logind: reject wall messages containing control characters

method_set_wall_message() and the property setter only checked the
message length but not its content. Since wall messages are broadcast
to all TTYs, control characters in the message could interfere with
terminal state. Reject messages containing control characters other
than newline and tab.

Follow-up for 9ef15026c0
Follow-up for e2fa5721c3

src/login/logind-dbus.c

@@ -59,6 +59,7 @@
 #include "signal-util.h"
 #include "sleep-config.h"
 #include "stdio-util.h"
+#include "string-util.h"
 #include "strv.h"
 #include "terminal-util.h"
 #include "tmpfile-util.h"
@@ -3582,6 +3583,46 @@ static int property_get_boot_loader_entries(
         return sd_bus_message_close_container(reply);
 }
 
+static int wall_message_validate(const char *wall_message, sd_bus_error *error) {
+        if (strlen(wall_message) > WALL_MESSAGE_MAX)
+                return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS,
+                                         "Wall message too long, maximum permitted length is %u characters.",
+                                         WALL_MESSAGE_MAX);
+
+        if (string_has_cc(wall_message, /* ok= */ "\n\t"))
+                return sd_bus_error_set(error,
+                                        SD_BUS_ERROR_INVALID_ARGS,
+                                        "Wall message contains control characters, refusing.");
+
+        return 0;
+}
+
+static int property_set_wall_message(
+                sd_bus *bus,
+                const char *path,
+                const char *interface,
+                const char *property,
+                sd_bus_message *value,
+                void *userdata,
+                sd_bus_error *error) {
+
+        char **p = ASSERT_PTR(userdata);
+        const char *s;
+        int r;
+
+        assert(value);
+
+        r = sd_bus_message_read(value, "s", &s);
+        if (r < 0)
+                return r;
+
+        r = wall_message_validate(s, error);
+        if (r < 0)
+                return r;
+
+        return free_and_strdup_warn(p, empty_to_null(s));
+}
+
 static int method_set_wall_message(
                 sd_bus_message *message,
                 void *userdata,
@@ -3598,10 +3639,9 @@ static int method_set_wall_message(
         if (r < 0)
                 return r;
 
-        if (strlen(wall_message) > WALL_MESSAGE_MAX)
-                return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS,
-                        "Wall message too long, maximum permitted length is %u characters.",
-                        WALL_MESSAGE_MAX);
+        r = wall_message_validate(wall_message, error);
+        if (r < 0)
+                return r;
 
         /* Short-circuit the operation if the desired state is already in place, to
          * avoid an unnecessary polkit permission check. */
@@ -3764,7 +3804,7 @@ static const sd_bus_vtable manager_vtable[] = {
         SD_BUS_VTABLE_START(0),
 
         SD_BUS_WRITABLE_PROPERTY("EnableWallMessages", "b", bus_property_get_bool, bus_property_set_bool, offsetof(Manager, wall_messages), 0),
-        SD_BUS_WRITABLE_PROPERTY("WallMessage", "s", NULL, NULL, offsetof(Manager, wall_message), 0),
+        SD_BUS_WRITABLE_PROPERTY("WallMessage", "s", NULL, property_set_wall_message, offsetof(Manager, wall_message), 0),
 
         SD_BUS_PROPERTY("NAutoVTs", "u", NULL, offsetof(Manager, n_autovts), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("KillOnlyUsers", "as", NULL, offsetof(Manager, kill_only_users), SD_BUS_VTABLE_PROPERTY_CONST),