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

[Kees Cook]

On Fri, May 20, 2016 at 2:46 PM, Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> On Fri, May 20, 2016 at 3:56 PM, Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
>> 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?
>
> 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?

(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