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

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loader configurations with outdated "lsm" kernel's command-line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: 265885daf3e5 ("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>

diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
index 898358f57fa0854416bfce396c519f826e432138..6788e73b6681b4127f6d4f77e3f74e45e08a531b 100644 (file)
--- 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;
 
        /*