ima: fix 'd-ng' comments and documentation

Initially the 'd-ng' template field did not prefix the digest with either
"md5" or "sha1" hash algorithms.  Prior to being upstreamed this changed,
but the comments and documentation were not updated.  Fix the comments
and documentation.

Fixes: 4d7aeee ("ima: define new template ima-ng and template fields d-ng and n-ng")
Reported-by: Eric Biggers <ebiggers@kernel.org>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

Author: mimizohar

Documentation/security/IMA-templates.rst

@@ -66,8 +66,7 @@ descriptors by adding their identifier to the format string
    calculated with the SHA1 or MD5 hash algorithm;
  - 'n': the name of the event (i.e. the file name), with size up to 255 bytes;
  - 'd-ng': the digest of the event, calculated with an arbitrary hash
-   algorithm (field format: [<hash algo>:]digest, where the digest
-   prefix is shown only if the hash algorithm is not SHA1 or MD5);
+   algorithm (field format: <hash algo>:digest);
  - 'd-modsig': the digest of the event without the appended modsig;
  - 'n-ng': the name of the event, without size limitations;
  - 'sig': the file signature, or the EVM portable signature if the file

security/integrity/ima/ima_template_lib.c

@@ -271,9 +271,11 @@ static int ima_eventdigest_init_common(const u8 *digest, u32 digestsize,
 	/*
 	 * digest formats:
 	 *  - DATA_FMT_DIGEST: digest
-	 *  - DATA_FMT_DIGEST_WITH_ALGO: [<hash algo>] + ':' + '\0' + digest,
-	 *    where <hash algo> is provided if the hash algorithm is not
-	 *    SHA1 or MD5
+	 *  - DATA_FMT_DIGEST_WITH_ALGO: <hash algo> + ':' + '\0' + digest,
+	 *
+	 *    where 'DATA_FMT_DIGEST' is the original digest format ('d')
+	 *      with a hash size limitation of 20 bytes,
+	 *    where <hash algo> is the hash_algo_name[] string.
 	 */
 	u8 buffer[CRYPTO_MAX_ALG_NAME + 2 + IMA_MAX_DIGEST_SIZE] = { 0 };
 	enum data_formats fmt = DATA_FMT_DIGEST;