+static bool tls_desc_okay(const struct user_desc *info)
+{
+ /*
+ * For historical reasons (i.e. no one ever documented how any
+ * of the segmentation APIs work), user programs can and do
+ * assume that a struct user_desc that's all zeros except for
+ * entry_number means "no segment at all". This never actually
+ * worked. In fact, up to Linux 3.19, a struct user_desc like
+ * this would create a 16-bit read-write segment with base and
+ * limit both equal to zero.
+ *
+ * That was close enough to "no segment at all" until we
+ * hardened this function to disallow 16-bit TLS segments. Fix
+ * it up by interpreting these zeroed segments the way that they
+ * were almost certainly intended to be interpreted.
+ *
+ * The correct way to ask for "no segment at all" is to specify
+ * a user_desc that satisfies LDT_empty. To keep everything
+ * working, we accept both.
+ *
+ * Note that there's a similar kludge in modify_ldt -- look at
+ * the distinction between modes 1 and 0x11.
+ */
+ if (LDT_empty(info) || LDT_zero(info))
+ return true;
+
+ /*
+ * espfix is required for 16-bit data segments, but espfix
+ * only works for LDT segments.
+ */
+ 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;
+
+ return true;
+}
+