fs-verity: use mempool for hash requests

When initializing an fs-verity hash algorithm, also initialize a mempool
that contains a single preallocated hash request object.  Then replace
the direct calls to ahash_request_alloc() and ahash_request_free() with
allocating and freeing from this mempool.

This eliminates the possibility of the allocation failing, which is
desirable for the I/O path.

This doesn't cause deadlocks because there's no case where multiple hash
requests are needed at a time to make forward progress.

Link: https://lore.kernel.org/r/20191231175545.20709-1-ebiggers@kernel.org
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Biggers <ebiggers@google.com>

1 files changed, 11 insertions, 5 deletions

diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
index ab9cfdd8f965..74768cf539da 100644
--- a/fs/verity/fsverity_private.h
+++ b/fs/verity/fsverity_private.h
@@ -16,6 +16,7 @@
 
 #include <crypto/sha.h>
 #include <linux/fsverity.h>
+#include <linux/mempool.h>
 
 struct ahash_request;
 
@@ -37,11 +38,12 @@ struct fsverity_hash_alg {
 	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 */
 };
 
 /* Merkle tree parameters: hash algorithm, initial hash state, and topology */
 struct merkle_tree_params {
-	const struct fsverity_hash_alg *hash_alg; /* the hash algorithm */
+	struct fsverity_hash_alg *hash_alg; /* the hash algorithm */
 	const u8 *hashstate;		/* initial hash state or NULL */
 	unsigned int digest_size;	/* same as hash_alg->digest_size */
 	unsigned int block_size;	/* size of data and tree blocks */
@@ -115,14 +117,18 @@ struct fsverity_signed_digest {
 
 extern 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,
+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_page(const struct merkle_tree_params *params,
 		       const struct inode *inode,
 		       struct ahash_request *req, struct page *page, u8 *out);
-int fsverity_hash_buffer(const struct fsverity_hash_alg *alg,
+int fsverity_hash_buffer(struct fsverity_hash_alg *alg,
 			 const void *data, size_t size, u8 *out);
 void __init fsverity_check_hash_algs(void);