x86/tls: Forcibly set the accessed bit in TLS segments

For mysterious historical reasons, struct user_desc doesn't indicate
whether segments are accessed.  set_thread_area() has always programmed
segments as non-accessed, so the first write will set the accessed bit.
This will fault if the GDT is read-only.

Fix it by making TLS segments start out accessed.

If this ends up breaking something, we could, in principle, leave TLS
segments non-accessed and fix them up when we get the page fault.  I'd be
surprised, though -- AFAIK all the nasty legacy segmented programs (DOSEMU,
Wine, things that run on DOSEMU and Wine, etc.) do their nasty segmented
things using the LDT and not the GDT.  I assume this is mainly because old
OSes (Linux and otherwise) didn't historically provide APIs to do nasty
things in the GDT.

Fixes: 45fc8757d1 ("x86: Make the GDT remapping read-only on 64-bit")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Thomas Garnier <thgarnie@google.com>
Link: http://lkml.kernel.org/r/62b7748542df0164af7e0a5231283b9b13858c45.1489900519.git.luto@kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/tls.c

@@ -92,10 +92,17 @@ static void set_tls_desc(struct task_struct *p, int idx,
 	cpu = get_cpu();
 
 	while (n-- > 0) {
-		if (LDT_empty(info) || LDT_zero(info))
+		if (LDT_empty(info) || LDT_zero(info)) {
 			desc->a = desc->b = 0;
-		else
+		} else {
 			fill_ldt(desc, info);
+
+			/*
+			 * Always set the accessed bit so that the CPU
+			 * doesn't try to write to the (read-only) GDT.
+			 */
+			desc->type |= 1;
+		}
 		++info;
 		++desc;
 	}