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:
- define stub integrity_init_keyring() function (reported-by Fengguang Wu)
- differentiate between regular and trusted keyring names.
- replace printk with pr_info (D. Kasatkin)
Signed-off-by: Mimi Zohar <zohar@us.ibm.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>
 
@@ -21,11 +23,19 @@
 
 static struct key *keyring[INTEGRITY_KEYRING_MAX];
 
+#ifdef CONFIG_IMA_TRUSTED_KEYRING
+static const char *keyring_name[INTEGRITY_KEYRING_MAX] = {
+	".evm",
+	".module",
+	".ima",
+};
+#else
 static const char *keyring_name[INTEGRITY_KEYRING_MAX] = {
 	"_evm",
 	"_module",
 	"_ima",
 };
+#endif
 
 int integrity_digsig_verify(const unsigned int id, const char *sig, int siglen,
 			    const char *digest, int digestlen)
@@ -56,3 +66,21 @@ 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();
+	const struct user_struct *user = cred->user;
+
+	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_ALLOC_NOT_IN_QUOTA, user->uid_keyring);
+	if (!IS_ERR(keyring[id]))
+		set_bit(KEY_FLAG_TRUSTED_ONLY, &keyring[id]->flags);
+	else
+		pr_info("Can't allocate %s keyring (%ld)\n",
+			keyring_name[id], PTR_ERR(keyring[id]));
+	return 0;
+}

security/integrity/ima/Kconfig

@@ -123,3 +123,11 @@ config IMA_APPRAISE
 	  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
+	default y
+	help
+	   This option requires that all keys added to the _ima
+	   keyring be signed by a key on the system trusted keyring.

security/integrity/ima/ima_appraise.c

@@ -381,3 +381,14 @@ int ima_inode_removexattr(struct dentry *dentry, const char *xattr_name)
 	}
 	return result;
 }
+
+#ifdef CONFIG_IMA_TRUSTED_KEYRING
+static int __init init_ima_keyring(void)
+{
+	int ret;
+
+	ret = integrity_init_keyring(INTEGRITY_KEYRING_IMA);
+	return 0;
+}
+late_initcall(init_ima_keyring);
+#endif

security/integrity/integrity.h

@@ -137,12 +137,19 @@ static inline int integrity_digsig_verify(const unsigned int id,
 #ifdef CONFIG_INTEGRITY_ASYMMETRIC_KEYS
 int asymmetric_verify(struct key *keyring, const char *sig,
 		      int siglen, const char *data, int datalen);
+
+int integrity_init_keyring(const unsigned int id);
 #else
 static inline int asymmetric_verify(struct key *keyring, const char *sig,
 				    int siglen, const char *data, int datalen)
 {
 	return -EOPNOTSUPP;
 }
+
+static int integrity_init_keyring(const unsigned int id)
+{
+	return 0;
+}
 #endif
 
 #ifdef CONFIG_INTEGRITY_AUDIT