x86/alternatives: Sync core before enabling interrupts

text_poke_early() does:

   local_irq_save(flags);
   memcpy(addr, opcode, len);
   local_irq_restore(flags);
   sync_core();

That's not really correct because the synchronization should happen before
interrupts are re-enabled to ensure that a pending interrupt observes the
complete update of the opcodes.

It's not entirely clear whether the interrupt entry provides enough
serialization already, but moving the sync_core() invocation into interrupt
disabled region does no harm and is obviously correct.

Fixes: 6fffacb ("x86/alternatives, jumplabel: Use text_poke_early() before mm_init()")
Signed-off-by: Thomas Gleixner <[email protected]>
Signed-off-by: Borislav Petkov (AMD) <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Cc: <[email protected]>
Link: https://lore.kernel.org/r/ZT6narvE%2BLxX%[email protected]

arch/x86/kernel/alternative.c

@@ -1685,8 +1685,8 @@ void __init_or_module text_poke_early(void *addr, const void *opcode,
 	} else {
 		local_irq_save(flags);
 		memcpy(addr, opcode, len);
-		local_irq_restore(flags);
 		sync_core();
+		local_irq_restore(flags);
 
 		/*
 		 * Could also do a CLFLUSH here to speed up CPU recovery; but