ima: Define new template evm-sig

With the recent introduction of the evmsig template field, remote verifiers
can obtain the EVM portable signature instead of the IMA signature, to
verify file metadata.

After introducing the new fields to include file metadata in the
measurement list, this patch finally defines the evm-sig template, whose
format is:

d-ng|n-ng|evmsig|xattrnames|xattrlengths|xattrvalues|iuid|igid|imode

xattrnames, xattrlengths and xattrvalues are populated only from defined
EVM protected xattrs, i.e. the ones that EVM considers to verify the
portable signature. xattrnames and xattrlengths are populated only if the
xattr is present.

xattrnames and xattrlengths are not necessary for verifying the EVM
portable signature, but they are included for completeness of information,
if a remote verifier wants to infer more from file metadata.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

Author: robertosassu

Documentation/security/IMA-templates.rst

@@ -91,6 +91,7 @@ Below, there is the list of defined template descriptors:
  - "ima-sig": its format is ``d-ng|n-ng|sig``;
  - "ima-buf": its format is ``d-ng|n-ng|buf``;
  - "ima-modsig": its format is ``d-ng|n-ng|sig|d-modsig|modsig``;
+ - "evm-sig": its format is ``d-ng|n-ng|evmsig|xattrnames|xattrlengths|xattrvalues|iuid|igid|imode``;
 
 
 Use

security/integrity/ima/ima_template.c

@@ -22,6 +22,8 @@ static struct ima_template_desc builtin_templates[] = {
 	{.name = "ima-sig", .fmt = "d-ng|n-ng|sig"},
 	{.name = "ima-buf", .fmt = "d-ng|n-ng|buf"},
 	{.name = "ima-modsig", .fmt = "d-ng|n-ng|sig|d-modsig|modsig"},
+	{.name = "evm-sig",
+	 .fmt = "d-ng|n-ng|evmsig|xattrnames|xattrlengths|xattrvalues|iuid|igid|imode"},
 	{.name = "", .fmt = ""},	/* placeholder for a custom format */
 };
 
@@ -69,7 +71,8 @@ static const struct ima_template_field supported_fields[] = {
  * need to be accounted for since they shouldn't be defined in the same template
  * description as 'd-ng' and 'n-ng' respectively.
  */
-#define MAX_TEMPLATE_NAME_LEN sizeof("d-ng|n-ng|sig|buf|d-modisg|modsig")
+#define MAX_TEMPLATE_NAME_LEN \
+	sizeof("d-ng|n-ng|evmsig|xattrnames|xattrlengths|xattrvalues|iuid|igid|imode")
 
 static struct ima_template_desc *ima_template;
 static struct ima_template_desc *ima_buf_template;