Message-ID: <20250630172224.46909-3-ebiggers@kernel.org>
Date: Mon, 30 Jun 2025 10:22:24 -0700
From: Eric Biggers <ebiggers@...nel.org>
To: fsverity@...ts.linux.dev
Cc: linux-crypto@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net,
	linux-btrfs@...r.kernel.org,
	Ard Biesheuvel <ardb@...nel.org>,
	"Jason A . Donenfeld" <Jason@...c4.com>,
	Theodore Ts'o <tytso@....edu>,
	Eric Biggers <ebiggers@...nel.org>
Subject: [PATCH 2/2] fsverity: Switch from crypto_shash to SHA-2 library

fsverity supports two hash algorithms: SHA-256 and SHA-512.  Since both
of these have a library API now, just use the library API instead of
crypto_shash.  Even with multiple algorithms, the library-based code
still ends up being quite a bit simpler, due to how clumsy the
old-school crypto API is.  The library-based code is also more
efficient, since it avoids overheads such as indirect calls.

Signed-off-by: Eric Biggers <ebiggers@...nel.org>
---
 Documentation/filesystems/fsverity.rst |   3 +-
 fs/verity/Kconfig                      |   6 +-
 fs/verity/enable.c                     |   8 +-
 fs/verity/fsverity_private.h           |  24 +--
 fs/verity/hash_algs.c                  | 194 +++++++++----------------
 fs/verity/open.c                       |  36 ++---
 fs/verity/verify.c                     |   7 +-
 7 files changed, 102 insertions(+), 176 deletions(-)

diff --git a/Documentation/filesystems/fsverity.rst b/Documentation/filesystems/fsverity.rst
index dacdbc1149e67..412cf11e32985 100644
--- a/Documentation/filesystems/fsverity.rst
+++ b/Documentation/filesystems/fsverity.rst
@@ -183,12 +183,11 @@ FS_IOC_ENABLE_VERITY can fail with the following errors:
 - ``EKEYREJECTED``: the builtin signature doesn't match the file
 - ``EMSGSIZE``: the salt or builtin signature is too long
 - ``ENOKEY``: the ".fs-verity" keyring doesn't contain the certificate
   needed to verify the builtin signature
 - ``ENOPKG``: fs-verity recognizes the hash algorithm, but it's not
-  available in the kernel's crypto API as currently configured (e.g.
-  for SHA-512, missing CONFIG_CRYPTO_SHA512).
+  available in the kernel as currently configured
 - ``ENOTTY``: this type of filesystem does not implement fs-verity
 - ``EOPNOTSUPP``: the kernel was not configured with fs-verity
   support; or the filesystem superblock has not had the 'verity'
   feature enabled on it; or the filesystem does not support fs-verity
   on this file.  (See `Filesystem support`_.)
diff --git a/fs/verity/Kconfig b/fs/verity/Kconfig
index 40569d3527a71..76d1c5971b826 100644
--- a/fs/verity/Kconfig
+++ b/fs/verity/Kconfig
@@ -1,14 +1,12 @@
 # SPDX-License-Identifier: GPL-2.0
 
 config FS_VERITY
 	bool "FS Verity (read-only file-based authenticity protection)"
-	select CRYPTO
 	select CRYPTO_HASH_INFO
-	# SHA-256 is selected as it's intended to be the default hash algorithm.
-	# To avoid bloat, other wanted algorithms must be selected explicitly.
-	select CRYPTO_SHA256
+	select CRYPTO_LIB_SHA256
+	select CRYPTO_LIB_SHA512
 	help
 	  This option enables fs-verity.  fs-verity is the dm-verity
 	  mechanism implemented at the file level.  On supported
 	  filesystems (currently ext4, f2fs, and btrfs), userspace can
 	  use an ioctl to enable verity for a file, which causes the
diff --git a/fs/verity/enable.c b/fs/verity/enable.c
index c284f46d1b535..0816ec295c380 100644
--- a/fs/verity/enable.c
+++ b/fs/verity/enable.c
@@ -5,11 +5,10 @@
  * Copyright 2019 Google LLC
  */
 
 #include "fsverity_private.h"
 
-#include <crypto/hash.h>
 #include <linux/mount.h>
 #include <linux/sched/signal.h>
 #include <linux/uaccess.h>
 
 struct block_buffer {
@@ -22,11 +21,10 @@ struct block_buffer {
 static int hash_one_block(struct inode *inode,
 			  const struct merkle_tree_params *params,
 			  struct block_buffer *cur)
 {
 	struct block_buffer *next = cur + 1;
-	int err;
 
 	/*
 	 * Safety check to prevent a buffer overflow in case of a filesystem bug
 	 * that allows the file size to change despite deny_write_access(), or a
 	 * bug in the Merkle tree logic itself
@@ -35,14 +33,12 @@ static int hash_one_block(struct inode *inode,
 		return -EINVAL;
 
 	/* Zero-pad the block if it's shorter than the block size. */
 	memset(&cur->data[cur->filled], 0, params->block_size - cur->filled);
 
-	err = fsverity_hash_block(params, inode, cur->data,
-				  &next->data[next->filled]);
-	if (err)
-		return err;
+	fsverity_hash_block(params, inode, cur->data,
+			    &next->data[next->filled]);
 	next->filled += params->digest_size;
 	cur->filled = 0;
 	return 0;
 }
 
diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
index b3506f56e180b..5fe854a5b9ad3 100644
--- a/fs/verity/fsverity_private.h
+++ b/fs/verity/fsverity_private.h
@@ -18,25 +18,30 @@
  */
 #define FS_VERITY_MAX_LEVELS		8
 
 /* A hash algorithm supported by fs-verity */
 struct fsverity_hash_alg {
-	struct crypto_shash *tfm; /* hash tfm, allocated on demand */
 	const char *name;	  /* crypto API name, e.g. sha256 */
 	unsigned int digest_size; /* digest size in bytes, e.g. 32 for SHA-256 */
 	unsigned int block_size;  /* block size in bytes, e.g. 64 for SHA-256 */
 	/*
 	 * The HASH_ALGO_* constant for this algorithm.  This is different from
 	 * FS_VERITY_HASH_ALG_*, which uses a different numbering scheme.
 	 */
 	enum hash_algo algo_id;
 };
 
+union fsverity_hash_ctx {
+	struct sha256_ctx sha256;
+	struct sha512_ctx sha512;
+};
+
 /* Merkle tree parameters: hash algorithm, initial hash state, and topology */
 struct merkle_tree_params {
 	const struct fsverity_hash_alg *hash_alg; /* the hash algorithm */
-	const u8 *hashstate;		/* initial hash state or NULL */
+	/* initial hash state if salted, NULL if unsalted */
+	const union fsverity_hash_ctx *hashstate;
 	unsigned int digest_size;	/* same as hash_alg->digest_size */
 	unsigned int block_size;	/* size of data and tree blocks */
 	unsigned int hashes_per_block;	/* number of hashes per tree block */
 	unsigned int blocks_per_page;	/* PAGE_SIZE / block_size */
 	u8 log_digestsize;		/* log2(digest_size) */
@@ -74,20 +79,21 @@ struct fsverity_info {
 #define FS_VERITY_MAX_SIGNATURE_SIZE	(FS_VERITY_MAX_DESCRIPTOR_SIZE - \
 					 sizeof(struct fsverity_descriptor))
 
 /* hash_algs.c */
 
-extern struct fsverity_hash_alg fsverity_hash_algs[];
+extern const struct fsverity_hash_alg fsverity_hash_algs[];
 
 const struct fsverity_hash_alg *fsverity_get_hash_alg(const struct inode *inode,
 						      unsigned int num);
-const u8 *fsverity_prepare_hash_state(const struct fsverity_hash_alg *alg,
-				      const u8 *salt, size_t salt_size);
-int fsverity_hash_block(const struct merkle_tree_params *params,
-			const struct inode *inode, const void *data, u8 *out);
-int fsverity_hash_buffer(const struct fsverity_hash_alg *alg,
-			 const void *data, size_t size, u8 *out);
+union fsverity_hash_ctx *
+fsverity_prepare_hash_state(const struct fsverity_hash_alg *alg,
+			    const u8 *salt, size_t salt_size);
+void fsverity_hash_block(const struct merkle_tree_params *params,
+			 const struct inode *inode, const void *data, u8 *out);
+void fsverity_hash_buffer(const struct fsverity_hash_alg *alg,
+			  const void *data, size_t size, u8 *out);
 void __init fsverity_check_hash_algs(void);
 
 /* init.c */
 
 void __printf(3, 4) __cold
diff --git a/fs/verity/hash_algs.c b/fs/verity/hash_algs.c
index 6b08b1d9a7d7c..9bb3c6344907e 100644
--- a/fs/verity/hash_algs.c
+++ b/fs/verity/hash_algs.c
@@ -5,14 +5,12 @@
  * Copyright 2019 Google LLC
  */
 
 #include "fsverity_private.h"
 
-#include <crypto/hash.h>
-
 /* The hash algorithms supported by fs-verity */
-struct fsverity_hash_alg fsverity_hash_algs[] = {
+const struct fsverity_hash_alg fsverity_hash_algs[] = {
 	[FS_VERITY_HASH_ALG_SHA256] = {
 		.name = "sha256",
 		.digest_size = SHA256_DIGEST_SIZE,
 		.block_size = SHA256_BLOCK_SIZE,
 		.algo_id = HASH_ALGO_SHA256,
@@ -23,144 +21,76 @@ struct fsverity_hash_alg fsverity_hash_algs[] = {
 		.block_size = SHA512_BLOCK_SIZE,
 		.algo_id = HASH_ALGO_SHA512,
 	},
 };
 
-static DEFINE_MUTEX(fsverity_hash_alg_init_mutex);
-
 /**
- * fsverity_get_hash_alg() - validate and prepare a hash algorithm
+ * fsverity_get_hash_alg() - get a hash algorithm by number
  * @inode: optional inode for logging purposes
  * @num: the hash algorithm number
  *
- * Get the struct fsverity_hash_alg for the given hash algorithm number, and
- * ensure it has a hash transform ready to go.  The hash transforms are
- * allocated on-demand so that we don't waste resources unnecessarily, and
- * because the crypto modules may be initialized later than fs/verity/.
+ * Get the struct fsverity_hash_alg for the given hash algorithm number.
  *
- * Return: pointer to the hash alg on success, else an ERR_PTR()
+ * Return: pointer to the hash alg if it's known, otherwise NULL.
  */
 const struct fsverity_hash_alg *fsverity_get_hash_alg(const struct inode *inode,
 						      unsigned int num)
 {
-	struct fsverity_hash_alg *alg;
-	struct crypto_shash *tfm;
-	int err;
-
 	if (num >= ARRAY_SIZE(fsverity_hash_algs) ||
 	    !fsverity_hash_algs[num].name) {
 		fsverity_warn(inode, "Unknown hash algorithm number: %u", num);
-		return ERR_PTR(-EINVAL);
-	}
-	alg = &fsverity_hash_algs[num];
-
-	/* pairs with smp_store_release() below */
-	if (likely(smp_load_acquire(&alg->tfm) != NULL))
-		return alg;
-
-	mutex_lock(&fsverity_hash_alg_init_mutex);
-
-	if (alg->tfm != NULL)
-		goto out_unlock;
-
-	tfm = crypto_alloc_shash(alg->name, 0, 0);
-	if (IS_ERR(tfm)) {
-		if (PTR_ERR(tfm) == -ENOENT) {
-			fsverity_warn(inode,
-				      "Missing crypto API support for hash algorithm \"%s\"",
-				      alg->name);
-			alg = ERR_PTR(-ENOPKG);
-			goto out_unlock;
-		}
-		fsverity_err(inode,
-			     "Error allocating hash algorithm \"%s\": %ld",
-			     alg->name, PTR_ERR(tfm));
-		alg = ERR_CAST(tfm);
-		goto out_unlock;
+		return NULL;
 	}
-
-	err = -EINVAL;
-	if (WARN_ON_ONCE(alg->digest_size != crypto_shash_digestsize(tfm)))
-		goto err_free_tfm;
-	if (WARN_ON_ONCE(alg->block_size != crypto_shash_blocksize(tfm)))
-		goto err_free_tfm;
-
-	pr_info("%s using implementation \"%s\"\n",
-		alg->name, crypto_shash_driver_name(tfm));
-
-	/* pairs with smp_load_acquire() above */
-	smp_store_release(&alg->tfm, tfm);
-	goto out_unlock;
-
-err_free_tfm:
-	crypto_free_shash(tfm);
-	alg = ERR_PTR(err);
-out_unlock:
-	mutex_unlock(&fsverity_hash_alg_init_mutex);
-	return alg;
+	return &fsverity_hash_algs[num];
 }
 
 /**
  * fsverity_prepare_hash_state() - precompute the initial hash state
  * @alg: hash algorithm
  * @salt: a salt which is to be prepended to all data to be hashed
- * @salt_size: salt size in bytes, possibly 0
+ * @salt_size: salt size in bytes
  *
- * Return: NULL if the salt is empty, otherwise the kmalloc()'ed precomputed
- *	   initial hash state on success or an ERR_PTR() on failure.
+ * Return: the kmalloc()'ed initial hash state, or NULL if out of memory.
  */
-const u8 *fsverity_prepare_hash_state(const struct fsverity_hash_alg *alg,
-				      const u8 *salt, size_t salt_size)
+union fsverity_hash_ctx *
+fsverity_prepare_hash_state(const struct fsverity_hash_alg *alg,
+			    const u8 *salt, size_t salt_size)
 {
-	u8 *hashstate = NULL;
-	SHASH_DESC_ON_STACK(desc, alg->tfm);
 	u8 *padded_salt = NULL;
 	size_t padded_salt_size;
-	int err;
-
-	desc->tfm = alg->tfm;
-
-	if (salt_size == 0)
-		return NULL;
-
-	hashstate = kmalloc(crypto_shash_statesize(alg->tfm), GFP_KERNEL);
-	if (!hashstate)
-		return ERR_PTR(-ENOMEM);
+	union fsverity_hash_ctx ctx;
+	void *res = NULL;
 
 	/*
 	 * Zero-pad the salt to the next multiple of the input size of the hash
 	 * algorithm's compression function, e.g. 64 bytes for SHA-256 or 128
 	 * bytes for SHA-512.  This ensures that the hash algorithm won't have
 	 * any bytes buffered internally after processing the salt, thus making
 	 * salted hashing just as fast as unsalted hashing.
 	 */
 	padded_salt_size = round_up(salt_size, alg->block_size);
 	padded_salt = kzalloc(padded_salt_size, GFP_KERNEL);
-	if (!padded_salt) {
-		err = -ENOMEM;
-		goto err_free;
-	}
+	if (!padded_salt)
+		return NULL;
 	memcpy(padded_salt, salt, salt_size);
-	err = crypto_shash_init(desc);
-	if (err)
-		goto err_free;
-
-	err = crypto_shash_update(desc, padded_salt, padded_salt_size);
-	if (err)
-		goto err_free;
-
-	err = crypto_shash_export(desc, hashstate);
-	if (err)
-		goto err_free;
-out:
-	kfree(padded_salt);
-	return hashstate;
 
-err_free:
-	kfree(hashstate);
-	hashstate = ERR_PTR(err);
-	goto out;
+	switch (alg->algo_id) {
+	case HASH_ALGO_SHA256:
+		sha256_init(&ctx.sha256);
+		sha256_update(&ctx.sha256, padded_salt, padded_salt_size);
+		res = kmemdup(&ctx.sha256, sizeof(ctx.sha256), GFP_KERNEL);
+		break;
+	case HASH_ALGO_SHA512:
+		sha512_init(&ctx.sha512);
+		sha512_update(&ctx.sha512, padded_salt, padded_salt_size);
+		res = kmemdup(&ctx.sha512, sizeof(ctx.sha512), GFP_KERNEL);
+		break;
+	default:
+		WARN_ON_ONCE(1);
+	}
+	kfree(padded_salt);
+	return res;
 }
 
 /**
  * fsverity_hash_block() - hash a single data or hash block
  * @params: the Merkle tree's parameters
@@ -168,50 +98,58 @@ const u8 *fsverity_prepare_hash_state(const struct fsverity_hash_alg *alg,
  * @data: virtual address of a buffer containing the block to hash
  * @out: output digest, size 'params->digest_size' bytes
  *
  * Hash a single data or hash block.  The hash is salted if a salt is specified
  * in the Merkle tree parameters.
- *
- * Return: 0 on success, -errno on failure
  */
-int fsverity_hash_block(const struct merkle_tree_params *params,
-			const struct inode *inode, const void *data, u8 *out)
+void fsverity_hash_block(const struct merkle_tree_params *params,
+			 const struct inode *inode, const void *data, u8 *out)
 {
-	SHASH_DESC_ON_STACK(desc, params->hash_alg->tfm);
-	int err;
-
-	desc->tfm = params->hash_alg->tfm;
-
-	if (params->hashstate) {
-		err = crypto_shash_import(desc, params->hashstate);
-		if (err) {
-			fsverity_err(inode,
-				     "Error %d importing hash state", err);
-			return err;
-		}
-		err = crypto_shash_finup(desc, data, params->block_size, out);
-	} else {
-		err = crypto_shash_digest(desc, data, params->block_size, out);
+	union fsverity_hash_ctx ctx;
+
+	if (!params->hashstate) {
+		fsverity_hash_buffer(params->hash_alg, data, params->block_size,
+				     out);
+		return;
+	}
+
+	switch (params->hash_alg->algo_id) {
+	case HASH_ALGO_SHA256:
+		ctx.sha256 = params->hashstate->sha256;
+		sha256_update(&ctx.sha256, data, params->block_size);
+		sha256_final(&ctx.sha256, out);
+		return;
+	case HASH_ALGO_SHA512:
+		ctx.sha512 = params->hashstate->sha512;
+		sha512_update(&ctx.sha512, data, params->block_size);
+		sha512_final(&ctx.sha512, out);
+		return;
+	default:
+		BUG();
 	}
-	if (err)
-		fsverity_err(inode, "Error %d computing block hash", err);
-	return err;
 }
 
 /**
  * fsverity_hash_buffer() - hash some data
  * @alg: the hash algorithm to use
  * @data: the data to hash
  * @size: size of data to hash, in bytes
  * @out: output digest, size 'alg->digest_size' bytes
- *
- * Return: 0 on success, -errno on failure
  */
-int fsverity_hash_buffer(const struct fsverity_hash_alg *alg,
-			 const void *data, size_t size, u8 *out)
+void fsverity_hash_buffer(const struct fsverity_hash_alg *alg,
+			  const void *data, size_t size, u8 *out)
 {
-	return crypto_shash_tfm_digest(alg->tfm, data, size, out);
+	switch (alg->algo_id) {
+	case HASH_ALGO_SHA256:
+		sha256(data, size, out);
+		return;
+	case HASH_ALGO_SHA512:
+		sha512(data, size, out);
+		return;
+	default:
+		BUG();
+	}
 }
 
 void __init fsverity_check_hash_algs(void)
 {
 	size_t i;
diff --git a/fs/verity/open.c b/fs/verity/open.c
index fdeb95eca3af3..0604cebe194cd 100644
--- a/fs/verity/open.c
+++ b/fs/verity/open.c
@@ -40,22 +40,22 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
 	int level;
 
 	memset(params, 0, sizeof(*params));
 
 	hash_alg = fsverity_get_hash_alg(inode, hash_algorithm);
-	if (IS_ERR(hash_alg))
-		return PTR_ERR(hash_alg);
+	if (!hash_alg)
+		return -EINVAL;
 	params->hash_alg = hash_alg;
 	params->digest_size = hash_alg->digest_size;
 
-	params->hashstate = fsverity_prepare_hash_state(hash_alg, salt,
-							salt_size);
-	if (IS_ERR(params->hashstate)) {
-		err = PTR_ERR(params->hashstate);
-		params->hashstate = NULL;
-		fsverity_err(inode, "Error %d preparing hash state", err);
-		goto out_err;
+	if (salt_size) {
+		params->hashstate =
+			fsverity_prepare_hash_state(hash_alg, salt, salt_size);
+		if (!params->hashstate) {
+			err = -ENOMEM;
+			goto out_err;
+		}
 	}
 
 	/*
 	 * fs/verity/ directly assumes that the Merkle tree block size is a
 	 * power of 2 less than or equal to PAGE_SIZE.  Another restriction
@@ -156,22 +156,19 @@ int fsverity_init_merkle_tree_params(struct merkle_tree_params *params,
 
 /*
  * Compute the file digest by hashing the fsverity_descriptor excluding the
  * builtin signature and with the sig_size field set to 0.
  */
-static int compute_file_digest(const struct fsverity_hash_alg *hash_alg,
-			       struct fsverity_descriptor *desc,
-			       u8 *file_digest)
+static void compute_file_digest(const struct fsverity_hash_alg *hash_alg,
+				struct fsverity_descriptor *desc,
+				u8 *file_digest)
 {
 	__le32 sig_size = desc->sig_size;
-	int err;
 
 	desc->sig_size = 0;
-	err = fsverity_hash_buffer(hash_alg, desc, sizeof(*desc), file_digest);
+	fsverity_hash_buffer(hash_alg, desc, sizeof(*desc), file_digest);
 	desc->sig_size = sig_size;
-
-	return err;
 }
 
 /*
  * Create a new fsverity_info from the given fsverity_descriptor (with optional
  * appended builtin signature), and check the signature if present.  The
@@ -199,16 +196,11 @@ struct fsverity_info *fsverity_create_info(const struct inode *inode,
 		goto fail;
 	}
 
 	memcpy(vi->root_hash, desc->root_hash, vi->tree_params.digest_size);
 
-	err = compute_file_digest(vi->tree_params.hash_alg, desc,
-				  vi->file_digest);
-	if (err) {
-		fsverity_err(inode, "Error %d computing file digest", err);
-		goto fail;
-	}
+	compute_file_digest(vi->tree_params.hash_alg, desc, vi->file_digest);
 
 	err = fsverity_verify_signature(vi, desc->signature,
 					le32_to_cpu(desc->sig_size));
 	if (err)
 		goto fail;
diff --git a/fs/verity/verify.c b/fs/verity/verify.c
index 4fcad0825a120..481a381128cb9 100644
--- a/fs/verity/verify.c
+++ b/fs/verity/verify.c
@@ -5,11 +5,10 @@
  * Copyright 2019 Google LLC
  */
 
 #include "fsverity_private.h"
 
-#include <crypto/hash.h>
 #include <linux/bio.h>
 
 static struct workqueue_struct *fsverity_read_workqueue;
 
 /*
@@ -200,12 +199,11 @@ verify_data_block(struct inode *inode, struct fsverity_info *vi,
 		struct page *hpage = hblocks[level - 1].page;
 		const void *haddr = hblocks[level - 1].addr;
 		unsigned long hblock_idx = hblocks[level - 1].index;
 		unsigned int hoffset = hblocks[level - 1].hoffset;
 
-		if (fsverity_hash_block(params, inode, haddr, real_hash) != 0)
-			goto error;
+		fsverity_hash_block(params, inode, haddr, real_hash);
 		if (memcmp(want_hash, real_hash, hsize) != 0)
 			goto corrupted;
 		/*
 		 * Mark the hash block as verified.  This must be atomic and
 		 * idempotent, as the same hash block might be verified by
@@ -220,12 +218,11 @@ verify_data_block(struct inode *inode, struct fsverity_info *vi,
 		kunmap_local(haddr);
 		put_page(hpage);
 	}
 
 	/* Finally, verify the data block. */
-	if (fsverity_hash_block(params, inode, data, real_hash) != 0)
-		goto error;
+	fsverity_hash_block(params, inode, data, real_hash);
 	if (memcmp(want_hash, real_hash, hsize) != 0)
 		goto corrupted;
 	return true;
 
 corrupted:
-- 
2.50.0

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