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

[Stephen Smalley]

On 05/20/2016 07:34 AM, Rafael J. Wysocki wrote:
> On Fri, May 20, 2016 at 9:15 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>>
>> * Logan Gunthorpe <logang@xxxxxxxxxxxx> 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?