
> u8 real_digest[HASH_MAX_DIGESTSIZE];
> u8 want_digest[HASH_MAX_DIGESTSIZE];
>
> /*
> - * 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().
> + * Temporary space for hashing. This is variable-length and must be
> at
> + * the end of the struct. struct shash_desc is just the fixed part;
> + * it's followed by a context of size
> crypto_shash_descsize(shash_tfm).
> */
> + struct shash_desc hash_desc;
> };
>
> -static inline void *verity_io_hash_req(struct dm_verity *v,
> - struct dm_verity_io *io)
> -{
> - return io + 1;
> -}
> -
> static inline u8 *verity_io_real_digest(struct dm_verity *v,
> struct dm_verity_io *io)
> {
> return io->real_digest;
> }
>
> base-commit: 846e9e999dd36ce5898d302d674e441e72c3a8cf
> --
> 2.50.1
>
