fsverity: use shash API instead of ahash API

The "ahash" API, like the other scatterlist-based crypto APIs such as
"skcipher", comes with some well-known limitations.  First, it can't
easily be used with vmalloc addresses.  Second, the request struct can't
be allocated on the stack.  This adds complexity and a possible failure
point that needs to be worked around, e.g. using a mempool.

The only benefit of ahash over "shash" is that ahash is needed to access
traditional memory-to-memory crypto accelerators, i.e. drivers/crypto/.
However, this style of crypto acceleration has largely fallen out of
favor and been superseded by CPU-based acceleration or inline crypto
engines.  Also, ahash needs to be used asynchronously to take full
advantage of such hardware, but fs/verity/ has never done this.

On all systems that aren't actually using one of these ahash-only crypto
accelerators, ahash just adds unnecessary overhead as it sits between
the user and the underlying shash algorithms.

Also, XFS is planned to cache fsverity Merkle tree blocks in the
existing XFS buffer cache.  As a result, it will be possible for a
single Merkle tree block to be split across discontiguous pages
(https://lore.kernel.org/r/20230405233753.GU3223426@dread.disaster.area).
This data will need to be hashed.  It is easiest to work with a vmapped
address in this case.  However, ahash is incompatible with this.

Therefore, let's convert fs/verity/ from ahash to shash.  This
simplifies the code, and it should also slightly improve performance for
everyone who wasn't actually using one of these ahash-only crypto
accelerators, i.e. almost everyone (or maybe even everyone)!

Link: https://lore.kernel.org/r/20230516052306.99600-1-ebiggers@kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 2 insertions, 11 deletions

diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
index d34dcc033d72..8527beca2a45 100644
--- a/fs/verity/fsverity_private.h
+++ b/fs/verity/fsverity_private.h
@@ -11,9 +11,6 @@
 #define pr_fmt(fmt) "fs-verity: " fmt
 
 #include <linux/fsverity.h>
-#include <linux/mempool.h>
-
-struct ahash_request;
 
 /*
  * Implementation limit: maximum depth of the Merkle tree.  For now 8 is plenty;
@@ -23,11 +20,10 @@ struct ahash_request;
 
 /* A hash algorithm supported by fs-verity */
 struct fsverity_hash_alg {
-	struct crypto_ahash *tfm; /* hash tfm, allocated on demand */
+	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 */
-	mempool_t req_pool;	  /* mempool with a preallocated hash request */
 	/*
 	 * The HASH_ALGO_* constant for this algorithm.  This is different from
 	 * FS_VERITY_HASH_ALG_*, which uses a different numbering scheme.
@@ -85,15 +81,10 @@ extern struct fsverity_hash_alg fsverity_hash_algs[];
 
 struct fsverity_hash_alg *fsverity_get_hash_alg(const struct inode *inode,
 						unsigned int num);
-struct ahash_request *fsverity_alloc_hash_request(struct fsverity_hash_alg *alg,
-						  gfp_t gfp_flags);
-void fsverity_free_hash_request(struct fsverity_hash_alg *alg,
-				struct ahash_request *req);
 const u8 *fsverity_prepare_hash_state(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, struct ahash_request *req,
-			struct page *page, unsigned int offset, u8 *out);
+			const struct inode *inode, const void *data, u8 *out);
 int fsverity_hash_buffer(struct fsverity_hash_alg *alg,
 			 const void *data, size_t size, u8 *out);
 void __init fsverity_check_hash_algs(void);