x86, tls: Interpret an all-zero struct user_desc as "no segment"

[pandora-kernel.git] / arch / x86 / kernel / tls.c

diff --git a/arch/x86/kernel/tls.c b/arch/x86/kernel/tls.c
index e7d6e9f..0c38d06 100644 (file)
--- a/arch/x86/kernel/tls.c
+++ b/arch/x86/kernel/tls.c
@@ -30,7 +30,28 @@ 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;
 
        /*
@@ -56,12 +77,6 @@ static bool tls_desc_okay(const struct user_desc *info)
        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;
 }
 
@@ -78,7 +93,7 @@ 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);