ima: allocate field pointers array on demand in template_desc_init_fields()

The allocation of a field pointers array is moved at the end of
template_desc_init_fields() and done only if the value of the 'fields'
and 'num_fields' parameters is not NULL. For just validating a template
format string, retrieved template field pointers are placed in a temporary
array.

Changelog:
 - v3:
   - do not check in this patch if 'fields' and 'num_fields' are NULL
     (suggested by Mimi Zohar)
Signed-off-by: Roberto Sassu <roberto.sassu@polito.it>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

security/integrity/ima/ima_template.c

@@ -117,8 +117,9 @@ static int template_desc_init_fields(const char *template_fmt,
 				     int *num_fields)
 {
 	const char *template_fmt_ptr;
+	struct ima_template_field *found_fields[IMA_TEMPLATE_NUM_FIELDS_MAX];
 	int template_num_fields = template_fmt_size(template_fmt);
-	int i, len, result = 0;
+	int i, len;
 
 	if (template_num_fields > IMA_TEMPLATE_NUM_FIELDS_MAX) {
 		pr_err("format string '%s' contains too many fields\n",
@@ -126,41 +127,32 @@ static int template_desc_init_fields(const char *template_fmt,
 		return -EINVAL;
 	}
 
-	*fields = kzalloc(template_num_fields * sizeof(*fields), GFP_KERNEL);
-	if (*fields == NULL) {
-		result = -ENOMEM;
-		goto out;
-	}
-
 	for (i = 0, template_fmt_ptr = template_fmt; i < template_num_fields;
 	     i++, template_fmt_ptr += len + 1) {
 		char tmp_field_id[IMA_TEMPLATE_FIELD_ID_MAX_LEN + 1];
-		struct ima_template_field *f;
 
 		len = strchrnul(template_fmt_ptr, '|') - template_fmt_ptr;
 		if (len == 0 || len > IMA_TEMPLATE_FIELD_ID_MAX_LEN) {
 			pr_err("Invalid field with length %d\n", len);
-			result = -EINVAL;
-			goto out;
+			return -EINVAL;
 		}
 
 		memcpy(tmp_field_id, template_fmt_ptr, len);
 		tmp_field_id[len] = '\0';
-		f = lookup_template_field(tmp_field_id);
-		if (!f) {
+		found_fields[i] = lookup_template_field(tmp_field_id);
+		if (!found_fields[i]) {
 			pr_err("field '%s' not found\n", tmp_field_id);
-			result = -ENOENT;
-			goto out;
+			return -ENOENT;
 		}
-		(*fields)[i] = f;
 	}
+
+	*fields = kmalloc_array(i, sizeof(*fields), GFP_KERNEL);
+	if (*fields == NULL)
+		return -ENOMEM;
+
+	memcpy(*fields, found_fields, i * sizeof(*fields));
 	*num_fields = i;
-out:
-	if (result < 0) {
-		kfree(*fields);
-		*fields = NULL;
-	}
-	return result;
+	return 0;
 }
 
 struct ima_template_desc *ima_template_desc_current(void)