security: Add hooks to rule on setting a watch

Add security hooks that will allow an LSM to rule on whether or not a watch
may be set. More than one hook is required as the watches watch different
types of object.

Signed-off-by: David Howells <[e-mail]>
Acked-by: James Morris <[e-mail]>
cc: Casey Schaufler <[e-mail]>
cc: Stephen Smalley <[e-mail]>
cc: [e-mail]

include/linux/lsm_hook_defs.h

···
 #if defined(CONFIG_SECURITY) && defined(CONFIG_WATCH_QUEUE)
 LSM_HOOK(int, 0, post_notification, const struct cred *w_cred,
 	 const struct cred *cred, struct watch_notification *n)
+#endif /* CONFIG_SECURITY && CONFIG_WATCH_QUEUE */
+
+#if defined(CONFIG_SECURITY) && defined(CONFIG_KEY_NOTIFICATIONS)
+LSM_HOOK(int, 0, watch_key, struct key *key)
 #endif /* CONFIG_SECURITY && CONFIG_KEY_NOTIFICATIONS */
 
 #ifdef CONFIG_SECURITY_NETWORK

include/linux/lsm_hooks.h

···
  *	@cred: The event-triggerer's credentials
  *	@n: The notification being posted
  *
+ * @watch_key:
+ *	Check to see if a process is allowed to watch for event notifications
+ *	from a key or keyring.
+ *	@key: The key to watch.
+ *
  * Security hooks for using the eBPF maps and programs functionalities through
  * eBPF syscalls.
  *

include/linux/security.h

···
 }
 #endif
 
+#if defined(CONFIG_SECURITY) && defined(CONFIG_KEY_NOTIFICATIONS)
+int security_watch_key(struct key *key);
+#else
+static inline int security_watch_key(struct key *key)
+{
+	return 0;
+}
+#endif
+
 #ifdef CONFIG_SECURITY_NETWORK
 
 int security_unix_stream_connect(struct sock *sock, struct sock *other, struct sock *newsk);

security/security.c

···
 }
 #endif /* CONFIG_WATCH_QUEUE */
 
+#ifdef CONFIG_KEY_NOTIFICATIONS
+int security_watch_key(struct key *key)
+{
+	return call_int_hook(watch_key, 0, key);
+}
+#endif
+
 #ifdef CONFIG_SECURITY_NETWORK
 
 int security_unix_stream_connect(struct sock *sock, struct sock *other, struct sock *newsk)