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Date: Thu, 11 Aug 2016 11:47:27 -0700
From: Thomas Garnier <>
To: "Rafael J. Wysocki" <>
Cc: Jiri Kosina <>, Borislav Petkov <>, 
	"Rafael J. Wysocki" <>, Linux PM list <>, 
	"the arch/x86 maintainers" <>, Linux Kernel Mailing List <>, 
	Yinghai Lu <>, Thomas Gleixner <>, Ingo Molnar <>, 
	"H . Peter Anvin" <>, Kees Cook <>, Pavel Machek <>, 
	Kernel Hardening <>
Subject: Re: [Resend][PATCH] x86/power/64: Always create temporary identity
 mapping correctly

On Wed, Aug 10, 2016 at 6:35 PM, Rafael J. Wysocki <> wrote:
> On Thu, Aug 11, 2016 at 3:17 AM, Thomas Garnier <> wrote:
>> On Wed, Aug 10, 2016 at 5:35 PM, Rafael J. Wysocki <> wrote:
>>> On Wed, Aug 10, 2016 at 11:59 PM, Jiri Kosina <> wrote:
>>>> On Wed, 10 Aug 2016, Rafael J. Wysocki wrote:
>>>>> So I used your .config to generate one for my test machine and with
>>>>> that I can reproduce.
>>>> Was that the config I've sent, or did Boris provide one as well? Which one
>>>> are you able to reproduce with please?
>>> It's the Boris' one.
>>> Moreover, I have found the options that make the difference: unsetting
>>> unset CONFIG_LOCKDEP AFAICS) in it makes hibernation work again with
>>> CONFIG_RANDOMIZE_MEMORY set and with the $subject patch applied.
>>> Unbelievable, but that's what I'm seeing.
>> Nice find!
>>> Now, that leads to a few questions:
>>> - How does lockdep change the picture so it matters for hibernation?
>>> - Why is hibernation the only piece that's affected?
>>> - Why is RANDOMIZE_MEMORY necessary to make this breakage show up?
>>> Thomas, any ideas?
>> No idea so far. I will investigate though.
>> We had an unrelated issue with CONFIG_DEBUG_PAGEALLOC on early boot. I
>> don't think it was related because it was on early boot and with
>> certain e820 memory layout (and PUD randomization that I disabled on
>> the previous patch test). The fix is on tip:
> Well, I don't think this is related.
> In the meantime, I went back to my original .config and verified that
> setting CONFIG_DEBUG_LOCK_ALLOC in it caused hibernation to fail (with
> CONFIG_RANDOMIZE_MEMORY set and with the $subject patch applied), so
> this really matters somehow.
> Besides, now that I have a reproducer, I can check various other
> things and for example this change (sorry for broken whitespace):
> Index: linux-pm/arch/x86/mm/kaslr.c
> ===================================================================
> --- linux-pm.orig/arch/x86/mm/kaslr.c
> +++ linux-pm/arch/x86/mm/kaslr.c
> @@ -122,7 +122,7 @@ void __init kernel_randomize_memory(void
>          prandom_bytes_state(&rand_state, &rand, sizeof(rand));
>          entropy = (rand % (entropy + 1)) & PUD_MASK;
>          vaddr += entropy;
> -        *kaslr_regions[i].base = vaddr;
> +        *kaslr_regions[i].base += PUD_SIZE;

I think it works because the address is fixed now (just PUD aligned).

>          /*
>           * Jump the region and add a minimum padding based on
> makes hibernation work for me again in the above configuration.  To
> me, this means that the $subject patch works as expected and the
> problem really is related to the vaddr value being too big.

I managed to debug the restoration and found that a first access
violation happens here:

(gdb) x/20i 0xffffffffb20a46de
   0xffffffffb20a46de <trace_suspend_resume+14>:
    mov    eax,DWORD PTR gs:[rip+0x4df65a4b]        # 0xa130 <cpu_number>

Handled by do_async_page_fault which will fault as well on this instruction:

=> 0xffffffffb2047ca1 <do_async_page_fault+1>:
    mov    eax,DWORD PTR gs:[rip+0x4dfc4e58]        # 0xcb00 <apf_reason+64>

So there is a problem with the gs register not being restored
correctly at this stage.

In create_image, there is tracing (trace_suspend_resume) before and
after the suspend. Except at this stage, gs was not yet restored. It
uses the old gs leading to the double fault.

I tested this fix to be correct:

diff --git a/kernel/power/hibernate.c b/kernel/power/hibernate.c
index a881c6a..33c79b6 100644
--- a/kernel/power/hibernate.c
+++ b/kernel/power/hibernate.c
@@ -300,12 +300,12 @@ static int create_image(int platform_mode)
  trace_suspend_resume(TPS("machine_suspend"), PM_EVENT_HIBERNATE, true);
  error = swsusp_arch_suspend();
+ /* Restore control flow magically appears here */
+ restore_processor_state();
  trace_suspend_resume(TPS("machine_suspend"), PM_EVENT_HIBERNATE, false);
  if (error)
  printk(KERN_ERR "PM: Error %d creating hibernation image\n",
- /* Restore control flow magically appears here */
- restore_processor_state();
  if (!in_suspend)
  events_check_enabled = false;

Let me know if it works for you. Note that I don't know why this issue
popup with the different config.

> Thanks,
> Rafael

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