fs-verity: define a function to return the integrity protected file digest

Define a function named fsverity_get_digest() to return the verity file
digest and the associated hash algorithm (enum hash_algo).

This assumes that before calling fsverity_get_digest() the file must have
been opened, which is even true for the IMA measure/appraise on file
open policy rule use case (func=FILE_CHECK).  do_open() calls vfs_open()
immediately prior to ima_file_check().

Acked-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

3 files changed, 44 insertions, 7 deletions

diff --git a/fs/verity/Kconfig b/fs/verity/Kconfig
index 24d1b54de807..54598cd80145 100644
--- a/fs/verity/Kconfig
+++ b/fs/verity/Kconfig
@@ -3,6 +3,7 @@
 config FS_VERITY
 	bool "FS Verity (read-only file-based authenticity protection)"
 	select CRYPTO
+	select CRYPTO_HASH_INFO
 	# SHA-256 is implied as it's intended to be the default hash algorithm.
 	# To avoid bloat, other wanted algorithms must be selected explicitly.
 	# Note that CRYPTO_SHA256 denotes the generic C implementation, but
diff --git a/fs/verity/fsverity_private.h b/fs/verity/fsverity_private.h
index a7920434bae5..c6fb62e0ef1a 100644
--- a/fs/verity/fsverity_private.h
+++ b/fs/verity/fsverity_private.h
@@ -14,7 +14,6 @@
 
 #define pr_fmt(fmt) "fs-verity: " fmt
 
-#include <crypto/sha2.h>
 #include <linux/fsverity.h>
 #include <linux/mempool.h>
 
@@ -26,12 +25,6 @@ struct ahash_request;
  */
 #define FS_VERITY_MAX_LEVELS		8
 
-/*
- * Largest digest size among all hash algorithms supported by fs-verity.
- * Currently assumed to be <= size of fsverity_descriptor::root_hash.
- */
-#define FS_VERITY_MAX_DIGEST_SIZE	SHA512_DIGEST_SIZE
-
 /* A hash algorithm supported by fs-verity */
 struct fsverity_hash_alg {
 	struct crypto_ahash *tfm; /* hash tfm, allocated on demand */
diff --git a/fs/verity/measure.c b/fs/verity/measure.c
index f0d7b30c62db..e99c00350c28 100644
--- a/fs/verity/measure.c
+++ b/fs/verity/measure.c
@@ -57,3 +57,46 @@ int fsverity_ioctl_measure(struct file *filp, void __user *_uarg)
 	return 0;
 }
 EXPORT_SYMBOL_GPL(fsverity_ioctl_measure);
+
+/**
+ * fsverity_get_digest() - get a verity file's digest
+ * @inode: inode to get digest of
+ * @digest: (out) pointer to the digest
+ * @alg: (out) pointer to the hash algorithm enumeration
+ *
+ * Return the file hash algorithm and digest of an fsverity protected file.
+ * Assumption: before calling fsverity_get_digest(), the file must have been
+ * opened.
+ *
+ * Return: 0 on success, -errno on failure
+ */
+int fsverity_get_digest(struct inode *inode,
+			u8 digest[FS_VERITY_MAX_DIGEST_SIZE],
+			enum hash_algo *alg)
+{
+	const struct fsverity_info *vi;
+	const struct fsverity_hash_alg *hash_alg;
+	int i;
+
+	vi = fsverity_get_info(inode);
+	if (!vi)
+		return -ENODATA; /* not a verity file */
+
+	hash_alg = vi->tree_params.hash_alg;
+	memset(digest, 0, FS_VERITY_MAX_DIGEST_SIZE);
+
+	/* convert the verity hash algorithm name to a hash_algo_name enum */
+	i = match_string(hash_algo_name, HASH_ALGO__LAST, hash_alg->name);
+	if (i < 0)
+		return -EINVAL;
+	*alg = i;
+
+	if (WARN_ON_ONCE(hash_alg->digest_size != hash_digest_size[*alg]))
+		return -EINVAL;
+	memcpy(digest, vi->file_digest, hash_alg->digest_size);
+
+	pr_debug("file digest %s:%*phN\n", hash_algo_name[*alg],
+		 hash_digest_size[*alg], digest);
+
+	return 0;
+}