Message-ID: <CAMj1kXGWzceNGV9OfML2NL5f3iyFktX+yhJGri00MdGX=QJJ=g@mail.gmail.com>
Date: Wed, 9 Jul 2025 17:51:58 +1000
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, 
	Gilad Ben-Yossef <gilad@...yossef.com>, "Jason A . Donenfeld" <Jason@...c4.com>
Subject: Re: [PATCH] dm-verity: remove support for asynchronous hashes

On Wed, 9 Jul 2025 at 16:48, Eric Biggers <ebiggers@...nel.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-ebiggers@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 <ebiggers@...nel.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 <ardb@...nel.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
>

Powered by blists - more mailing lists