landlock: Refactor filesystem access mask management

Replace get_raw_handled_fs_accesses() with a generic
landlock_union_access_masks(), and replace get_fs_domain() with a
generic landlock_get_applicable_domain().  These helpers will also be
useful for other types of access.

Cc: Mikhail Ivanov <ivanov.mikhail1@huawei-partners.com>
Reviewed-by: Günther Noack <gnoack@google.com>
Link: https://lore.kernel.org/r/20241109110856.222842-2-mic@digikod.net
[mic: Slightly improve doc as suggested by Günther]
Signed-off-by: Mickaël Salaün <mic@digikod.net>

security/landlock/fs.c

@@ -388,38 +388,22 @@ static bool is_nouser_or_private(const struct dentry *dentry)
 		unlikely(IS_PRIVATE(d_backing_inode(dentry))));
 }
 
-static access_mask_t
-get_raw_handled_fs_accesses(const struct landlock_ruleset *const domain)
-{
-	access_mask_t access_dom = 0;
-	size_t layer_level;
-
-	for (layer_level = 0; layer_level < domain->num_layers; layer_level++)
-		access_dom |=
-			landlock_get_raw_fs_access_mask(domain, layer_level);
-	return access_dom;
-}
-
 static access_mask_t
 get_handled_fs_accesses(const struct landlock_ruleset *const domain)
 {
 	/* Handles all initially denied by default access rights. */
-	return get_raw_handled_fs_accesses(domain) |
+	return landlock_union_access_masks(domain).fs |
 	       LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
 }
 
-static const struct landlock_ruleset *
+static const struct access_masks any_fs = {
-get_fs_domain(const struct landlock_ruleset *const domain)
+	.fs = ~0,
-{
+};
-	if (!domain || !get_raw_handled_fs_accesses(domain))
-		return NULL;
-
-	return domain;
-}
 
 static const struct landlock_ruleset *get_current_fs_domain(void)
 {
-	return get_fs_domain(landlock_get_current_domain());
+	return landlock_get_applicable_domain(landlock_get_current_domain(),
+					      any_fs);
 }
 
 /*
@@ -1517,7 +1501,8 @@ static int hook_file_open(struct file *const file)
 	access_mask_t open_access_request, full_access_request, allowed_access,
 		optional_access;
 	const struct landlock_ruleset *const dom =
-		get_fs_domain(landlock_cred(file->f_cred)->domain);
+		landlock_get_applicable_domain(
+			landlock_cred(file->f_cred)->domain, any_fs);
 
 	if (!dom)
 		return 0;

security/landlock/ruleset.h

@@ -11,6 +11,7 @@
 
 #include <linux/bitops.h>
 #include <linux/build_bug.h>
+#include <linux/kernel.h>
 #include <linux/mutex.h>
 #include <linux/rbtree.h>
 #include <linux/refcount.h>
@@ -47,6 +48,15 @@ struct access_masks {
 	access_mask_t scope : LANDLOCK_NUM_SCOPE;
 };
 
+union access_masks_all {
+	struct access_masks masks;
+	u32 all;
+};
+
+/* Makes sure all fields are covered. */
+static_assert(sizeof(typeof_member(union access_masks_all, masks)) ==
+	      sizeof(typeof_member(union access_masks_all, all)));
+
 typedef u16 layer_mask_t;
 /* Makes sure all layers can be checked. */
 static_assert(BITS_PER_TYPE(layer_mask_t) >= LANDLOCK_MAX_NUM_LAYERS);
@@ -260,6 +270,61 @@ static inline void landlock_get_ruleset(struct landlock_ruleset *const ruleset)
 		refcount_inc(&ruleset->usage);
 }
 
+/**
+ * landlock_union_access_masks - Return all access rights handled in the
+ *				 domain
+ *
+ * @domain: Landlock ruleset (used as a domain)
+ *
+ * Returns: an access_masks result of the OR of all the domain's access masks.
+ */
+static inline struct access_masks
+landlock_union_access_masks(const struct landlock_ruleset *const domain)
+{
+	union access_masks_all matches = {};
+	size_t layer_level;
+
+	for (layer_level = 0; layer_level < domain->num_layers; layer_level++) {
+		union access_masks_all layer = {
+			.masks = domain->access_masks[layer_level],
+		};
+
+		matches.all |= layer.all;
+	}
+
+	return matches.masks;
+}
+
+/**
+ * landlock_get_applicable_domain - Return @domain if it applies to (handles)
+ *				    at least one of the access rights specified
+ *				    in @masks
+ *
+ * @domain: Landlock ruleset (used as a domain)
+ * @masks: access masks
+ *
+ * Returns: @domain if any access rights specified in @masks is handled, or
+ * NULL otherwise.
+ */
+static inline const struct landlock_ruleset *
+landlock_get_applicable_domain(const struct landlock_ruleset *const domain,
+			       const struct access_masks masks)
+{
+	const union access_masks_all masks_all = {
+		.masks = masks,
+	};
+	union access_masks_all merge = {};
+
+	if (!domain)
+		return NULL;
+
+	merge.masks = landlock_union_access_masks(domain);
+	if (merge.all & masks_all.all)
+		return domain;
+
+	return NULL;
+}
+
 static inline void
 landlock_add_fs_access_mask(struct landlock_ruleset *const ruleset,
 			    const access_mask_t fs_access_mask,
@@ -295,19 +360,12 @@ landlock_add_scope_mask(struct landlock_ruleset *const ruleset,
 	ruleset->access_masks[layer_level].scope |= mask;
 }
 
-static inline access_mask_t
-landlock_get_raw_fs_access_mask(const struct landlock_ruleset *const ruleset,
-				const u16 layer_level)
-{
-	return ruleset->access_masks[layer_level].fs;
-}
-
 static inline access_mask_t
 landlock_get_fs_access_mask(const struct landlock_ruleset *const ruleset,
 			    const u16 layer_level)
 {
 	/* Handles all initially denied by default access rights. */
-	return landlock_get_raw_fs_access_mask(ruleset, layer_level) |
+	return ruleset->access_masks[layer_level].fs |
 	       LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
 }
 

security/landlock/syscalls.c

@@ -329,7 +329,7 @@ static int add_rule_path_beneath(struct landlock_ruleset *const ruleset,
 		return -ENOMSG;
 
 	/* Checks that allowed_access matches the @ruleset constraints. */
-	mask = landlock_get_raw_fs_access_mask(ruleset, 0);
+	mask = ruleset->access_masks[0].fs;
 	if ((path_beneath_attr.allowed_access | mask) != mask)
 		return -EINVAL;