KEYS: Add a lookup_restriction function for the asymmetric key type

Look up asymmetric keyring restriction information using the key-type
lookup_restrict hook.
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>

Documentation/crypto/asymmetric-keys.txt

@@ -311,3 +311,38 @@ Functions are provided to register and unregister parsers:
 
 Parsers may not have the same name.  The names are otherwise only used for
 displaying in debugging messages.
+
+
+=========================
+KEYRING LINK RESTRICTIONS
+=========================
+
+Keyrings created from userspace using add_key can be configured to check the
+signature of the key being linked.
+
+Several restriction methods are available:
+
+ (1) Restrict using the kernel builtin trusted keyring
+
+     - Option string used with KEYCTL_RESTRICT_KEYRING:
+       - "builtin_trusted"
+
+     The kernel builtin trusted keyring will be searched for the signing
+     key. The ca_keys kernel parameter also affects which keys are used for
+     signature verification.
+
+ (2) Restrict using the kernel builtin and secondary trusted keyrings
+
+     - Option string used with KEYCTL_RESTRICT_KEYRING:
+       - "builtin_and_secondary_trusted"
+
+     The kernel builtin and secondary trusted keyrings will be searched for the
+     signing key. The ca_keys kernel parameter also affects which keys are used
+     for signature verification.
+
+In all of these cases, if the signing key is found the signature of the key to
+be linked will be verified using the signing key.  The requested key is added
+to the keyring only if the signature is successfully verified.  -ENOKEY is
+returned if the parent certificate could not be found, or -EKEYREJECTED is
+returned if the signature check fails or the key is blacklisted.  Other errors
+may be returned if the signature check could not be performed.

crypto/asymmetric_keys/asymmetric_type.c

@@ -17,6 +17,7 @@
 #include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/ctype.h>
+#include <keys/system_keyring.h>
 #include "asymmetric_keys.h"
 
 MODULE_LICENSE("GPL");
@@ -451,6 +452,40 @@ static void asymmetric_key_destroy(struct key *key)
 	asymmetric_key_free_kids(kids);
 }
 
+static struct key_restriction *asymmetric_restriction_alloc(
+	key_restrict_link_func_t check,
+	struct key *key)
+{
+	struct key_restriction *keyres =
+		kzalloc(sizeof(struct key_restriction), GFP_KERNEL);
+
+	if (!keyres)
+		return ERR_PTR(-ENOMEM);
+
+	keyres->check = check;
+	keyres->key = key;
+	keyres->keytype = &key_type_asymmetric;
+
+	return keyres;
+}
+
+/*
+ * look up keyring restrict functions for asymmetric keys
+ */
+static struct key_restriction *asymmetric_lookup_restriction(
+	const char *restriction)
+{
+	if (strcmp("builtin_trusted", restriction) == 0)
+		return asymmetric_restriction_alloc(
+			restrict_link_by_builtin_trusted, NULL);
+
+	if (strcmp("builtin_and_secondary_trusted", restriction) == 0)
+		return asymmetric_restriction_alloc(
+			restrict_link_by_builtin_and_secondary_trusted, NULL);
+
+	return ERR_PTR(-EINVAL);
+}
+
 struct key_type key_type_asymmetric = {
 	.name			= "asymmetric",
 	.preparse		= asymmetric_key_preparse,
@@ -460,6 +495,7 @@ struct key_type key_type_asymmetric = {
 	.match_free		= asymmetric_key_match_free,
 	.destroy		= asymmetric_key_destroy,
 	.describe		= asymmetric_key_describe,
+	.lookup_restriction	= asymmetric_lookup_restriction,
 };
 EXPORT_SYMBOL_GPL(key_type_asymmetric);