landlock: Clarify BUILD_BUG_ON check in scoping logic

The BUILD_BUG_ON check in domain_is_scoped() and
unmask_scoped_access() should check that the loop that counts down
client_layer finishes.  We therefore check that the numbers
LANDLOCK_MAX_NUM_LAYERS-1 and -1 are both representable by that
integer.  If they are representable, the numbers in between are
representable too, and the loop finishes.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260327164838.38231-6-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>

Author: gnoack

security/landlock/fs.c

@@ -1595,10 +1595,13 @@ static void unmask_scoped_access(const struct landlock_ruleset *const client,
 		return;
 
 	/*
-	 * client_layer must be a signed integer with greater capacity
-	 * than client->num_layers to ensure the following loop stops.
+	 * client_layer must be able to represent all numbers from
+	 * LANDLOCK_MAX_NUM_LAYERS - 1 to -1 for the loop below to terminate.
+	 * (It must be large enough, and it must be signed.)
 	 */
-	BUILD_BUG_ON(sizeof(client_layer) > sizeof(client->num_layers));
+	BUILD_BUG_ON(!is_signed_type(typeof(client_layer)));
+	BUILD_BUG_ON(LANDLOCK_MAX_NUM_LAYERS - 1 >
+		     type_max(typeof(client_layer)));
 
 	client_layer = client->num_layers - 1;
 	client_walker = client->hierarchy;

security/landlock/task.c

@@ -191,10 +191,13 @@ static bool domain_is_scoped(const struct landlock_ruleset *const client,
 	client_layer = client->num_layers - 1;
 	client_walker = client->hierarchy;
 	/*
-	 * client_layer must be a signed integer with greater capacity
-	 * than client->num_layers to ensure the following loop stops.
+	 * client_layer must be able to represent all numbers from
+	 * LANDLOCK_MAX_NUM_LAYERS - 1 to -1 for the loop below to terminate.
+	 * (It must be large enough, and it must be signed.)
 	 */
-	BUILD_BUG_ON(sizeof(client_layer) > sizeof(client->num_layers));
+	BUILD_BUG_ON(!is_signed_type(typeof(client_layer)));
+	BUILD_BUG_ON(LANDLOCK_MAX_NUM_LAYERS - 1 >
+		     type_max(typeof(client_layer)));
 
 	server_layer = server ? (server->num_layers - 1) : -1;
 	server_walker = server ? server->hierarchy : NULL;