KEYS: validate certificate trust only with builtin keys

Instead of allowing public keys, with certificates signed by any
key on the system trusted keyring, to be added to a trusted keyring,
this patch further restricts the certificates to those signed only by
builtin keys on the system keyring.

This patch defines a new option 'builtin' for the kernel parameter
'keys_ownerid' to allow trust validation using builtin keys.

Simplified Mimi's "KEYS: define an owner trusted keyring" patch

Changelog v7:
- rename builtin_keys to use_builtin_keys

Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index bdb193a..90c12c5 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -569,7 +569,7 @@
 	ca_keys=	[KEYS] This parameter identifies a specific key(s) on
 			the system trusted keyring to be used for certificate
 			trust validation.
-			format: id:<keyid>
+			format: { id:<keyid> | builtin }
 
 	ccw_timeout_log [S390]
 			See Documentation/s390/CommonIO for details.