x86/microcode: Fix again accessing initrd after having been freed

Commit 24c25032 ("x86/microcode: Do not access the initrd after it has
been freed") fixed attempts to access initrd from the microcode loader
after it has been freed. However, a similar KASAN warning was reported
(stack trace edited):

  smpboot: Booting Node 0 Processor 1 APIC 0x11
  ==================================================================
  BUG: KASAN: use-after-free in find_cpio_data+0x9b5/0xa50
  Read of size 1 at addr ffff880035ffd000 by task swapper/1/0

  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.14.8-slack #7
  Hardware name: System manufacturer System Product Name/A88X-PLUS, BIOS 3003 03/10/2016
  Call Trace:
   dump_stack
   print_address_description
   kasan_report
   ? find_cpio_data
   __asan_report_load1_noabort
   find_cpio_data
   find_microcode_in_initrd
   __load_ucode_amd
   load_ucode_amd_ap
      load_ucode_ap

After some investigation, it turned out that a merge was done using the
wrong side to resolve, leading to picking up the previous state, before
the 24c25032 fix. Therefore the Fixes tag below contains a merge
commit.

Revert the mismerge by catching the save_microcode_in_initrd_amd()
retval and thus letting the function exit with the last return statement
so that initrd_gone can be set to true.

Fixes: f26483ea ("Merge branch 'x86/urgent' into x86/microcode, to resolve conflicts")
Reported-by: <higuita@gmx.net>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://bugzilla.kernel.org/show_bug.cgi?id=198295
Link: https://lkml.kernel.org/r/20180123104133.918-2-bp@alien8.de

arch/x86/kernel/cpu/microcode/core.c

@@ -239,7 +239,7 @@ static int __init save_microcode_in_initrd(void)
 		break;
 	case X86_VENDOR_AMD:
 		if (c->x86 >= 0x10)
-			return save_microcode_in_initrd_amd(cpuid_eax(1));
+			ret = save_microcode_in_initrd_amd(cpuid_eax(1));
 		break;
 	default:
 		break;