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favor and been superseded by CPU-based acceleration or inline crypto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, 6 insertions, 13 deletions

diff --git a/fs/verity/enable.c b/fs/verity/enable.c
index fc4c50e5219d..bd86b25ac084 100644
--- a/fs/verity/enable.c
+++ b/fs/verity/enable.c
@@ -7,6 +7,7 @@
 
 #include "fsverity_private.h"
 
+#include <crypto/hash.h>
 #include <linux/mount.h>
 #include <linux/sched/signal.h>
 #include <linux/uaccess.h>
@@ -20,7 +21,7 @@ struct block_buffer {
 /* Hash a block, writing the result to the next level's pending block buffer. */
 static int hash_one_block(struct inode *inode,
 			  const struct merkle_tree_params *params,
-			  struct ahash_request *req, struct block_buffer *cur)
+			  struct block_buffer *cur)
 {
 	struct block_buffer *next = cur + 1;
 	int err;
@@ -36,8 +37,7 @@ static int hash_one_block(struct inode *inode,
 	/* 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, req, virt_to_page(cur->data),
-				  offset_in_page(cur->data),
+	err = fsverity_hash_block(params, inode, cur->data,
 				  &next->data[next->filled]);
 	if (err)
 		return err;
@@ -76,7 +76,6 @@ static int build_merkle_tree(struct file *filp,
 	struct inode *inode = file_inode(filp);
 	const u64 data_size = inode->i_size;
 	const int num_levels = params->num_levels;
-	struct ahash_request *req;
 	struct block_buffer _buffers[1 + FS_VERITY_MAX_LEVELS + 1] = {};
 	struct block_buffer *buffers = &_buffers[1];
 	unsigned long level_offset[FS_VERITY_MAX_LEVELS];
@@ -90,9 +89,6 @@ static int build_merkle_tree(struct file *filp,
 		return 0;
 	}
 
-	/* This allocation never fails, since it's mempool-backed. */
-	req = fsverity_alloc_hash_request(params->hash_alg, GFP_KERNEL);
-
 	/*
 	 * Allocate the block buffers.  Buffer "-1" is for data blocks.
 	 * Buffers 0 <= level < num_levels are for the actual tree levels.
@@ -130,7 +126,7 @@ static int build_merkle_tree(struct file *filp,
 			fsverity_err(inode, "Short read of file data");
 			goto out;
 		}
-		err = hash_one_block(inode, params, req, &buffers[-1]);
+		err = hash_one_block(inode, params, &buffers[-1]);
 		if (err)
 			goto out;
 		for (level = 0; level < num_levels; level++) {
@@ -141,8 +137,7 @@ static int build_merkle_tree(struct file *filp,
 			}
 			/* Next block at @level is full */
 
-			err = hash_one_block(inode, params, req,
-					     &buffers[level]);
+			err = hash_one_block(inode, params, &buffers[level]);
 			if (err)
 				goto out;
 			err = write_merkle_tree_block(inode,
@@ -162,8 +157,7 @@ static int build_merkle_tree(struct file *filp,
 	/* Finish all nonempty pending tree blocks. */
 	for (level = 0; level < num_levels; level++) {
 		if (buffers[level].filled != 0) {
-			err = hash_one_block(inode, params, req,
-					     &buffers[level]);
+			err = hash_one_block(inode, params, &buffers[level]);
 			if (err)
 				goto out;
 			err = write_merkle_tree_block(inode,
@@ -183,7 +177,6 @@ static int build_merkle_tree(struct file *filp,
 out:
 	for (level = -1; level < num_levels; level++)
 		kfree(buffers[level].data);
-	fsverity_free_hash_request(params->hash_alg, req);
 	return err;
 }