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>

security/integrity/digsig.c

@@ -13,7 +13,9 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/err.h>
+#include <linux/sched.h>
 #include <linux/rbtree.h>
+#include <linux/cred.h>
 #include <linux/key-type.h>
 #include <linux/digsig.h>
 
@@ -24,7 +26,11 @@ static struct key *keyring[INTEGRITY_KEYRING_MAX];
 static const char *keyring_name[INTEGRITY_KEYRING_MAX] = {
 	"_evm",
 	"_module",
+#ifndef CONFIG_IMA_TRUSTED_KEYRING
 	"_ima",
+#else
+	".ima",
+#endif
 };
 
 int integrity_digsig_verify(const unsigned int id, const char *sig, int siglen,
@@ -56,3 +62,25 @@ int integrity_digsig_verify(const unsigned int id, const char *sig, int siglen,
 
 	return -EOPNOTSUPP;
 }
+
+int integrity_init_keyring(const unsigned int id)
+{
+	const struct cred *cred = current_cred();
+	int err = 0;
+
+	keyring[id] = keyring_alloc(keyring_name[id], KUIDT_INIT(0),
+				    KGIDT_INIT(0), cred,
+				    ((KEY_POS_ALL & ~KEY_POS_SETATTR) |
+				     KEY_USR_VIEW | KEY_USR_READ |
+				     KEY_USR_WRITE | KEY_USR_SEARCH),
+				    KEY_ALLOC_NOT_IN_QUOTA, NULL);
+	if (!IS_ERR(keyring[id]))
+		set_bit(KEY_FLAG_TRUSTED_ONLY, &keyring[id]->flags);
+	else {
+		err = PTR_ERR(keyring[id]);
+		pr_info("Can't allocate %s keyring (%d)\n",
+			keyring_name[id], err);
+		keyring[id] = NULL;
+	}
+	return err;
+}