certs: Only allow certs signed by keys on the builtin keyring

Originally the secondary trusted keyring provided a keyring to which extra
keys may be added, provided those keys were not blacklisted and were
vouched for by a key built into the kernel or already in the secondary
trusted keyring.

On systems with the machine keyring configured, additional keys may also
be vouched for by a key on the machine keyring.

Prevent loading additional certificates directly onto the secondary
keyring, vouched for by keys on the machine keyring, yet allow these
certificates to be loaded onto other trusted keyrings.

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

Author: mimizohar

certs/Kconfig

@@ -88,7 +88,21 @@ config SECONDARY_TRUSTED_KEYRING
 	help
 	  If set, provide a keyring to which extra keys may be added, provided
 	  those keys are not blacklisted and are vouched for by a key built
-	  into the kernel or already in the secondary trusted keyring.
+	  into the kernel, machine keyring (if configured), or already in the
+	  secondary trusted keyring.
+
+config SECONDARY_TRUSTED_KEYRING_SIGNED_BY_BUILTIN
+	bool "Only allow additional certs signed by keys on the builtin trusted keyring"
+	depends on SECONDARY_TRUSTED_KEYRING
+	help
+	  If set, only certificates signed by keys on the builtin trusted
+	  keyring may be loaded onto the secondary trusted keyring.
+
+	  Note: The machine keyring, if configured, will be linked to the
+	  secondary keyring.  When enabling this option, it is recommended
+	  to also configure INTEGRITY_CA_MACHINE_KEYRING_MAX to prevent
+	  linking code signing keys with imputed trust to the secondary
+	  trusted keyring.
 
 config SYSTEM_BLACKLIST_KEYRING
 	bool "Provide system-wide ring of blacklisted keys"

crypto/asymmetric_keys/restrict.c

@@ -102,6 +102,10 @@ int restrict_link_by_signature(struct key *dest_keyring,
 
 	if (use_builtin_keys && !test_bit(KEY_FLAG_BUILTIN, &key->flags))
 		ret = -ENOKEY;
+	else if (IS_BUILTIN(CONFIG_SECONDARY_TRUSTED_KEYRING_SIGNED_BY_BUILTIN) &&
+		 !strcmp(dest_keyring->description, ".secondary_trusted_keys") &&
+		 !test_bit(KEY_FLAG_BUILTIN, &key->flags))
+		ret = -ENOKEY;
 	else
 		ret = verify_signature(key, sig);
 	key_put(key);