2 files changed, 36 insertions, 2 deletions

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index fc237fd0259a..a94b82e8f156 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -262,6 +262,19 @@ static inline void native_load_tls(struct thread_struct *t, unsigned int cpu)
          (info)->seg_not_present        == 1    &&      \
          (info)->useable                == 0)
 
+/* Lots of programs expect an all-zero user_desc to mean "no segment at all". */
+static inline bool LDT_zero(const struct user_desc *info)
+{
+        return (info->base_addr         == 0 &&
+                info->limit             == 0 &&
+                info->contents          == 0 &&
+                info->read_exec_only    == 0 &&
+                info->seg_32bit         == 0 &&
+                info->limit_in_pages    == 0 &&
+                info->seg_not_present   == 0 &&
+                info->useable           == 0);
+}
+
 static inline void clear_LDT(void)
 {
         set_ldt(NULL, 0);
diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index 4e942f31b1a7..7fc5e843f247 100644
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -29,7 +29,28 @@ static int get_free_idx(void)
 
 static bool tls_desc_okay(const struct user_desc *info)
 {
-        if (LDT_empty(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;
 
         /*
@@ -71,7 +92,7 @@ static void set_tls_desc(struct task_struct *p, int idx,
         cpu = get_cpu();
 
         while (n-- > 0) {
-                if (LDT_empty(info))
+                if (LDT_empty(info) || LDT_zero(info))
                         desc->a = desc->b = 0;
                 else
                         fill_ldt(desc, info);