Re: [PATCH] Prefer kASLR over Hibernation

[Ingo Molnar]

* Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> 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.

So as I suggested it in the previous mail, the right solution would be to pass in 
the randomization seed via a new kasl_seed=xyz boot option, and thus have the same 
addresses as prior hibernation.

That should make hibernation work as-is, with very little effort.

Two details I can think of:

1) the new option has to be hidden from /proc/cmdline, due to:

  triton:~/tip> ll /proc/cmdline
  -r--r--r-- 1 root root 0 Apr  6 23:45 /proc/cmdline

2)

another detail is that the new boot option has to be checked in 
choose_kernel_location(), to make sure it's done at the right point during bootup. 
That's a good place to remove it from the boot options string as well.

Thanks,

	Ingo