ima: load x509 certificate from the kernel

Define configuration option to load X509 certificate into the
IMA trusted kernel keyring. It implements ima_load_x509() hook
to load X509 certificate into the .ima trusted kernel keyring
from the root filesystem.

Changes in v3:
* use ima_policy_flag in ima_get_action()
  ima_load_x509 temporarily clears ima_policy_flag to disable
  appraisal to load key. Use it to skip appraisal rules.
* Key directory path changed to /etc/keys (Mimi)
* Expand IMA_LOAD_X509 Kconfig help

Changes in v2:
* added '__init'
* use ima_policy_flag to disable appraisal to load keys

Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

security/integrity/ima/Kconfig

@@ -131,3 +131,21 @@ config IMA_TRUSTED_KEYRING
 	help
 	   This option requires that all keys added to the .ima
 	   keyring be signed by a key on the system trusted keyring.
+
+config IMA_LOAD_X509
+	bool "Load X509 certificate onto the '.ima' trusted keyring"
+	depends on IMA_TRUSTED_KEYRING
+	default n
+	help
+	   File signature verification is based on the public keys
+	   loaded on the .ima trusted keyring. These public keys are
+	   X509 certificates signed by a trusted key on the
+	   .system keyring.  This option enables X509 certificate
+	   loading from the kernel onto the '.ima' trusted keyring.
+
+config IMA_X509_PATH
+	string "IMA X509 certificate path"
+	depends on IMA_LOAD_X509
+	default "/etc/keys/x509_ima.der"
+	help
+	   This option defines IMA X509 certificate path.