KEYS: validate certificate trust only with selected key

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 by a
particular key on the system keyring.

This patch defines a new kernel parameter 'ca_keys' to identify the
specific key which must be used for trust validation of certificates.

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

Changelog:
- support for builtin x509 public keys only
- export "asymmetric_keyid_match"
- remove ifndefs MODULE
- rename kernel boot parameter from keys_ownerid to ca_keys

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

Documentation/kernel-parameters.txt

@@ -566,6 +566,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			possible to determine what the correct size should be.
 			This option provides an override for these situations.
 
+	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>
+
 	ccw_timeout_log [S390]
 			See Documentation/s390/CommonIO for details.
 

crypto/asymmetric_keys/asymmetric_type.c

@@ -49,6 +49,7 @@ int asymmetric_keyid_match(const char *kid, const char *id)
 
 	return 1;
 }
+EXPORT_SYMBOL_GPL(asymmetric_keyid_match);
 
 /*
  * Match asymmetric keys on (part of) their name

crypto/asymmetric_keys/x509_public_key.c

@@ -24,6 +24,22 @@
 #include "public_key.h"
 #include "x509_parser.h"
 
+static char *ca_keyid;
+
+#ifndef MODULE
+static int __init ca_keys_setup(char *str)
+{
+	if (!str)		/* default system keyring */
+		return 1;
+
+	if (strncmp(str, "id:", 3) == 0)
+		ca_keyid = str;	/* owner key 'id:xxxxxx' */
+
+	return 1;
+}
+__setup("ca_keys=", ca_keys_setup);
+#endif
+
 /*
  * Find a key in the given keyring by issuer and authority.
  */
@@ -171,6 +187,9 @@ static int x509_validate_trust(struct x509_certificate *cert,
 	if (!trust_keyring)
 		return -EOPNOTSUPP;
 
+	if (ca_keyid && !asymmetric_keyid_match(cert->authority, ca_keyid))
+		return -EPERM;
+
 	key = x509_request_asymmetric_key(trust_keyring,
 					  cert->issuer, strlen(cert->issuer),
 					  cert->authority,