x86/alternatives: Make optimize_nops() interrupt safe and synced

Richard reported the following crash:

[    0.036000] BUG: unable to handle kernel paging request at 55501e06
[    0.036000] IP: [<c0aae48b>] common_interrupt+0xb/0x38
[    0.036000] Call Trace:
[    0.036000]  [<c0409c80>] ? add_nops+0x90/0xa0
[    0.036000]  [<c040a054>] apply_alternatives+0x274/0x630

Chuck decoded:

 "  0:   8d 90 90 83 04 24       lea    0x24048390(%eax),%edx
    6:   80 fc 0f                cmp    $0xf,%ah
    9:   a8 0f                   test   $0xf,%al
 >> b:   a0 06 1e 50 55          mov    0x55501e06,%al
   10:   57                      push   %edi
   11:   56                      push   %esi

 Interrupt 0x30 occurred while the alternatives code was replacing the
 initial 0x90,0x90,0x90 NOPs (from the ASM_CLAC macro) with the
 optimized version, 0x8d,0x76,0x00. Only the first byte has been
 replaced so far, and it makes a mess out of the insn decoding."

optimize_nops() is buggy in two aspects:

- It's not disabling interrupts across the modification
- It's lacking a sync_core() call

Add both.

Fixes: 4fd4b6e5537c 'x86/alternatives: Use optimized NOPs for padding'
Reported-and-tested-by: "Richard W.M. Jones" <rjones@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard W.M. Jones <rjones@redhat.com>
Cc: Chuck Ebbert <cebbert.lkml@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/alpine.DEB.2.11.1509031232340.15006@nanos
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/alternative.c

@@ -338,10 +338,15 @@ done:
 
 static void __init_or_module optimize_nops(struct alt_instr *a, u8 *instr)
 {
+	unsigned long flags;
+
 	if (instr[0] != 0x90)
 		return;
 
+	local_irq_save(flags);
 	add_nops(instr + (a->instrlen - a->padlen), a->padlen);
+	sync_core();
+	local_irq_restore(flags);
 
 	DUMP_BYTES(instr, a->instrlen, "%p: [%d:%d) optimized NOPs: ",
 		   instr, a->instrlen - a->padlen, a->padlen);