ima: define '.ima' as a builtin 'trusted' keyring

Require all keys added to the IMA keyring be signed by an
existing trusted key on the system trusted keyring.

Changelog v6:
- remove ifdef CONFIG_IMA_TRUSTED_KEYRING in C code - Dmitry
- update Kconfig dependency and help
- select KEYS_DEBUG_PROC_KEYS - Dmitry

Changelog v5:
- Move integrity_init_keyring() to init_ima() - Dmitry
- reset keyring[id] on failure - Dmitry

Changelog v1:
- don't link IMA trusted keyring to user keyring

Changelog:
- define stub integrity_init_keyring() function (reported-by Fengguang Wu)
- differentiate between regular and trusted keyring names.
- replace printk with pr_info (D. Kasatkin)
- only make the IMA keyring a trusted keyring (reported-by D. Kastatkin)
- define stub integrity_init_keyring() definition based on
  CONFIG_INTEGRITY_SIGNATURE, not CONFIG_INTEGRITY_ASYMMETRIC_KEYS.
  (reported-by Jim Davis)

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Acked-by: David Howells <dhowells@redhat.com>
diff --git a/security/integrity/ima/Kconfig b/security/integrity/ima/Kconfig
index 81a2797..08758fb 100644
--- a/security/integrity/ima/Kconfig
+++ b/security/integrity/ima/Kconfig
@@ -123,3 +123,13 @@
 	  For more information on integrity appraisal refer to:
 	  <http://linux-ima.sourceforge.net>
 	  If unsure, say N.
+
+config IMA_TRUSTED_KEYRING
+	bool "Require all keys on the .ima keyring be signed"
+	depends on IMA_APPRAISE && SYSTEM_TRUSTED_KEYRING
+	depends on INTEGRITY_ASYMMETRIC_KEYS
+	select KEYS_DEBUG_PROC_KEYS
+	default y
+	help
+	   This option requires that all keys added to the .ima
+	   keyring be signed by a key on the system trusted keyring.