x86/vdso: Make the PER_CPU segment 32 bits

IMO users ought not to be able to use 16-bit segments without
using modify_ldt.  Fortunately, it's impossible to break
espfix64 by loading the PER_CPU segment into SS because it's
PER_CPU is marked read-only and SS cannot contain an RO segment,
but marking PER_CPU as 32-bit is less fragile.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/179f490d659307873eefd09206bebd417e2ab5ad.1411494540.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/vdso/vma.c

@@ -268,6 +268,7 @@ static void vsyscall_set_cpu(int cpu)
 		.dpl = 3,		/* Visible to user code */
 		.s = 1,			/* Not a system segment */
 		.p = 1,			/* Present */
+		.d = 1,			/* 32-bit */
 	};
 
 	write_gdt_entry(get_cpu_gdt_table(cpu), GDT_ENTRY_PER_CPU, &d, DESCTYPE_S);