landlock: Warn once if a Landlock action is requested while disabled

[ Upstream commit 782191c74875cc33b50263e21d76080b1411884d ]

Because sandboxing can be used as an opportunistic security measure,
user space may not log unsupported features.  Let the system
administrator know if an application tries to use Landlock but failed
because it isn't enabled at boot time.  This may be caused by boot
loader configurations with outdated "lsm" kernel's command-line
parameter.

Cc: stable@vger.kernel.org
Fixes: 265885daf3 ("landlock: Add syscall implementations")
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20240227110550.3702236-2-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>

security/landlock/syscalls.c

@@ -32,6 +32,18 @@
 #include "ruleset.h"
 #include "setup.h"
 
+static bool is_initialized(void)
+{
+	if (likely(landlock_initialized))
+		return true;
+
+	pr_warn_once(
+		"Disabled but requested by user space. "
+		"You should enable Landlock at boot time: "
+		"https://docs.kernel.org/userspace-api/landlock.html#boot-time-configuration\n");
+	return false;
+}
+
 /**
  * copy_min_struct_from_user - Safe future-proof argument copying
  *
@@ -165,7 +177,7 @@ SYSCALL_DEFINE3(landlock_create_ruleset,
 	/* Build-time checks. */
 	build_check_abi();
 
-	if (!landlock_initialized)
+	if (!is_initialized())
 		return -EOPNOTSUPP;
 
 	if (flags) {
@@ -311,7 +323,7 @@ SYSCALL_DEFINE4(landlock_add_rule, const int, ruleset_fd,
 	struct landlock_ruleset *ruleset;
 	int res, err;
 
-	if (!landlock_initialized)
+	if (!is_initialized())
 		return -EOPNOTSUPP;
 
 	/* No flag for now. */
@@ -402,7 +414,7 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
 	struct landlock_cred_security *new_llcred;
 	int err;
 
-	if (!landlock_initialized)
+	if (!is_initialized())
 		return -EOPNOTSUPP;
 
 	/*