landlock: Simplify initially denied access rights

Upgrade domain's handled access masks when creating a domain from a
ruleset, instead of converting them at runtime.  This is more consistent
and helps with audit support.

Cc: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20250108154338.1129069-7-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: l0kod

security/landlock/access.h

@@ -20,7 +20,8 @@
 /*
  * All access rights that are denied by default whether they are handled or not
  * by a ruleset/layer.  This must be ORed with all ruleset->access_masks[]
- * entries when we need to get the absolute handled access masks.
+ * entries when we need to get the absolute handled access masks, see
+ * landlock_upgrade_handled_access_masks().
  */
 /* clang-format off */
 #define _LANDLOCK_ACCESS_FS_INITIALLY_DENIED ( \
@@ -59,4 +60,18 @@ typedef u16 layer_mask_t;
 /* Makes sure all layers can be checked. */
 static_assert(BITS_PER_TYPE(layer_mask_t) >= LANDLOCK_MAX_NUM_LAYERS);
 
+/* Upgrades with all initially denied by default access rights. */
+static inline struct access_masks
+landlock_upgrade_handled_access_masks(struct access_masks access_masks)
+{
+	/*
+	 * All access rights that are denied by default whether they are
+	 * explicitly handled or not.
+	 */
+	if (access_masks.fs)
+		access_masks.fs |= _LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
+
+	return access_masks;
+}
+
 #endif /* _SECURITY_LANDLOCK_ACCESS_H */

security/landlock/fs.c

@@ -389,14 +389,6 @@ static bool is_nouser_or_private(const struct dentry *dentry)
 		unlikely(IS_PRIVATE(d_backing_inode(dentry))));
 }
 
-static access_mask_t
-get_handled_fs_accesses(const struct landlock_ruleset *const domain)
-{
-	/* Handles all initially denied by default access rights. */
-	return landlock_union_access_masks(domain).fs |
-	       _LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
-}
-
 static const struct access_masks any_fs = {
 	.fs = ~0,
 };
@@ -788,7 +780,7 @@ static bool is_access_to_paths_allowed(
 		 * a superset of the meaningful requested accesses).
 		 */
 		access_masked_parent1 = access_masked_parent2 =
-			get_handled_fs_accesses(domain);
+			landlock_union_access_masks(domain).fs;
 		is_dom_check = true;
 	} else {
 		if (WARN_ON_ONCE(dentry_child1 || dentry_child2))

security/landlock/ruleset.c

@@ -387,7 +387,8 @@ static int merge_ruleset(struct landlock_ruleset *const dst,
 		err = -EINVAL;
 		goto out_unlock;
 	}
-	dst->access_masks[dst->num_layers - 1] = src->access_masks[0];
+	dst->access_masks[dst->num_layers - 1] =
+		landlock_upgrade_handled_access_masks(src->access_masks[0]);
 
 	/* Merges the @src inode tree. */
 	err = merge_tree(dst, src, LANDLOCK_KEY_INODE);