linux-kernel - [PATCH 2/2] x86/microcode: Fix again accessing initrd after having been freed

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Date:   Tue, 23 Jan 2018 11:41:33 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     X86 ML <x86@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>
Subject: [PATCH 2/2] x86/microcode: Fix again accessing initrd after having been freed

From: Borislav Petkov <bp@...e.de>

Commit

  24c2503255d3 ("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 24c2503255d3 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.

Reported-by: <higuita@....net>
Cc: <stable@...r.kernel.org> # 4.11
Fixes: f26483eaedec ("Merge branch 'x86/urgent' into x86/microcode, to resolve conflicts")
Link: https://bugzilla.kernel.org/show_bug.cgi?id=198295
Signed-off-by: Borislav Petkov <bp@...e.de>
---
 arch/x86/kernel/cpu/microcode/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/microcode/core.c b/arch/x86/kernel/cpu/microcode/core.c
index c4fa4a85d4cb..e4fc595cd6ea 100644
--- a/arch/x86/kernel/cpu/microcode/core.c
+++ b/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;
-- 
2.13.0