@@ -39,6 +39,28 @@ static bool tls_desc_okay(const struct user_desc *info)
if (!info->seg_32bit)
return false;
+ /* Only allow data segments in the TLS array. */
+ if (info->contents > 1)
+ return false;
+
+ /*
+ * Non-present segments with DPL 3 present an interesting attack
+ * surface. The kernel should handle such segments correctly,
+ * but TLS is very difficult to protect in a sandbox, so prevent
+ * such segments from being created.
+ *
+ * If userspace needs to remove a TLS entry, it can still delete
+ * it outright.
+ */
+ if (info->seg_not_present)
+ return false;
+
+#ifdef CONFIG_X86_64
+ /* The L bit makes no sense for data. */
+ if (info->lm)
+ return false;
+#endif
+
return true;
}