nvme-keyring: implement nvme_tls_psk_default()

Implement a function to select the preferred PSK for TLS.
A 'retained' PSK should be preferred over a 'generated' PSK,
and SHA-384 should be preferred to SHA-256.
Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>

drivers/nvme/common/keyring.c

@@ -5,6 +5,7 @@
 
 #include <linux/module.h>
 #include <linux/seq_file.h>
+#include <linux/key.h>
 #include <linux/key-type.h>
 #include <keys/user-type.h>
 #include <linux/nvme.h>
@@ -103,6 +104,53 @@ static struct key *nvme_tls_psk_lookup(struct key *keyring,
 	return key_ref_to_ptr(keyref);
 }
 
+/*
+ * NVMe PSK priority list
+ *
+ * 'Retained' PSKs (ie 'generated == false')
+ * should be preferred to 'generated' PSKs,
+ * and SHA-384 should be preferred to SHA-256.
+ */
+struct nvme_tls_psk_priority_list {
+	bool generated;
+	enum nvme_tcp_tls_cipher cipher;
+} nvme_tls_psk_prio[] = {
+	{ .generated = false,
+	  .cipher = NVME_TCP_TLS_CIPHER_SHA384, },
+	{ .generated = false,
+	  .cipher = NVME_TCP_TLS_CIPHER_SHA256, },
+	{ .generated = true,
+	  .cipher = NVME_TCP_TLS_CIPHER_SHA384, },
+	{ .generated = true,
+	  .cipher = NVME_TCP_TLS_CIPHER_SHA256, },
+};
+
+/*
+ * nvme_tls_psk_default - Return the preferred PSK to use for TLS ClientHello
+ */
+key_serial_t nvme_tls_psk_default(struct key *keyring,
+		      const char *hostnqn, const char *subnqn)
+{
+	struct key *tls_key;
+	key_serial_t tls_key_id;
+	int prio;
+
+	for (prio = 0; prio < ARRAY_SIZE(nvme_tls_psk_prio); prio++) {
+		bool generated = nvme_tls_psk_prio[prio].generated;
+		enum nvme_tcp_tls_cipher cipher = nvme_tls_psk_prio[prio].cipher;
+
+		tls_key = nvme_tls_psk_lookup(keyring, hostnqn, subnqn,
+					      cipher, generated);
+		if (!IS_ERR(tls_key)) {
+			tls_key_id = tls_key->serial;
+			key_put(tls_key);
+			return tls_key_id;
+		}
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(nvme_tls_psk_default);
+
 int nvme_keyring_init(void)
 {
 	int err;

include/linux/nvme-keyring.h

@@ -8,12 +8,20 @@
 
 #ifdef CONFIG_NVME_KEYRING
 
+key_serial_t nvme_tls_psk_default(struct key *keyring,
+		const char *hostnqn, const char *subnqn);
+
 key_serial_t nvme_keyring_id(void);
 int nvme_keyring_init(void);
 void nvme_keyring_exit(void);
 
 #else
 
+static inline key_serial_t nvme_tls_psk_default(struct key *keyring,
+		const char *hostnqn, const char *subnqn)
+{
+	return 0;
+}
 static inline key_serial_t nvme_keyring_id(void)
 {
 	return 0;