Re: [PATCH] Prefer kASLR over Hibernation

[Kees Cook]

On Wed, Apr 6, 2016 at 1:56 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Apr 6, 2016 at 1:17 PM, Pavel Machek <pavel@xxxxxxx> wrote:
>>
>> Why is kASLR incompatible with hibernation? We can hibernate have
>> 4.3 kernel resume hibernation image of 4.2 kernel (on x86-64, and I
>> have patches for x86). Resuming kernel with different randomization
>> does not look that much different...
>
> Oh, I'd absolutely prefer to just allow kaslr together with
> hibernation if it actually works.
>
> Could the people who piped up to say that they actually use
> hibernation just try passing in the "kaslr" command line option on
> their machine, and see if it works for them? We could just remove the
> "no kaslr with hibername" code - or at least limit it to 32-bit for
> now..
>
> Because that would be lovely.

This is where our original investigation of having them coexist ended:
https://lkml.org/lkml/2014/6/15/180

To quote Rafael Wysocki:
> We're jumping from the boot kernel into the image kernel.  The virtual address
> comes from the image kernel, but the boot kernel has to use it.  The only way
> we can ensure that we'll jump to the right place is to pass the physical address
> in the header (otherwise we de facto assume that the virtual address of the
> target page frame will be the same in both the boot and the image kernels).
>
> The missing piece is that the code in swsusp_arch_resume() sets up temporary
> page tables to ensure that they won't be overwritten while copying the last
> remaining image kernel pages to the right page frames (those page tables
> have to be stored in page frames that are free from the kernel image perspective).
>
> But if the kernel address space is randomized, set_up_temporary_mappings()
> really should duplicate the existing layout instead of creating a new one from
> scratch.  Otherwise, virtual addresses before set_up_temporary_mappings() may
> be different from the ones after it.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security