[PATCH v2 2/2] landlock: Warn once if a Landlock action is requested while disabled

["Mickaël Salaün" <mic@digikod.net>]

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 bootloader
configurations with outdated "lsm" kernel's command-line parameter.

Cc: stable@vger.kernel.org
Fixes: 265885daf3e5 ("landlock: Add syscall implementations")
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
---

Changes since v1:
* Add Kees's and Günther's Reviewed-by.
* Rename is_not_initialized() to not_initialized() and invert the logic,
  as suggested by Günther.  This is a cosmetic change without global
  behavioral changed.
* Update link to point to a new subsection.
---
 security/landlock/syscalls.c | 18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
index 898358f57fa0..6788e73b6681 100644
--- a/security/landlock/syscalls.c
+++ b/security/landlock/syscalls.c
@@ -33,6 +33,18 @@
 #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
  *
@@ -173,7 +185,7 @@ SYSCALL_DEFINE3(landlock_create_ruleset,
 	/* Build-time checks. */
 	build_check_abi();
 
-	if (!landlock_initialized)
+	if (!is_initialized())
 		return -EOPNOTSUPP;
 
 	if (flags) {
@@ -398,7 +410,7 @@ SYSCALL_DEFINE4(landlock_add_rule, const int, ruleset_fd,
 	struct landlock_ruleset *ruleset;
 	int err;
 
-	if (!landlock_initialized)
+	if (!is_initialized())
 		return -EOPNOTSUPP;
 
 	/* No flag for now. */
@@ -458,7 +470,7 @@ 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;
 
 	/*
-- 
2.44.0