x86/vdso: Make the PER_CPU segment start out accessed

The first userspace attempt to read or write the PER_CPU segment
will write the accessed bit to the GDT.  This is visible to
userspace using the LAR instruction, and it also pointlessly
dirties a cache line.

Set the segment's accessed bit at boot to prevent userspace
access to segments from having side effects.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/ac63814ca4c637a08ec2fd0360d67ca67560a9ee.1411494540.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/vdso/vma.c

@@ -264,7 +264,7 @@ static void vsyscall_set_cpu(int cpu)
 	d = (struct desc_struct) {
 		.limit0 = cpu | ((node & 0xf) << 12),
 		.limit = node >> 4,
-		.type = 4,		/* RO data, expand down */
+		.type = 5,		/* RO data, expand down, accessed */
 		.dpl = 3,		/* Visible to user code */
 		.s = 1,			/* Not a system segment */
 		.p = 1,			/* Present */