Re: [kernel-hardening] Re: [RFC v2][PATCH 04/11] x86: Implement __arch_rare_write_begin/unmap()

[Thomas Gleixner]

On Fri, 7 Apr 2017, Mathias Krause wrote:
> On 6 April 2017 at 17:59, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > On Wed, Apr 5, 2017 at 5:14 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >> static __always_inline rare_write_begin(void)
> >> {
> >>     preempt_disable();
> >>     local_irq_disable();
> >>     barrier();
> >>     __arch_rare_write_begin();
> >>     barrier();
> >> }
> >
> > Looks good, except you don't need preempt_disable().
> > local_irq_disable() also disables preemption.  You might need to use
> > local_irq_save(), though, depending on whether any callers already
> > have IRQs off.
> 
> Well, doesn't look good to me. NMIs will still be able to interrupt
> this code and will run with CR0.WP = 0.
> 
> Shouldn't you instead question yourself why PaX can do it "just" with
> preempt_disable() instead?!

That's silly. Just because PaX does it, doesn't mean it's correct. To be
honest, playing games with the CR0.WP bit is outright stupid to begin with.

Whether protected by preempt_disable or local_irq_disable, to make that
work it needs CR0 handling in the exception entry/exit at the lowest
level. And that's just a nightmare maintainence wise as it's prone to be
broken over time. Aside of that it's pointless overhead for the normal case.

The proper solution is:

write_rare(ptr, val)
{
	mp = map_shadow_rw(ptr);
	*mp = val;
	unmap_shadow_rw(mp);
}

map_shadow_rw() is essentially the same thing as we do in the highmem case
where the kernel creates a shadow mapping of the user space pages via
kmap_atomic().

It's valid (at least on x86) to have a shadow map with the same page
attributes but write enabled. That does not require any fixups of CR0 and
just works.

Thanks,

	tglx