Re: PROBLEM: Resume form hibernate broken by setting NX on gap

[Kees Cook <keescook@chromium.org>]

On Fri, May 20, 2016 at 2:59 PM, Kees Cook <keescook@chromium.org> wrote:
> On Fri, May 20, 2016 at 2:46 PM, Rafael J. Wysocki <rafael@kernel.org> wrote:
>> On Fri, May 20, 2016 at 3:56 PM, Stephen Smalley <sds@tycho.nsa.gov> wrote:
>>> On 05/20/2016 07:34 AM, Rafael J. Wysocki wrote:
>>>> On Fri, May 20, 2016 at 9:15 AM, Ingo Molnar <mingo@kernel.org> wrote:
>>>>>
>>>>> * Logan Gunthorpe <logang@deltatee.com> wrote:
>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> I have been working on a bug that causes my laptop to freeze during
>>>>>> resume from hibernation. I did a bisect to find the offending commit:
>>>>>>
>>>>>> [ab76f7b4ab] x86/mm: Set NX on gap between __ex_table and rodata
>>>>>>
>>>>>> There is more information in the bugzilla report [1] that
>>>>>> I've been working on but I will summarize things below.
>>>>>>
>>>>>> I've experienced intermittent but reproducible freezes when resuming
>>>>>> from hibernation since about kernel version 3.19. The freeze was
>>>>>> significantly more reproducible when a few applications were loaded
>>>>>> before hibernation and would largely not happen if hibernated
>>>>>> immediately after booting to a desktop. I did some tracing work to find
>>>>>> that the kernel gets as far as the resume_image call in
>>>>>> swsusp_arch_resume and I could not find any response from the image
>>>>>> kernel when I hit the bug. I also did testing that seemed to rule out
>>>>>> this being caused by a problematic driver.
>>>>>>
>>>>>> I did a successful bisect between 3.18 and 3.19 which found a bug in
>>>>>> commit f5b2831d6 that was then later fixed by commit 55696b1f66 in 4.4.
>>>>>> Then, I did a second bisect with a ported version of the fix to the
>>>>>> first bug and found commit ab76f7b4ab in 4.3 to also break hibernation
>>>>>> with what appears to be the exact same symptoms. Reverting that commit
>>>>>> in recent kernels up to and including 4.6 fixes the issue and restores
>>>>>> reliable hibernation. However, it's not at all clear to me why that
>>>>>> commit would cause this issue or how to fix the issue without reverting.
>>>>>
>>>>> I've attached that commit below and also Cc:-ed a few more people who might have
>>>>> an idea about why this regressed. Worst-case we'll have to revert it.
>>>>
>>>> Without looking deep into mm, my theory would be that after this patch
>>>> the final jump from the boot kernel to the image kernel's trampoline
>>>> code during resume may crash the kernel if the trampoline page turns
>>>> out to be NX in the boot kernel (it has to be executable in both the
>>>> boot and the image kernels).
>>>
>>> So, pardon my ignorance, but where is this trampoline page placed in
>>> kernel memory?
>>
>> On 32-bit its location has to be the same in both the boot and the
>> image kernels and that's within kernel text in both cases, so that
>> shouldn't be a problem.
>>
>> On 64-bit its location depends on the image kernel and specifically on
>> the location of the restore_registers routine in it.  The (virtual)
>> address of that routine is stored in the restore_jump_address
>> variable, so the page containing it (the trampoline page) can be found
>> with the help of that.
>>
>> swsusp_arch_resume() sets up a temporary kernel mapping to finalize
>> the image restoration and that page must not be NX in that mapping for
>> things to work.
>
> It looks like nothing in the swsusp_arch_resume() -> get_safe_page()
> -> get_image_page() path sets the page executable...
>
> Untested, but I wonder if this work work in swsusp_arch_resume()
> before the memcpy?

I can't type today, it seems. It should read "... if this would work ..."

If you can test this and it works for you, I'll send a proper patch... :P

-Kees

>
> (apologies for any gmail-based whitespace mangling...)
>
> diff --git a/arch/x86/power/hibernate_64.c b/arch/x86/power/hibernate_64.c
> index 009947d419a6..c2f3ecc45bd4 100644
> --- a/arch/x86/power/hibernate_64.c
> +++ b/arch/x86/power/hibernate_64.c
> @@ -12,6 +12,7 @@
>  #include <linux/smp.h>
>  #include <linux/suspend.h>
>
> +#include <asm/cacheflush.h>
>  #include <asm/init.h>
>  #include <asm/proto.h>
>  #include <asm/page.h>
> @@ -89,6 +90,7 @@ int swsusp_arch_resume(void)
>         relocated_restore_code = (void *)get_safe_page(GFP_ATOMIC);
>         if (!relocated_restore_code)
>                 return -ENOMEM;
> +       set_memory_x((unsigned long)relocated_restore_code, 1);
>         memcpy(relocated_restore_code, &core_restore_code,
>                &restore_registers - &core_restore_code);
>
>
> -Kees
>
> --
> Kees Cook
> Chrome OS & Brillo Security



-- 
Kees Cook
Chrome OS & Brillo Security