ima: fix buffer overrun in ima_eventdigest_init_common

Function ima_eventdigest_init() calls ima_eventdigest_init_common()
with HASH_ALGO__LAST which is then used to access the array
hash_digest_size[] leading to buffer overrun. Have a conditional
statement to handle this.

Fixes: 9fab303 ("ima: fix violation measurement list record")
Signed-off-by: Samasth Norway Ananda <samasth.norway.ananda@oracle.com>
Tested-by: Enrico Bravi (PhD at polito.it) <enrico.bravi@huawei.com>
Cc: stable@vger.kernel.org # 5.19+
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

Author: samasth-norway

security/integrity/ima/ima_template_lib.c

@@ -318,15 +318,21 @@ static int ima_eventdigest_init_common(const u8 *digest, u32 digestsize,
 				      hash_algo_name[hash_algo]);
 	}
 
-	if (digest)
+	if (digest) {
 		memcpy(buffer + offset, digest, digestsize);
-	else
+	} else {
 		/*
 		 * If digest is NULL, the event being recorded is a violation.
 		 * Make room for the digest by increasing the offset by the
-		 * hash algorithm digest size.
+		 * hash algorithm digest size. If the hash algorithm is not
+		 * specified increase the offset by IMA_DIGEST_SIZE which
+		 * fits SHA1 or MD5
 		 */
-		offset += hash_digest_size[hash_algo];
+		if (hash_algo < HASH_ALGO__LAST)
+			offset += hash_digest_size[hash_algo];
+		else
+			offset += IMA_DIGEST_SIZE;
+	}
 
 	return ima_write_template_field_data(buffer, offset + digestsize,
 					     fmt, field_data);