+static bool use_builtin_keys;
+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' */
+ else if (strcmp(str, "builtin") == 0)
+ use_builtin_keys = true;
+
+ return 1;
+}
+__setup("ca_keys=", ca_keys_setup);
+#endif
+
+/**
+ * x509_request_asymmetric_key - Request a key by X.509 certificate params.
+ * @keyring: The keys to search.
+ * @subject: The name of the subject to whom the key belongs.
+ * @key_id: The subject key ID as a hex string.
+ *
+ * Find a key in the given keyring by subject name and key ID. These might,
+ * for instance, be the issuer name and the authority key ID of an X.509
+ * certificate that needs to be verified.
+ */
+struct key *x509_request_asymmetric_key(struct key *keyring,
+ const char *subject,
+ const char *key_id)
+{
+ key_ref_t key;
+ size_t subject_len = strlen(subject), key_id_len = strlen(key_id);
+ char *id;
+
+ /* Construct an identifier "<subjname>:<keyid>". */
+ id = kmalloc(subject_len + 2 + key_id_len + 1, GFP_KERNEL);
+ if (!id)
+ return ERR_PTR(-ENOMEM);
+
+ memcpy(id, subject, subject_len);
+ id[subject_len + 0] = ':';
+ id[subject_len + 1] = ' ';
+ memcpy(id + subject_len + 2, key_id, key_id_len);
+ id[subject_len + 2 + key_id_len] = 0;
+
+ pr_debug("Look up: \"%s\"\n", id);
+
+ key = keyring_search(make_key_ref(keyring, 1),
+ &key_type_asymmetric, id);
+ if (IS_ERR(key))
+ pr_debug("Request for key '%s' err %ld\n", id, PTR_ERR(key));
+ kfree(id);
+
+ if (IS_ERR(key)) {
+ switch (PTR_ERR(key)) {
+ /* Hide some search errors */
+ case -EACCES:
+ case -ENOTDIR:
+ case -EAGAIN:
+ return ERR_PTR(-ENOKEY);
+ default:
+ return ERR_CAST(key);
+ }
+ }
+
+ pr_devel("<==%s() = 0 [%x]\n", __func__,
+ key_serial(key_ref_to_ptr(key)));
+ return key_ref_to_ptr(key);
+}
+EXPORT_SYMBOL_GPL(x509_request_asymmetric_key);
+