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

[Mickaël Salaün]

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configurations with outdated "lsm" kernel's command-line parameter.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: Günther Noack <gnoack@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 265885daf3e5 ("landlock: Add syscall implementations")
Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx>
---
 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 f0bc50003b46..b5b424819dee 100644
--- a/security/landlock/syscalls.c
+++ b/security/landlock/syscalls.c
@@ -33,6 +33,18 @@
 #include "ruleset.h"
 #include "setup.h"
 
+static bool is_not_initialized(void)
+{
+	if (likely(landlock_initialized))
+		return false;
+
+	pr_warn_once(
+		"Disabled but requested by user space. "
+		"You should enable Landlock at boot time: "
+		"https://docs.kernel.org/userspace-api/landlock.html#kernel-support\n";);
+	return true;
+}
+
 /**
  * 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_not_initialized())
 		return -EOPNOTSUPP;
 
 	if (flags) {
@@ -407,7 +419,7 @@ SYSCALL_DEFINE4(landlock_add_rule, const int, ruleset_fd,
 	struct landlock_ruleset *ruleset;
 	int err;
 
-	if (!landlock_initialized)
+	if (is_not_initialized())
 		return -EOPNOTSUPP;
 
 	/* No flag for now. */
@@ -467,7 +479,7 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
 	struct landlock_cred_security *new_llcred;
 	int err;
 
-	if (!landlock_initialized)
+	if (is_not_initialized())
 		return -EOPNOTSUPP;
 
 	/*
-- 
2.43.0