[PATCH v1] landlock: Clean up ruleset validation checks

["Mickaël Salaün" <mic@digikod.net>]

landlock_merge_ruleset() checks for a NULL ruleset after dereferencing
it in lockdep_assert_held().  Move the assertion after the check so the
defensive path remains effective.

The mask-validation comment originated in landlock_add_fs_access_mask()
to explain that its WARN_ON_ONCE() checked a caller invariant.  It
became self-referential when this helper and its network and scope
counterparts were inlined into landlock_create_ruleset().  Restate the
invariant without naming the caller.

Keep both as defensive callee checks.  Moving the assertion preserves
the NULL check's ability to warn and return -EINVAL, while invalid masks
remain warned about and masked.

Reported-by: Günther Noack <gnoack@google.com>
Closes: https://patch.msgid.link/aobYhIt3vcs2xN0b@google.com
Closes: https://patch.msgid.link/aobasxUDQ8b7GYXl@google.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>
---
 security/landlock/domain.c  | 3 ++-
 security/landlock/ruleset.c | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/security/landlock/domain.c b/security/landlock/domain.c
index 93c7104fd6b2..4031b581be07 100644
--- a/security/landlock/domain.c
+++ b/security/landlock/domain.c
@@ -439,10 +439,11 @@ landlock_merge_ruleset(struct landlock_domain *const parent,
 	int err;
 
 	might_sleep();
-	lockdep_assert_held(&ruleset->lock);
 	if (WARN_ON_ONCE(!ruleset))
 		return ERR_PTR(-EINVAL);
 
+	lockdep_assert_held(&ruleset->lock);
+
 	if (parent) {
 		if (parent->num_layers >= LANDLOCK_MAX_NUM_LAYERS)
 			return ERR_PTR(-E2BIG);
diff --git a/security/landlock/ruleset.c b/security/landlock/ruleset.c
index 0d07707523cd..a5d135d085cb 100644
--- a/security/landlock/ruleset.c
+++ b/security/landlock/ruleset.c
@@ -58,7 +58,7 @@ landlock_create_ruleset(const access_mask_t fs_access_mask,
 	new_ruleset->id = landlock_get_id_range(1);
 #endif /* CONFIG_TRACEPOINTS */
 
-	/* Should already be checked in landlock_create_ruleset(). */
+	/* The caller must only pass supported access rights and scopes. */
 	if (fs_access_mask) {
 		const access_mask_t mask = fs_access_mask &
 					   LANDLOCK_MASK_ACCESS_FS;
-- 
2.55.0